First Course in Biology

PREFACE

CONTENTS

GENERAL INTRODUCTION — PRELIMINARY EXPERIMENTS — How a Candle Burns — I. Oxygen — II. Carbon — III. Carbon dioxid

Elements and Compounds.—The material world is made up of elements and compounds. An element is a substance that cannot be separated into two or more substances. A compound is formed by the union of two or more elements. All the material or substance of which the earth and its inhabitants is composed is formed of the chemical elements; this substance taken all together is known as matter.

Carbon and iron are examples of elements. Compare a bit of charcoal, which is one form of carbon, with a new iron nail. Which is brighter? Heavier for its size? Tougher? More brittle? Harder? More readily combustible? Resistant to change when left exposed to air and dampness? There are two other forms of carbon: graphite or black lead (used in pencils and stove polish); and diamond, which occurs in crystals and is the hardest known substance. Iron does not have varied forms like carbon. Sulfur is another element. What is its color? Has it odor? Taste? Will it dissolve in water? Is it heavy or light? Will it burn? What is the color of the flame? Of the fumes? Phosphorus, another element, burns so readily that it ignites by friction and is used in matches. Rub the tip of a match with the finger. What is the odor of phosphorus? Phosphorus exists in nature only in combination with other elements. Lead, tin, silver, gold, copper, zinc, nickel, platinum, are elements.

There are less than eighty known elements; but the compounds formed of them are innumerable. Carbon is found in all substances formed by the growth of living things. That there is carbon in sugar, for example, can easily be shown by charring it on a hot shovel or a stove until its water is driven off and only charcoal is left. Part of the starch in a biscuit remains as charcoal when it has been half burned.

Oxygen and the Air.—The great activity of pure oxygen in attacking other substances can be shown by passing into a fruit-jar a lighted splinter, a piece of lighted magnesium ribbon, an old watch spring (or a bit of picture wire), the end of which has been dipped in sulfur and lighted. About one fifth of the air is oxygen and about four fifths is nitrogen and other inactive gases. Pure nitrogen will quickly extinguish a lighted splinter thrust into it. It is the oxygen in the air that supports all forms of burning. Less than one half of one per cent of the air is an inactive gas called carbon dioxid, a compound of carbon and oxygen. It is formed not only when wood or coal is burned, but also by the life processes of animals and plants.

Favorable and Unfavorable Conditions for Evaporation.—Pour the same quantity of water (half a glassful) into three saucers and two bottles. Place one saucer near a hot stove; place the other two in a cool place, having first covered one of them with a dish. Place one of the bottles by the stove and the other by the remaining saucers. After some hours, examine the saucers and bottles and compare and record the results. Explain. State three conditions that are favorable to evaporation. State three ways in which evaporation may be prevented or decreased.

Tests for Acid, Alkaline, and Neutral Substances.—For acid tests, use sour buttermilk (which contains lactic acid), or hydrochloric acid diluted in ten parts water, or strong vinegar (which contains acetic acid). Has the acid a characteristic (“sour”) odor and taste (test it only when very dilute)? Rub dilute acid between the fingers; how does it feel? Is there any effect on the fingers? Obtain litmus paper at a druggist’s. Dip a strip of red litmus and of blue litmus paper into the acid. What result?

For alkaline tests, dissolve in a glass of water a spoonful of baking soda or some laundry soap; or dissolve an inch stick of caustic soda in a glass of water. Test odor and “feel” of last solution as with the acid; likewise test effect of alkaline solution on red and blue litmus paper. Record results. Alkalies are strong examples of a more general class of substances called bases, which have the opposite effect from acids.

Test pure water. Has it odor? A taste? Test it with red and blue litmus paper. Water is a neutral substance: that is, it is neither an acid nor an alkali (or base).

After making appropriate tests, write ac, al, or neu after each name in the following list (or write in three columns): vinegar, soda, saliva, sugar, juice of apple, lemon, and other fruits, milk, baking powder, buttermilk, ammonia, salt water.

Pour some of the alkaline solution into a dish, gradually add dilute acid (or sour buttermilk), stirring with glass rod and testing with litmus until the mixture does not turn red litmus blue nor blue litmus red. The acid and alkali are then said to have neutralized each other, and the resulting substance is called a salt. The salt may be obtained by evaporating the water of the solution. Most common minerals are salts. If the last experiment is tried with soda and sour buttermilk, the demonstration will show some of the facts involved in bread making with the use of these substances.

Test for Starch.—Starch turns blue with iodine. The color may be driven away by heat, but will return again as the temperature lowers. Procure a few cents’ worth of tincture of iodine and dilute it. Get a half dozen pieces of paper and cardboard, all different, and test each for starch by placing it over mouth of bottle and tipping the bottle up. If much starch is present the spot will be blue-black or dark blue; if little starch, pale blue; if no starch, brown or yellowish.

Make pastes with wheat flour, potato starch, and corn starch. Treat a little of each with a solution of rather dilute tincture of iodine. Try grains from crushed rice with the same solution. Are they the same color? Cut a thin section from a potato, treat with iodine and examine under the microscope.

To study Starch Grains.—Mount in cold water a few grains of starch from each of the following: potato, wheat, arrowroot (buy at drug store), rice, oats, corn. Study under microscope the sizes, forms, layers, fissures, and location of nuclei, and make a drawing of a few grains of each.

Test for Grape Sugar.—Make a thick section of a bit of the edible part of a pear and place it in a bath of Fehling’s solution. After a few moments boil the liquid containing the section for one or two minutes. It will turn to an orange color, showing a deposit of an oxid of copper and perhaps a little copper in the metallic form. A thin section treated in like manner may be examined under the microscope, and the fine particles, precipitated from the sugar of the pear, may be clearly seen. (Fehling’s solution is made by taking one part each of these three solutions and two parts of water: (1) Copper sulfate, 9 grams in 250 cubic centimeters of water; (2) sodium hydroxid, 30 grams in 250 c.c. water; (3) Rochelle salts, 43 grams in 250 c.c. water.)

Test for Nitrogenous Substances, or Proteids.—Put a little white of egg into a test tube and heat slowly. What change takes place in the egg? Put another part of the white of egg into a test tube and add dilute nitric acid. Compare the results of the two experiments. White of egg is an example of a proteid; that is, it is the form of nitrogen most commonly found in plant and animal tissue, and it can be formed only by life processes. Do acid and heat harden or soften most substances? Either of the above tests reveal proteid, if present. Does cooking tend to soften or toughen lean meat?

Another test for proteid is nitric acid, which turns proteid (and hardly anything else) yellow. Proteid when burned has a characteristic odor; this will be noticed if lean meat or cheese is charred in a spoon. The offensive odor from decomposing proteid is also characteristic, whether it comes from stale beans, meat, mushrooms, or other things containing proteid.

Test for Fats and Oils.—Place a little tallow from a candle on unglazed paper and warm. Hold the paper up to the light and examine it. What effect has the fat had on the paper? Place a little starch, sugar, powdered chalk, or white of egg on paper and repeat the experiment; is the effect the same? Place some of the tallow in a spoon, and heat. Compare the effect of heat on fat and proteid. Water also makes paper semi-transparent, but it soon evaporates: fat does not evaporate.

Another test for fats is to mount a thin section of the endosperm of castor-oil seed in water and examine with high power. Small drops of oil will be quite abundant. Treat the mount with alcanin (henna root in alcohol). The drops of oil will stain red. This is a standard test for fats and oils.

To make or liberate Oxygen.—If there is a chemistry class in school, one of its members will doubtless be glad to prepare some of the gas called oxygen, and furnish several glass jars filled with it to the biology class. If it is desired to make oxygen, the following method may be employed: Provide a dry glass flask of three to four ounces capacity. It should have a glass delivery tube, inserted through a one-holed rubber stopper, and so bent as to pass under the surface of water contained in a deep dish. Fill several pint fruit-jars with water, cover with pieces of stiff pasteboard, and turn mouth downwards in the dish of water. From one half to two thirds ounces of an equal mixture of potassium chlorate and manganese dioxid (procured at drug store) is put in the flask and heated by means of a gas or alcohol lamp. When the oxygen begins to form, collect some in jars by inserting the end of delivery tube under the jars as they stand in water. Caution: Remove delivery tube from water before cooling the flask, to prevent any water being drawn back.

Oxidation.—That something besides wood or coal is necessary to a fire can be shown by shutting off entirely the draught of a stove. Fire and other forms of combustion depend on a process called oxidation. This consists in the uniting of oxygen with other substances. When wood decays, the carbon in it oxidizes (unites with oxygen) and carbon dioxid gas is formed. When wood burns, the oxidation is more rapid. When iron oxidizes, iron rust is formed. When hydrogen is oxidized, water is formed. Kerosene oil contains hydrogen, and water is formed when it is burned. Almost every one has noticed the cloud of moisture which collects on the chimney when the lamp is first lighted. By using a chimney which has been kept in a cold place, the moisture becomes apparent; soon the chimney becomes hot and the water no longer collects, but it continues to pass into the room as long as the lamp burns. Fats also contain hydrogen. Hold a piece of cold glass or an inverted tumbler above the flame of a tallow candle. Does water collect on it?

Oxidation may be said to be the basis of all life processes for this reason: oxidation gives rise to heat and sets free energy, and all living things need heat and energy in order to grow and live. The heat of animals is very noticeable. The oxidation in plants also forms a slight amount of heat. In both animals and plants oxidation is much slower than in ordinary fires. That heat is formed even in slow oxidation is shown by fires which arise spontaneously in masses of decaying material. The rotting of wood is not only accompanied by heat but sometimes by light, as when “fox fire” is emitted. Rub the end of a match on your finger in the dark. Explain the result. Strike a match and notice the white fumes which rise for an instant. These fumes are not ordinary smoke (particles of carbon), but they are oxid of phosphorus. Why will water (oxid of hydrogen) not burn? Sand is oxid of silicon. Explain how throwing sand on a fire puts it out. [See also experiments with candle and breath, in The Principles of Biology.]

Inorganic and Organic Matter.—Test for Minerals.—The earth was once in a molten condition, which would have destroyed any combustible material if any had then existed. Before plants and animals existed, the earth consisted mostly of incombustible minerals, known as inorganic matter. Substances formed by animals and plants are organic matter, so called because built up by organized or organ-bearing or living things; starch is an example, being formed in plants. Organic substances are composed chiefly of carbon, oxygen, hydrogen, and nitrogen. (See page 1 of “Animal Biology.”) Coal-oil, and all combustible materials have their origin in life. Hence, burning to find whether there is an incombustible residue is also a test for minerals. Meat, bread, oatmeal, bone, wood, may be tested for mineral matter by burning in a spoon held over a hot fire, or flame of gas or lamp. The substance being tested should be burned until all black material (which is organic carbon and not a mineral) has disappeared. Any residue will be mineral matter.

Protoplasm.—Inside the cells of plants and animals is the living substance, known as protoplasm. It is a structureless, nearly or quite colorless, transparent jelly-like substance of very complex and unstable composition. Eighty per cent or more is water; the remainder is proteid, fats, oils, sugars, and salts. Protoplasm has the power of growth and reproduction; it can make living substance from dead or lifeless substances. It has the power of movement within the cell, and it is influenced (or is irritable) by heat, light, touch, and other stimuli. When protoplasm dies the organism dies.

Physics is the science that treats of the properties and phenomena (or behavior) of matter or of objects; as of such properties or phenomena or agencies as heat, light, force, electricity, sound, friction, density, weight, and the like.

Chemistry is the science that treats of the composition of matter. All matter is made up, as we have seen, of elements. Very few elements exist in nature in a free or uncombined form. The nitrogen and oxygen of the air are the leading uncombined elements.

In order to express the chemical combinations clearly, symbols are used to represent each element, and these symbols are then combined to represent the proportions of each in the compound. If C stands for carbon and O for oxygen, the carbon dioxid might be represented by the formula COO. In order to avoid the repetition of any letter, however, a number is used to denote how many times the element is taken: thus the formula always used for carbon dioxid is CO₂. The formula for hydrogen oxid, or water, is H₂O; that for starch is C₆H₁₀O₅. N stands for nitrogen; P, for phosphorus; K, potassium; Fe, iron; S, sulfur.

Biology is the science that treats of life; that is, of all knowledge of plants and animals of all kinds. (See page 1, “Animal Biology.”)

Light the candle and place it on a piece of blotting paper (A). What do you see burning? Is anything burning besides the candle? The answer will probably be “no.” Let us see.

Place the lamp chimney over the lighted candle, and partly cover the top by a piece of stiff paper, as in Fig. A. Ask the pupils to observe and describe how the flame goes out; i.e. that it is gradually extinguished and does not go out instantly. Why did the flame go out? The probable thought will be, “Because there was no air.” (If there was no air within the chimney, some could have entered at the top.)

A.—The Beginning of the Candle Experiment.

B.—Supplying Air underneath the Chimney.

 

Place two pencils beside the relighted candle and on them the chimney (B). What is the difference between the way in which the candle burns now and before the chimney was placed over it? It flickers, or dances about more. What makes boys and girls feel like dancing about when they go out from a warm schoolroom? What makes the flame dance or flicker when the chimney is raised by the pencils? Because it gets fresh air under the chimney.

Repeat the first experiment, in which the flame grows gradually smaller till it is extinguished. Why does the flame die out now? Is it really necessary to have fresh air in order to keep a flame burning?

To prove this further, let the candle be relighted. Place the chimney over it, now having the top completely closed by a piece of paper. Have ready a lighted splinter or match, and just as soon as the candle is extinguished remove the paper from the chimney top and thrust in the lighted splinter. Why does the light on the splinter go out? What became of the freshness that was in the air? It was destroyed by the burning candle.

Evidently there is some decided difference between unburned air and burned air, since a flame can continue to burn only in air that has the quality known as freshness. This quality of fresh air is due to oxygen, represented by O. Why was the splinter put out instantly, while the candle flame died out gradually? When the splinter was thrust in, the air had no freshness or oxygen at all, while when the candle was placed under the chimney, it had whatever oxygen was originally in the air within the chimney.

Endeavor to have this point clearly understood: that the candle did not go out as long as the air had any oxygen and that the splinter was extinguished immediately because there was no oxygen left.

Relight the candle. A former question may now be repeated: Is anything else burning besides the candle?

When the subject of the necessity of fresh air and consequently of oxygen for the burning of the candle seems to be understood, the following questions, together with any others which suggest themselves, may be asked: What is the reason that draughts are opened in stoves? Why is the bottom of a “burner” on a lamp always full of holes?

Let us now observe the blackened end of a burned match or splinter. This black substance is usually known by the name of charcoal. If handled, it will blacken the fingers. Try this. The same substance is found on the bottoms of kettles which have been used over a wood fire, but it is there a fine powder.

Let us see what was burning when the candle was lighted, besides the oxygen in the air. Relight the candle and hold the porcelain or glass about an inch above the bright part of the flame. What happens to it there? Next, lower it directly into the flame (C). What is the black stuff that gets on the glass? Look closely and see whether it is not deposited here also as a fine powder. Will this deposit from the candle blacken the fingers?

C.—The Carbon (or Soot) is deposited on the Glass.

Instead of using the name charcoal for this black substance, let us call it carbon, the better name, because there are several kinds of carbon, and charcoal is only that kind which is rather light and easily blackens the hands.

The carbon from the candle flame came mostly from the wax or tallow; only a very small part came from the wick. It cannot be seen in the tallow, neither can it be seen in unburned wood, and yet it can be found when the wood is partly burned.

Why, now, is the glass blackened when held in the flame and not when held directly above it? It is because the carbon from the candle has not been completely burned at the middle of the flame; but it is burned beyond the bright part of the flame. When the glass is held in the flame, the carbon that is not yet completely burned is deposited on it, because it is cooler than that in the surrounding flame.

A fine deposit of carbon can be had from any of the luminous parts of the flame; and it is these thousands of little particles of carbon, getting white hot, which glow like coals in the stove and make the light. Just as soon as they are completely burned, there is no more light, as coals cease to glow when burned to ashes.

D.—The Test with the Suspended Film of Limewater.

Let us now inquire what becomes of the carbon that we find in the bright part of the flame and of the oxygen that was in the air in the lamp chimney. When the candle was extinguished within the chimney, there was no oxygen left, as shown by the lighted splinter, which was put out immediately. Neither could any of the particles of carbon be found except on the wick. Yet they both still exist within the chimney, but in an entirely different condition. While the candle was burning, the little particles of carbon that we find ascending in the flame are joining with the oxygen of the air and making an entirely new substance. This new substance is a gas and cannot be seen in the air.

Of what two substances is this new substance made? It is CO₂.

Place a bit of quicklime in about half a glass of water on the day previous to the experiment. When ready for use there will be a white sediment at the bottom and a thin white scum on the top of the clear limewater. The pupils should see this white scum, as a question about it will follow. Make a loop in the end of the piece of wire by turning it around the point of a lead pencil. Remove the scum from the limewater with a piece of paper and insert the loop into the clear water. When withdrawn, the loop ought to hold a film of clear water. Pass the wire through a piece of cardboard or stiff paper, and arrange as shown in D.

Place the chimney over the lighted candle. Lower the loop into the chimney and cover the top of the chimney with the paper. Withdraw the wire two minutes after the candle goes out. Note the cloudy appearance of the film of water on the wire. The cloudiness was caused by the carbon dioxid formed while the candle was burning.

Omitting the candle, hang the freshly wetted wire in the empty chimney. Let the film of limewater remain within the chimney for the same length of time as when the candle was used. It does not become cloudy now. The cloudiness in clear limewater is a test or indication that carbon dioxid is present.

What caused the white scum on the limewater which stood overnight?

How does the CO₂ get into the air? It is formed whenever wood, coal, oil, or gas is burned.

The amount of CO₂ in ordinary air is very small, being only three parts in ten thousand. If the limewater in the loop be left long enough in the air, it will become cloudy. The reason it clouds so quickly when the candle is being burned is that a large amount of CO₂ is formed. Besides being made by real flames, CO₂ is formed every time we breathe out air. Renew the film of water in the loop and breathe against it gently for two or three minutes.

The presence of CO₂ in the breath may be shown better by pouring off some of the clear limewater into a clean glass and blowing into it through a straw.

Why does water put out a fire? The answer is, not alone because it wets and shuts off the supply of free oxygen, but because it cools the carbon, which must be hot in order to unite with the oxygen, and prevents the oxygen of the air from getting as near the carbon as before.

PLANT BIOLOGY — CHAPTER I NO TWO PLANTS OR PARTS ARE ALIKE

Fig. 1.—No Two Branches are Alike. (Hemlock.)

If one compares any two plants of the same kind ever so closely, it will be found that they differ from each other. The difference is apparent in size, form, color, mode of branching, number of leaves, number of flowers, vigor, season of maturity, and the like; or, in other words, all plants and animals vary from an assumed or standard type.

If one compares any two branches or twigs on a tree, it will be found that they differ in size, age, form, vigor, and in other ways (Fig. 1).

If one compares any two leaves, it will be found that they are unlike in size, shape, color, veining, hairiness, markings, cut of the margins, or other small features. In some cases (as in Fig. 2) the differences are so great as to be readily seen in a small black-and-white drawing.

Fig. 2.—No Two Leaves are Alike.

If the pupil extends his observation to animals, he will still find the same truth; for probably no two living objects are exact duplicates. If any person finds two objects that he thinks to be exactly alike, let him set to work to discover the differences, remembering that nothing in nature is so small or apparently trivial as to be overlooked.

Variation, or differences between organs and also between organisms, is one of the most significant facts in nature.

CHAPTER II THE STRUGGLE TO LIVE

Every plant and animal is exposed to unfavorable conditions. It is obliged to contend with these conditions in order to live.

Fig. 3.—A Battle for Life.

No two plants or parts of plants are identically exposed to the conditions in which they live. The large branches in Fig. 1 probably had more room and a better exposure to light than the smaller ones. Probably no two of the leaves in Fig. 2 are equally exposed to light, or enjoy identical advantages in relation to the food that they receive from the tree.

Examine any tree to determine under what advantages or disadvantages any of the limbs may live. Examine similarly the different plants in a garden row (Fig. 3); or the different bushes in a thicket; or the different trees in a wood.

The plant meets its conditions by succumbing to them (that is, by dying), or by adapting itself to them.

The tree meets the cold by ceasing its active growth, hardening its tissues, dropping its leaves. Many herbaceous or soft-stemmed plants meet the cold by dying to the ground and withdrawing all life into the root parts. Some plants meet the cold by dying outright and providing abundance of seeds to perpetuate the kind next season.

Fig. 4.—The Reach for Light of a Tree on the Edge of a Wood.

Plants adapt themselves to light by growing toward it (Fig. 4); or by hanging their leaves in such position that they catch the light; or, in less sunny places, by expanding their leaf surface, or by greatly lengthening their stems so as to overtop their fellows, as do trees and vines.

The adaptations of plants will afford a fertile field of study as we proceed.

Struggle for existence and adaptation to conditions are among the most significant facts in nature.

The sum of all the conditions in which a plant or an animal is placed is called its environment, that is, its surroundings. The environment comprises the conditions of climate, soil, moisture, exposure to light, relation to food supply, contention with other plants or animals. The organism adapts itself to its environment, or else it weakens or dies. Every weak branch or plant has undergone some hardship that it was not wholly able to withstand.

CHAPTER III THE SURVIVAL OF THE FIT

The plants that most perfectly meet their conditions are able to persist. They perpetuate themselves. Their offspring are likely to inherit some of the attributes that enabled them successfully to meet the battle of life. The fit (those best adapted to their conditions) tend to survive.

Adaptation to conditions depends on the fact of variation; that is, if plants were perfectly rigid or invariable (all exactly alike) they could not meet new conditions. Conditions are necessarily new for every organism. It is impossible to picture a perfectly inflexible and stable succession of plants or animals.

Breeding.—Man is able to modify plants and animals. All our common domestic animals are very unlike their original ancestors. So all our common and long-cultivated plants have varied from their ancestors. Even in some plants that have been in cultivation less than a century the change is marked: compare the common black-cap raspberry with its common wild ancestor, or the cultivated blackberry with the wild form.

Fig. 5.—Desirable and Undesirable Types of Cotton Plants. Why?

By choosing seeds from a plant that pleases him, the breeder may be able, under given conditions, to produce numbers of plants with more or less of the desired qualities; from the best of these, he may again choose; and so on until the race becomes greatly improved (Figs. 5, 6, 7). This process of continuously choosing the most suitable plants is known as selection. A somewhat similar process proceeds in wild nature, and it is then known as natural selection.

Fig. 6.—Flax Breeding.

A is a plant grown for seed production; B, for fiber production. Why?

Fig. 7.—Breeding.

A, effect from breeding from smallest grains (after four years), average head; B, result from breeding from the plumpest and heaviest grains (after four years), average head.

CHAPTER IV PLANT SOCIETIES

In the long course of time in which plants have been accommodating themselves to the varying conditions in which they are obliged to grow, they have become adapted to every different environment. Certain plants, therefore, may live together or near each other, all enjoying the same general conditions and surroundings. These aggregations of plants that are adapted to similar general conditions are known as plant societies.

Moisture and temperature are the leading factors in determining plant societies. The great geographical societies or aggregations of the plant world may conveniently be associated chiefly with the moisture supply, as: wet-region societies, comprising aquatic and bog vegetation (Fig. 8); arid-region societies, comprising desert and most sand-region vegetation; mid-region societies, comprising the mixed vegetation in intermediate regions (Fig. 9), this being the commonest type. Much of the characteristic scenery of any place is due to its plant societies. Arid-region plants usually have small and hard leaves, apparently preventing too rapid loss of water. Usually, also, they are characterized by stiff growth, hairy covering, spines, or a much-contracted plant-body, and often by large underground parts for the storage of water.

Plant societies may also be distinguished with reference to latitude and temperature. There are tropical societies, temperate-region societies, boreal or cold-region societies. With reference to altitude, societies might be classified as lowland (which are chiefly wet-region), intermediate (chiefly mid-region), subalpine or mid-mountain (which are chiefly boreal), alpine or high-mountain.

The above classifications have reference chiefly to great geographical floras or societies. But there are societies within societies. There are small societies coming within the experience of every person who has ever seen plants growing in natural conditions. There are roadside, fence-row, lawn, thicket, pasture, dune, woods, cliff, barn-yard societies. Every different place has its characteristic vegetation. Note the smaller societies in Figs. 8 and 9. In the former is a water-lily society and a cat-tail society. In the latter there are grass and bush and woods societies.

Fig. 8.—A Wet-region Society.

Some Details of Plant Societies.—Societies may be composed of scattered and intermingled plants, or of dense clumps or groups of plants. Dense clumps or groups are usually made up of one kind of plant, and they are then called colonies. Colonies of most plants are transient: after a short time other plants gain a foothold amongst them, and an intermingled society is the outcome. Marked exceptions to this are grass colonies and forest colonies, in which one kind of plant may hold its own for years and centuries.

In a large newly cleared area, plants usually first establish themselves in dense colonies. Note the great patches of nettles, jewel-weeds, smart-weeds, clot-burs, fire-weeds in recently cleared but neglected swales, also the fire-weeds in recently burned areas, the rank weeds in the neglected garden, and the ragweeds and May-weeds along the recently worked highway. The competition amongst themselves and with their neighbors finally breaks up the colonies, and a mixed and intermingled flora is generally the result.

Fig. 9.—A Mid-region Society.

In many parts of the world the general tendency of neglected areas is to run into forest. All plants rush for the cleared area. Here and there bushes gain a foothold. Young trees come up; in time these shade the bushes and gain the mastery. Sometimes the area grows to poplars or birches, and people wonder why the original forest trees do not return; but these forest trees may be growing unobserved here and there in the tangle, and in the slow processes of time the poplars perish—for they are short-lived—and the original forest may be replaced. Whether one kind of forest or another returns will depend partly on the kinds that are most seedful in that vicinity and which, therefore, have sown themselves most profusely. Much depends, also, on the kind of undergrowth that first springs up, for some young trees can endure more or less shade than others.

Fig. 10.—Overgrowth and Undergrowth in Three Series,—trees, bushes, grass.

Some plants associate. They grow together. This is possible largely because they diverge or differ in character. Plants associate in two ways: by growing side by side; by growing above or beneath. In sparsely populated societies, plants may grow alongside each other. In most cases, however, there is overgrowth and undergrowth: one kind grows beneath another. Plants that have become adapted to shade are usually undergrowths. In a cat-tail swamp, grasses and other narrow-leaved plants grow in the bottom, but they are usually unseen by the casual observer. Note the undergrowth in woods or under trees (Fig. 10). Observe that in pine and spruce forests there is almost no undergrowth, partly because there is very little light.

On the same area the societies may differ at different times of the year. There are spring, summer, and fall societies. The knoll which is cool with grass and strawberries in June may be aglow with goldenrod in September. If the bank is examined in May, look for the young plants that are to cover it in July and October; if in September, find the dead stalks of the flora of May. What succeeds the skunk cabbage, hepaticas, trilliums, phlox, violets, buttercups of spring? What precedes the wild sunflowers, ragweed, asters, and goldenrod of fall?

The Landscape.—To a large extent the color of the landscape is determined by the character of the plant societies. Evergreen societies remain green, but the shade of green varies from season to season; it is bright and soft in spring, becomes dull in midsummer and fall, and assumes a dull yellow-green or a black-green in winter. Deciduous societies vary remarkably in color—from the dull browns and grays of winter to the brown greens and olive-greens of spring, the staid greens of summer, and the brilliant colors of autumn.

The autumn colors are due to intermingled shades of green, yellow, and red. The coloration varies with the kind of plant, the special location, and the season. Even in the same species or kind, individual plants differ in color; and this individuality usually distinguishes the plant year by year. That is, an oak which is maroon red this autumn is likely to exhibit that range of color every year. The autumn color is associated with the natural maturity and death of the leaf, but it is most brilliant in long and open falls—largely because the foliage ripens more gradually and persists longer in such seasons. It is probable that the autumn tints are of no utility to the plant. Autumn colors are not caused by frost. Because of the long, dry falls and the great variety of plants, the autumnal color of the American landscape is phenomenal.

Ecology.—The study of the relationships of plants and animals to each other and to seasons and environments is known as ecology (still written œcology in the dictionaries). It considers the habits, habitats, and modes of life of living things—the places in which they grow, how they migrate or are disseminated, means of collecting food, their times and seasons of flowering, producing young, and the like.

CHAPTER V THE PLANT BODY

The Parts of a Plant.—Our familiar plants are made up of several distinct parts. The most prominent of these parts are root, stem, leaf, flower, fruit, and seed. Familiar plants differ wonderfully in size and shape,—from fragile mushrooms, delicate waterweeds and pond-scums, to floating leaves, soft grasses, coarse weeds, tall bushes, slender climbers, gigantic trees, and hanging moss.

The Stem Part.—In most plants there is a main central part or shaft on which the other or secondary parts are borne. This main part is the plant axis. Above ground, in most plants, the main plant axis bears the branches, leaves, and flowers; below ground, it bears the roots.

The rigid part of the plant, which persists over winter and which is left after leaves and flowers are fallen, is the framework of the plant. The framework is composed of both root and stem. When the plant is dead, the framework remains for a time, but it slowly decays. The dry winter stems of weeds are the framework, or skeleton of the plant (Figs. 11 and 12). The framework of trees is the most conspicuous part of the plant.

The Root Part.—The root bears the stem at its apex, but otherwise it normally bears only root-branches. The stem, however, bears leaves, flowers, and fruits. Those living surfaces of the plant which are most exposed to light are green or highly colored. The root tends to grow downward, but the stem tends to grow upward toward light and air. The plant is anchored or fixed in the soil by the roots. Plants have been called “earth parasites.”

The Foliage Part.—The leaves precede the flowers in point of time or life of the plant. The flowers always precede the fruits and seeds. Many plants die when the seeds have matured. The whole mass of leaves of any plant or any branch is known as its foliage. In some cases, as in crocuses, the flowers seem to precede the leaves; but the leaves that made the food for these flowers grew the preceding year.

Fig. 11.—Plant of a Wild Sunflower.

Fig. 12.—Framework of Fig. 11.

 

The Plant Generation.—The course of a plant’s life, with all the events through which the plant naturally passes, is known as the plant’s life-history. The life-history embraces various stages, or epochs, as dormant seed, germination, growth, flowering, fruiting. Some plants run their course in a few weeks or months, and some live for centuries.

The entire life-period of a plant is called a generation. It is the whole period from birth to normal death, without reference to the various stages or events through which it passes.

A generation begins with the young seed, not with germination. It ends with death—that is, when no life is left in any part of the plant, and only the seed or spore remains to perpetuate the kind. In a bulbous plant, as a lily or an onion, the generation does not end until the bulb dies, even though the top is dead.

When the generation is of only one season’s duration, the plant is said to be annual. When it is of two seasons, it is biennial. Biennials usually bloom the second year. When of three or more seasons, the plant is perennial. Examples of annuals are pigweed, bean, pea, garden sunflower; of biennials, evening primrose, mullein, teasel; of perennials, dock, most meadow grasses, cat-tail, and all shrubs and trees.

Duration of the Plant Body.—Plant structures which are more or less soft and which die at the close of the season are said to be herbaceous, in contradistinction to being ligneous or woody. A plant which is herbaceous to the ground is called an herb; but an herb may have a woody or perennial root, in which case it is called an herbaceous perennial. Annual plants are classed as herbs. Examples of herbaceous perennials are buttercups, bleeding heart, violet, water lily, Bermuda grass, horse-radish, dock, dandelion, golden rod, asparagus, rhubarb, many wild sunflowers (Figs. 11, 12).

Many herbaceous perennials have short generations. They become weak with one or two seasons of flowering and gradually die out. Thus, red clover usually begins to fail after the second year. Gardeners know that the best bloom of hollyhock, larkspur, pink, and many other plants, is secured when the plants are only two or three years old.

Herbaceous perennials which die away each season to bulbs or tubers, are sometimes called pseud-annuals (that is, false annuals). Of such are lily, crocus, onion, potato, bull nettle, and false indigo of the Southern states.

True annuals reach old age the first year. Plants which are normally perennial may become annual in a shorter-season climate by being killed by frost, rather than by dying naturally at the end of a season of growth. They are climatic annuals. Such plants are called plur-annuals in the short-season region. Many tropical perennials are plur-annuals when grown in the north, but they are treated as true annuals because they ripen sufficient of their crop the same season in which the seeds are sown to make them worth cultivating, as tomato, red pepper, castor bean, cotton. Name several vegetables that are planted in gardens with the expectation that they will bear till frost comes.

Fig. 13.—A Shrub or Bush. Dogwood osier.

Woody or ligneous plants are usually longer lived than herbs. Those that remain low and produce several or many similar shoots from the base are called shrubs, as lilac, rose, elder, osier (Fig. 13). Low and thick shrubs are bushes. Plants that produce one main trunk and a more or less elevated head are trees (Fig. 14). All shrubs and trees are perennial.

Fig. 14.—A Tree. The weeping birch.

Every plant makes an effort to propagate, or to perpetuate its kind; and, as far as we can see, this is the end for which the plant itself lives. The seed or spore is the final product of the plant.

CHAPTER VI SEEDS AND GERMINATION

Fig. 15.—Parts of the Bean.

R, cotyledon; O, caulicle; A, plumule; F, first node.

The seed contains a miniature plant, or embryo. The embryo usually has three parts that have received names: the stemlet, or caulicle; the seed-leaf, or cotyledon (usually 1 or 2); the bud, or plumule, lying between or above the cotyledons. These parts are well seen in the common bean (Fig. 15), particularly when the seed has been soaked for a few hours. One of the large cotyledons—comprising half of the bean—is shown at R. The caulicle is at O. The plumule is shown at A. The cotyledons are attached to the caulicle at F: this point may be taken as the first node or joint.

The Number of Seed-leaves.—All plants having two seed-leaves belong to the group called dicotyledons. Such seeds in many cases split readily in halves, e.g. a bean. Some plants have only one seed-leaf in a seed. They form a group of plants called monocotyledons. Indian corn is an example of a plant with only one seed-leaf: a grain of corn does not split into halves as a bean does. Seeds of the pine family contain more than two cotyledons, but for our purposes they may be associated with the dicotyledons, although really forming a different group.

These two groups—the dicotyledons and the monocotyledons—represent two great natural divisions of the vegetable kingdom. The dicotyledons contain the woody bark-bearing trees and bushes (except conifers), and most of the herbs of temperate climates except the grasses, sedges, rushes, lily tribes, and orchids. The flower-parts are usually in fives or multiples of five, the leaves mostly netted-veined, the bark or rind distinct, and the stem often bearing a pith at the center. The monocotyledons usually have the flower-parts in threes or multiples of three, the leaves long and parallel-veined, the bark not separable, and the stem without a central pith.

Every seed is provided with food to support the germinating plant. Commonly this food is starch. The food may be stored in the cotyledons, as in bean, pea, squash; or outside the cotyledons, as in castor bean, pine, Indian corn. When the food is outside or around the embryo, it is usually called endosperm.

Fig. 16.—External Parts of Bean.

Seed-coats; Markings on Seed.—The embryo and endosperm are inclosed within a covering made of two or more layers and known as the seed-coats. Over the point of the caulicle is a minute hole or a thin place in the coats known as the micropyle. This is the point at which the pollen-tube entered the forming ovule and through which the caulicle breaks in germination. The micropyle is shown at M in Fig. 16. The scar where the seed broke from its funiculus (or stalk that attached it to its pod) is named the hilum. It occupies a third of the length of the bean in Fig. 16. The hilum and micropyle are always present in seeds, but they are not always close together. In many cases it is difficult to identify the micropyle in the dormant seed, but its location is at once shown by the protruding caulicle as germination begins. Opposite the micropyle in the bean (at the other end of the hilum) is an elevation known as the raphe. This is formed by a union of the funiculus, or seed-stalk, with the seed-coats, and through it food was transferred for the development of the seed, but it is now functionless.

Seeds differ wonderfully in size, shape, color, and other characteristics. They also vary in longevity. These characteristics are peculiar to the species or kind. Some seeds maintain life only a few weeks or even days, whereas others will “keep” for ten or twenty years. In special cases, seeds have retained vitality longer than this limit, but the stories that live seeds, several thousand years old, have been taken from the wrappings of mummies are unfounded.

Germination.—The embryo is not dead; it is only dormant. When supplied with moisture, warmth, and oxygen (air), it awakes and grows: this growth is germination. The embryo lives for a time on the stored food, but gradually the plantlet secures a foothold in the soil and gathers food for itself. When the plantlet is finally able to shift for itself, germination is complete.

Early Stages of Seedling.—The germinating seed first absorbs water, and swells. The starchy matters gradually become soluble. The seed-coats are ruptured, the caulicle and plumule emerge. During this process the seed respires freely, throwing off carbon dioxid (CO₂).

The caulicle usually elongates, and from its lower end roots are emitted. The elongating caulicle is known as the hypocotyl (“below the cotyledons”). That is, the hypocotyl is that part of the stem of the plantlet lying between the roots and the cotyledon. The general direction of the young hypocotyl, or emerging caulicle, is downwards. As soon as roots form, it becomes fixed and its subsequent growth tends to raise the cotyledons above the ground, as in the bean. When cotyledons rise into the air, germination is said to be epigeal (“above the earth”). Bean and pumpkin are examples. When the hypocotyl does not elongate greatly and the cotyledons remain under ground, the germination is hypogeal (“beneath the earth”). Pea and scarlet runner bean are examples (Fig. 48). When the germinating seed lies on a hard surface, as on closely compacted soil, the hypocotyl and rootlets may not be able to secure a foothold and they assume grotesque forms. (Fig. 17.) Try this with peas and beans.

Fig. 17.—Pea. Grotesque forms assumed when the roots cannot gain entrance to the soil.

The first internode (“between nodes”) above the cotyledons is the epicotyl. It elevates the plumule into the air, and the plumule-leaves expand into the first true leaves of the plant. These first true leaves, however, may be very unlike the later leaves in shape.

Fig. 18.—Cotyledons of Germinating Bean spread apart to show Elongating Caulicle and Plumule.

Germination of Bean.—The common bean, as we have seen (Fig. 15), has cotyledons that occupy all the space inside the seed-coats. When the hypocotyl, or elongated caulicle, emerges, the plumule-leaves have begun to enlarge, and to unfold (Fig. 18). The hypocotyl elongates rapidly. One end of it is held by the roots. The other is held by the seed-coats in the soil. It therefore takes the form of a loop, and the central part of the loop “comes up” first (a, Fig. 19). Presently the cotyledons come out of the seed-coats, and the plant straightens and the cotyledons expand. These cotyledons, or “halves of the bean,” persist for some time (b, Fig. 19). They often become green and probably perform some function of foliage. Because of its large size, the Lima bean shows all these parts well.

Fig. 19.—Germination Of Bean.

Fig. 20.—Sprouting of Castor Bean.

Fig. 21.—Germination of Castor Bean.

Endosperm at a.

Fig. 22.—Castor Bean.

Endosperm at a, a; cotyledons at b.

 

Fig. 23.—Germination Complete in Castor Bean.

Germination of Castor Bean.—In the castor bean the hilum and micropyle are at the smaller end (Fig. 20). The bean “comes up” with a loop, which indicates that the hypocotyl greatly elongates. On examining germinating seed, however, it will be found that the cotyledons are contained inside a fleshy body, or sac (a, Fig. 21). This sac is the endosperm. Against its inner surface the thin, veiny cotyledons are very closely pressed, absorbing its substance (Fig. 22). The cotyledons increase in size as they reach the air (Fig. 23), and become functional leaves.

Fig. 24.—Sprouting Indian Corn.

Hilum at h; micropyle at d.

Fig. 25.—Kernel of Indian Corn.

Caulicle at b; cotyledon at a; plumule at p.

Fig. 26.—Indian Corn.

Caulicle at c; roots emerging at m; plumule at p.

 

Fig. 27.—Indian Corn.

o, plumule: n to p, epicotyl.

Germination of Monocotyledons.—Thus far we have studied dicotyledonous seeds; we may now consider the monocotyledonous group. Soak kernels of corn. Note that the micropyle and hilum are at the smaller end (Fig. 24). Make a longitudinal section through the narrow diameter; Fig. 25 shows it. The single cotyledon is at a, the caulicle at b, the plumule at p. The cotyledon remains in the seed. The food is stored both in the cotyledon and as endosperm, chiefly the latter. The emerging shoot is the plumule, with a sheathing leaf (p, Fig. 26). The root is emitted from the tip of the caulicle, c. The caulicle is held in a sheath (formed mostly from the seed-coats), and some of the roots escape through the upper end of this sheath (m, Fig. 26). The epicotyl elongates, particularly if the seed is planted deep or if it is kept for a time confined. In Fig. 27 the epicotyl has elongated from n to p. The true plumule-leaf is at o, but other leaves grow from its sheath. In Fig. 28 the roots are seen emerging from the two ends of the caulicle sheath, c, m; the epicotyl has grown to p; the first plumule-leaf is at o.

Fig. 28.—Germination is Complete.

p, top of epicotyl; o, plumule-leaf; m, roots; c, lower roots.

In studying corn or other fruits or seeds, the pupil should note how the seeds are arranged, as on the cob. Count the rows on a corn cob. Odd or even in number? Always the same number? The silk is the style: find where it was attached to the kernel. Did the ear have any coverings? Explain. Describe colors and markings of kernels of corn; and of peas, beans, castor bean.

Gymnosperms.—The seeds in the pine cone, not being inclosed in a seed-vessel, readily fall out when the cone dries and the scales separate. Hence it is difficult to find cones with seeds in them after autumn has passed (Fig. 29). The cedar is also a gymnosperm.

Fig. 29.—Cones of Hemlock (above), White Pine, Pitch Pine.

Remove a scale from a pine cone and draw it and the seeds as they lie in place on the upper side of the scale. Examine the seed, preferably with a magnifying glass. Is there a hilum? The micropyle is at the bottom or little end of the seed. Toss a seed upward into the air. Why does it fall so slowly? Can you explain the peculiar whirling motion by the shape of the wing? Repeat the experiment in the wind. Remove the wing from a seed and toss it and an uninjured seed into the air together. What do you infer from these experiments?

Fig. 30.—Muskmelon Seedlings, with the unlike seed-leaves and true leaves.

Fig. 31.—A Home-made Seed-tester.

A.—Improvised Stand for Lens.

B.—Dissecting Needle ¹⁄₂ natural size.

C.—Dissecting Glass.

D.—Dissecting Stand.

CHAPTER VII THE ROOT—THE FORMS OF ROOTS

The Root System.—The offices of the root are to hold the plant in place, and to gather food. Not all the food materials, however, are gathered by the roots.

 

Fig. 32.—Tap-root System of Alfalfa.

Fig. 33.—Tap-root of the Dandelion.

 

The entire mass of roots of any plant is called its root system. The root system may be annual, biennial or perennial, herbaceous or woody, deep or shallow, large or small.

Kinds of Roots.—A strong leading central root, which runs directly downwards, is a tap-root. The tap-root forms an axis from which the side roots may branch. The side or spreading roots are usually smaller. Plants that have such a root system are said to be tap-rooted. Examples are red clover, alfalfa, beet, turnip, radish, burdock, dandelion, hickory (Figs. 32, 33).

A fibrous root system is one that is composed of many nearly equal slender branches. The greater number of plants have fibrous roots. Examples are many common grasses, wheat, oats, corn. The buttercup in Fig. 34 has a fibrous root system. Many trees have a strong tap-root when very young, but after a while it ceases to extend strongly and the side roots develop until finally the tap-root character disappears.

Fig. 34.—A Buttercup Plant, with fibrous roots.

Shape and Extent of the Root System.—The depth to which roots extend depends on the kind of plant, and the nature of the soil. Of most plants the roots extend far in all directions and lie comparatively near the surface. The roots usually radiate from a common point just beneath the surface of the ground.

The roots grow here and there in search of food, often extending much farther in all directions than the spread of the top of the plant. Roots tend to spread farther in poor soil than in rich soil, for the same size of plant. The root has no such definite form as the stem has. Roots are usually very crooked, because they are constantly turned aside by obstacles. Examine roots in stony soil.

The extent of root surface is usually very large, for the feeding roots are fine and very numerous. An ordinary plant of Indian corn may have a total length of root (measured as if the roots were placed end to end) of several hundred feet.

The fine feeding roots are most abundant in the richest part of the soil. They are attracted by the food materials. Roots often will completely surround a bone or other morsel. When roots of trees are exposed, observe that most of them are horizontal and lie near the top of the ground. Some roots, as of willows, extend far in search of water. They often run into wells and drains, and into the margins of creeks and ponds. Grow plants in a long narrow box, in one end of which the soil is kept very dry and in the other moist: observe where the roots grow.

Buttresses.—With the increase in diameter, the upper roots often protrude above the ground and become bracing buttresses. These buttresses are usually largest in trees which always have been exposed to strong winds (Fig. 35). Because of growth and thickening, the roots elevate part of their diameter, and the washing away of the soil makes them to appear as if having risen out of the ground.

Fig. 35.—The Bracing Base of a Field Pine.

Aërial Roots.—Although roots usually grow underground, there are some that naturally grow above ground. These usually occur on climbing plants, the roots becoming supports or fulfilling the office of tendrils. These aërial roots usually turn away from the light, and therefore enter the crevices and dark places of the wall or tree over which the plant climbs. The trumpet creeper (Fig. 36), true or English ivy, and poison ivy climb by means of roots.

Fig. 36.—Aërial Roots of Trumpet Creeper or Tecoma.

Fig. 37.—Aërial Roots of an Orchid.

In some plants all the roots are aërial; that is, the plant grows above ground, and the roots gather food from the air. Such plants usually grow on trees. They are known as epiphytes or air-plants. The most familiar examples are some of the tropical orchids, which are grown in glass-houses (Fig. 37). Rootlike organs of dodder and other parasites are discussed in a future chapter.

Some plants bear aërial roots, that may propagate the plant or may act as braces. They are often called prop-roots. The roots of Indian corn are familiar (Fig. 38). Many ficus trees, as the banyan of India, send out roots from their branches; when these roots reach the ground they take hold and become great trunks, thus spreading the top of the parent tree over large areas. The muscadine grape of the Southern states often sends down roots from its stems. The mangrove tree of the tropics grows along seashores and sends down roots from the overhanging branches (and from the fruits) into the shallow water, and thereby gradually marches into the sea. The tangled mass behind catches the drift, and soil is formed.

Fig. 38.—Indian Corn, showing the brace roots at oo.

Adventitious Roots.—Sometimes roots grow from the stem or other unusual places as the result of some accident to the plant, being located without known method or law. They are called adventitious (chance) roots. Cuttings of the stems of roses, figs, geraniums, and other plants, when planted, send out adventitious roots and form new plants. The ordinary roots, or soil roots, are of course not classed as adventitious roots. The adventitious roots arise on occasion, and not as a normal or regular course in the growth of the plant.

No two roots are alike; that is, they vary among themselves as stems and leaves do. Each kind of plant has its own form or habit of root (Fig. 39). Carefully wash away the soil from the roots of any two related plants, as oats and wheat, and note the differences in size, depth, direction, mode of branching, number of fibrils, color, and other features. The character of the root system often governs the treatment that the farmer should give the soil in which the plant or crop grows.

Fig. 39.—Roots of Barley at A and Corn at B.

Carefully trace the differences.

Roots differ not only in their form and habit, but also in color of tissue, character of bark or rind, and other features. It is excellent practice to try to identify different plants by means of their roots. Let each pupil bring to school two plants with the roots very carefully dug up, as cotton, corn, potato, bean, wheat, rye, timothy, pumpkin, clover, sweet pea, raspberry, strawberry, or other common plants.

Root Systems of Weeds.—Some weeds are pestiferous because they seed abundantly, and others because their underground parts run deep or far and are persistent. Make out the root systems in the six worst weeds in your locality.

CHAPTER VIII THE ROOT.—FUNCTION AND STRUCTURE

The function of roots is twofold,—to provide support or anchorage for the plant, and to collect and convey food materials. The first function is considered in Chapter VII; we may now give attention in more detail to the second.

The feeding surface of the roots is near their ends. As the roots become old and hard, they serve only as channels through which food passes and as hold-fasts or supports for the plant. The root-hold of a plant is very strong. Slowly pull upwards on some plant, and note how firmly it is anchored in the soil.

Fig. 40.—Wheat growing under Different Soil Treatments. Soil deficient in nitrogen; commercial nitrogen applied to pot 3 (on right).

Roots have power to choose their food; that is, they do not absorb all substances with which they come in contact. They do not take up great quantities of useless or harmful materials, even though these materials may be abundant in the soil; but they may take up a greater quantity of some of the plant-foods than the plant can use to advantage. Plants respond very quickly to liberal feeding,—that is, to the application of plant-food to the soil (Fig. 40). The poorer the soil, the more marked are the results, as a rule, of the application of fertilizers. Certain substances, as common salt, will kill the roots.

Roots absorb Substances only in Solution.—Substances cannot be taken in solid particles. These materials are in solution in the soil water, and the roots themselves also have the power to dissolve the soil materials to some extent by means of substances that they excrete. The materials that come into the plant through the roots are water and mostly the mineral substances, as compounds of potassium, iron, phosphorus, calcium, magnesium, sulfur, and chlorine. These mineral substances compose the ash when the plant is burned. The carbon is derived from the air through the green parts. Oxygen is derived from the air and the soil water.

Fig. 41.—Nodules on Roots of Red Clover.

Nitrogen enters through the Roots.—All plants must have nitrogen; yet, although about four fifths of the air is nitrogen, plants are not able, so far as we know, to take it in through their leaves. It enters through the roots in combination with other elements, chiefly in the form of nitrates (certain combinations with oxygen and a mineral base). The great family of leguminous plants, however (as peas, beans, cowpea, clover, alfalfa, vetch), use the nitrogen contained in the air in the soil. They are able to utilize it through the agency of nodules on their roots (Figs. 41, 42). These nodules contain bacteria, which appropriate the free or uncombined nitrogen and pass it on to the plant. The nitrogen becomes incorporated in the plant tissue, so that these crops are high in their nitrogen content. Inasmuch as nitrogen in any form is expensive to purchase in fertilizers, the use of leguminous crops to plow under is a very important agricultural practice in preparing the land for other crops. In order that leguminous crops may acquire atmospheric nitrogen more freely and thereby thrive better, the land is sometimes sown or inoculated with the nodule-forming bacteria.

Fig. 42.—Nodules on Vetch.

Fig. 43.—Two Kinds of Soil that have been Wet and then Dried. The loamy soil above remains loose and capable of growing plants; the clay soil below has baked and cracked.

Roots require moisture in order to serve the plant. The soil water that is valuable to the plant is not the free water, but the thin film of moisture which adheres to each little particle of soil. The finer the soil, the greater the number of particles, and therefore the greater is the quantity of film moisture that it can hold. This moisture surrounding the grains may not be perceptible, yet the plant can use it. Root absorption may continue in a soil which seems to be dust dry. Soils that are very hard and “baked” (Fig. 43) contain very little moisture or air,—not so much as similar soils that are granular or mellow.

Proper Temperature for Root Action.—The root must be warm in order to perform its functions. Should the soil of fields or greenhouses be much colder than the air, the plant suffers. When in a warm atmosphere, or in a dry atmosphere, plants need to absorb much water from the soil, and the roots must be warm if the root-hairs are to supply the water as rapidly as it is needed. If the roots are chilled, the plant may wilt or die.

Roots need Air.—Corn on land that has been flooded by heavy rains loses its green color and turns yellow. Besides diluting plant-food, the water drives the air from the soil, and this suffocation of the roots is very soon apparent in the general ill health of the plant. Stirring or tilling the soil aërates it. Water plants and bog plants have adapted themselves to their particular conditions. They get their air either by special surface roots, or from the water through stems and leaves.

Rootlets.—Roots divide into the thinnest and finest fibrils: there are roots and there are rootlets. The smallest rootlets are so slender and delicate that they break off even when the plant is very carefully lifted from the soil.

Fig. 44.—Root-hairs of the Radish.

The rootlets, or fine divisions, are clothed with the root-hairs (Figs. 44, 45, 46). These root-hairs attach to the soil particles, and a great amount of soil is thus brought into actual contact with the plant. These are very delicate prolonged surface cells of the roots. They are borne for a short distance just back of the tip of the root.

Rootlet and root-hair differ. The rootlet is a compact cellular structure. The root-hair is a delicate tubular cell (Fig. 45), within which is contained living matter (protoplasm); and the protoplasmic lining membrane of the wall governs the entrance of water and substances in solution. Being long and tube-like, these root-hairs are especially adapted for taking in the largest quantity of solutions; and they are the principal means by which plant-food is absorbed from the soil, although the surfaces of the rootlets themselves do their part. Water plants do not produce an abundant system of root-hairs, and such plants depend largely on their rootlets.

Fig. 45.—Cross-section of Root, enlarged, showing root-hairs.

Fig. 46.—Root-hair, much enlarged, in contact with the soil particles (s). Air-spaces at a; water-films on the particles, as at w.

The root-hairs are very small, often invisible. They, with the young roots, are usually broken off when the plant is pulled up. They are best seen when seeds are germinated between layers of dark blotting paper or flannel. On the young roots, they will be seen as a mold-like or gossamer-like covering. Root-hairs soon die: they do not grow into roots. New ones form as the root grows.

Osmosis.—The water with its nourishment goes through the thin walls of the root-hairs and rootlets by the process of osmosis. If there are two liquids of different density on the inside and outside of an organic (either vegetable or animal) membrane, the liquids tend to mix through the membrane. The law of osmosis is that the most rapid flow is toward the denser solution. The protoplasmic lining of the cell wall is such a membrane. The soil water being a weaker solution than the sap in the roots, the flow is into the root. A strong fertilizer sometimes causes a plant to wither, or “burns it.” Explain.

Structure of Roots.—The root that grows from the lower end of the caulicle is the first or primary root. Secondary roots branch from the primary root. Branches of secondary roots are sometimes called tertiary roots. Do the secondary roots grow from the cortex, or from the central cylinder of the primary root? Trim or peel the cortex from a root and its branches and determine whether the branches still hold to the central cylinder of the main root.

Internal Structure of Roots.—A section of a root shows that it consists of a central cylinder (see Fig. 45) surrounded by a layer. This layer is called the cortex. The outer layer of cells in the cortex is called the epidermis, and some of the cells of the epidermis are prolonged and form the delicate root-hairs. The cortex resembles the bark of the stem in its nature. The central cylinder contains many tube-like canals, or “vessels” that convey water and food (Fig. 45). Cut a sweet potato across (also a radish and a turnip) and distinguish the central cylinder, cortex and epidermis. Notice the hard cap on the tip of roots. Roots differ from stems in having no real pith.

Microscopic Structure of Roots.—Near the end of any young root or shoot the cells are found to differ from each other more or less, according to the distance from the point. This differentiation takes place in the region just back of the growing point. To study growing points, use the hypocotyl of Indian corn which has grown about one half inch. Make a longitudinal section. Note these points (Fig. 47): (a) the tapering root-cap beyond the growing point; (b) the blunt end of the root proper and the rectangular shape of the cells found there; (c) the group of cells in the middle of the first layers beneath the root-cap,—this group is the growing point; (d) study the slight differences in the tissues a short distance back of the growing point. There are four regions: the central cylinder, made up of several rows of cells in the center (pl); the endodermis, (e) composed of a single layer on each side which separates the central cylinder from the bark; the cortex, or inner bark, (e) of several layers outside the endodermis; and the epidermis, or outer layer of bark on the outer edges (d). Make a drawing of the section. If a series of the cross-sections of the hypocotyl should be made and studied, beginning near the growing point and going upward, it would be found that these four tissues become more distinctly marked, for at the tip the tissues have not yet assumed their characteristic form. The central cylinder contains the ducts and vessels which convey the sap.

Fig. 47.—Growing Point of Root of Indian Corn.

d, d, cells which will form the epidermis; p, p, cells that will form bark; e, e, endodermis; pl, cells which will form the axis cylinder; i, initial group of cells, or growing point proper; c, root-cap.

The Root-cap.—Note the form of the root-cap shown in the microscopic section drawn in Fig. 47. Growing cells, and especially those which are forming tissue by subdividing, are very delicate and are easily injured. The cells forming the root-cap are older and tougher and are suited for pushing aside the soil that the root may penetrate it.

Region of most Rapid Growth.—The roots of a seedling bean may be marked at equal distances by waterproof ink or by bits of black thread tied moderately tight. The seedling is then replanted and left undisturbed for two days. When it is dug up, the region of most rapid growth in the root can be determined. Give a reason why a root cannot elongate throughout its length,—whether there is anything to prevent a young root from doing so.

Fig. 48.—The Marking of the Stem and Root.

In Fig. 48 is shown a germinating scarlet runner bean with a short root upon which are marks made with waterproof ink; and the same root (Fig. 49) is shown after it has grown longer. Which part of it did not lengthen at all? Which part lengthened slightly? Where is the region of most rapid growth?

Fig. 49.—The Result.

Geotropism.—Roots turn toward the earth, even if the seed is planted with the micropyle up. This phenomenon is called positive geotropism. Stems grow away from the earth. This is negative geotropism.

Fig. 50.—The Grasp of a Plant on the Particles Of Earth. A grass plant pulled in a garden.

Fig. 51.—Plant growing in Inverted Pot.

Fig. 52.—Holes in Soil made by Roots, now decayed. Somewhat magnified.

CHAPTER IX THE STEM—KINDS AND FORMS; PRUNING

The Stem System.—The stem of a plant is the part that bears the buds, leaves, flowers, and fruits. Its office is to hold these parts up to the light and air; and through its tissues the various food-materials and the life-giving fluids are distributed to the growing and working parts.

The entire mass or fabric of stems of any plant is called its stem system. It comprises the trunk, branches, and twigs, but not the stalks of leaves and flowers that die and fall away. The stem system may be herbaceous or woody, annual, biennial, or perennial; and it may assume many sizes and shapes.

Stems are of Many Forms.—The general way in which a plant grows is called its habit. The habit is the appearance or general form. Its habit may be open or loose, dense, straight, crooked, compact, straggling, climbing, erect, weak, strong, and the like. The roots and leaves are the important functional or working parts; the stem merely connects them, and its form is exceedingly variable.

Kinds of Stems.—The stem may be so short as to be scarcely distinguishable. In such cases the crown of the plant—that part just at the surface of the ground—bears the leaves and flowers; but this crown is really a very short stem. The dandelion, Fig. 33, is an example. Such plants are often said to be stemless, however, in order to distinguish them from plants that have long or conspicuous stems. These so-called stemless plants die to the ground every year.

Fig. 55.—Trailing Stem of Wild Morning Glory (Convolvulus arvensis).

Stems are erect when they grow straight up (Figs. 53, 54). They are trailing when they run along on the ground, as melon, wild morning-glory (Fig. 55). They are creeping when they run on the ground and take root at places, as the strawberry. They are decumbent when they lop over to the ground. They are ascending when they lie mostly or in part on the ground but stand more or less upright at their ends; example, a tomato. They are climbing when they cling to other objects for support (Figs. 36, 56).

Fig. 56.—A Climbing Plant (a twiner).

Trees in which the main trunk or the “leader” continues to grow from its tip are said to be excurrent in growth. The branches are borne along the sides of the trunk, as in common pines (Fig. 57) and spruces. Excurrent means running out or running up.

Fig. 57.—Excurrent Trunk. A pine.

Trees in which the main trunk does not continue are said to be deliquescent. The branches arise from one common point or from each other. The stem is lost in the branches. The apple tree, plum (Fig. 58), maple, elm, oak, China tree, are familiar examples. Deliquescent means dissolving or melting away.

Fig. 58.—Deliquescent Trunk of Plum Tree.

Each kind of plant has its own peculiar habit or direction of growth; spruces always grow to a single stem or trunk, pear trees are always deliquescent, morning-glories are always trailing or climbing, strawberries are always creeping. We do not know why each plant has its own habit, but the habit is in some way associated with the plant’s genealogy or with the way in which it has been obliged to live.

The stem may be simple or branched. A simple stem usually grows from the terminal bud, and side branches either do not start, or, if they start, they soon perish. Mulleins (Fig. 53) are usually simple. So are palms.

Branched stems may be of very different habit and shape. Some stem systems are narrow and erect; these are said to be strict (Fig. 54). Others are diffuse, open, branchy, twiggy.

Nodes and Internodes.—The parts of the stem at which buds grow are called nodes or joints and the spaces between the buds are internodes. The stem at nodes is usually enlarged, and the pith is usually interrupted. The distance between the nodes is influenced by the vigor of the plant: how?

Fig. 59.—Rhizome or Rootstock.

Stems vs. Roots.—Roots sometimes grow above ground (Chap. VII); so, also, stems sometimes grow underground, and they are then known as subterranean stems, rhizomes, or rootstocks (Fig. 59).

Stems normally bear leaves and buds, and thereby are they distinguished from roots; usually, also, they contain a pith. The leaves, however, may be reduced to mere scales, and the buds beneath them may be scarcely visible. Thus the “eyes” on a white potato are cavities with a bud or buds at the bottom (Fig. 60). Sweet potatoes have no evident “eyes” when first dug (but they may develop adventitious buds before the next growing-season). The white potato is a stem: the sweet potato is probably a root.

Fig. 60.—Sprouts arising from the Buds, or eyes, of a potato tuber.

How Stems elongate.—Roots elongate by growing near the tip. Stems elongate by growing more or less throughout the young or soft part or “between joints” (Figs. 48, 49). But any part of the stem soon reaches a limit beyond which it cannot grow, or becomes “fixed”; and the new parts beyond elongate until they, too, become rigid. When a part of the stem once becomes fixed or hard, it never increases in length: that is, the trunk or woody parts never grow longer or higher; branches do not become farther apart or higher from the ground.

Stems are modified in form by the particular or incidental conditions under which they grow. The struggle for light is the chief factor in determining the shape and direction of any limb (Chap. II). This is well illustrated in any tree or bush that grows against a building or on the margin of a forest (Fig. 4). In a very dense thicket the innermost trees shoot up over the others or they perish. Examine any stem and endeavor to determine why it took its particular form.

The stem is cylindrical, the outer part being bark and the inner part being wood or woody tissue. In the dicotyledonous plants, the bark is usually easily separated from the remainder of the cylinder at some time of the year; in monocotyledonous plants the bark is not free. Growth in thickness takes place inside the covering and not on the very outside of the plant cylinder. It is evident, then, that the covering of bark must expand in order to allow of the expansion of the woody cylinder within it. The tissues, therefore, must be under constant pressure or tension. It has been determined that the pressure within a growing trunk is often as much as fifty pounds to the square inch. The lower part of the limb in Fig. 61 shows that the outer layers of bark (which are long since dead, and serve only as protective tissue) have reached the limit of their expanding capacity and have begun to split. The pupil will now be interested in the bark on the body of an old elm tree (Fig. 62); and he should be able to suggest one reason why stems remain cylindrical, and why the old bark becomes marked with furrows, scales, and plates.

Fig. 61.—Cracking of the Bark on an Elm Branch.

Fig. 62.—Piece of Bark from an Old Elm Trunk.

Most woody plants increase in diameter by the addition of an annual layer or “ring” on the outside of the woody cylinder, underneath the bark. The monocotyledonous plants comprise very few trees and shrubs in temperate climates (the palms, yuccas, and other tree-like plants are of this class), and they do not increase greatly in diameter and they rarely branch to any extent. Consult the woodpile for information as to the annual rings.

Bark-bound Trees.—If, for any reason, the bark should become so dense and strong that the trunk cannot expand, the tree is said to be “bark-bound.” Such condition is not rare in orchard trees that have been neglected. When good tillage is given to such trees, they may not be able to overcome the rigidity of the old bark, and, therefore, do not respond to the treatment. Sometimes the thinner-barked parts may outgrow in diameter the trunk or the old branches below them. The remedy is to release the tension. This may be done either by softening the bark (by washes of soap or lye), or by separating it. The latter is done by slitting the bark-bound part (in spring), thrusting the point of a knife through the bark to the wood and then drawing the blade down the entire length of the bark-bound part. The slit is scarcely discernible at first, but it opens with the growth of the tree, filling up with new tissue beneath. Let the pupil consider the ridges which he now and then finds on trees, and determine whether they have any significance—whether the tree has ever been released or injured by natural agencies.

Fig. 63.—Proper Cutting of a Branch. The wound will soon be “healed.”

The Tissue covers the Wounds and “heals” them.—This is seen in Fig. 63, in which a ring of tissue rolls out over the wound. This ring of healing tissue forms most rapidly and uniformly when the wound is smooth and regular. Observe the healing on broken and splintered limbs; also the difference in rapidity of healing between wounds on strong and weak limbs. There is difference in the rapidity of the healing process in different kinds of trees. Compare the apple tree and the peach. This tissue may in turn become bark-bound, and the healing may stop. On large wounds it progresses more rapidly the first few years than it does later. This roll or ring of tissue is called a callus.

The callus grows from the living tissue of the stem just about the wound. It cannot cover long dead stubs or very rough broken branches (Fig. 64). Therefore, in pruning the branches should be cut close to the trunk and made even and smooth; all long stubs must be avoided. The seat of the wound should be close to the living part of the trunk, for the stub of the limb that is severed has no further power in itself of making healing tissue. The end of the remaining stub is merely covered over by the callus, and usually remains a dead piece of wood sealed inside the trunk (Fig. 65). If wounds do not heal over speedily, germs and fungi obtain foothold in the dying wood and rot sets in. Hollow trees are those in which the decay-fungi have progressed into the inner wood of the trunk; they have been infected (Fig. 66).

Fig. 64.—Erroneous Pruning.

Fig. 65.—Knot in a Hemlock Log.

Large wounds should be protected with a covering of paint, melted wax, or other adhesive and lasting material, to keep out the germs and fungi. A covering of sheet iron or tin may keep out the rain, but it will not exclude the germs of decay; in fact, it may provide the very moist conditions that such germs need for their growth. Deep holes in trees should be treated by having all the decayed parts removed down to the clean wood, the surfaces painted or otherwise sterilized, and the hole filled with wax or cement.

Fig. 66.—A Knot Hole, and the beginning of a hollow trunk.

Stems and roots are living, and they should not be wounded or mutilated unnecessarily. Horses should never be hitched to trees. Supervision should be exercised over persons who run telephone, telegraph, and electric light wires, to see that they do not mutilate trees. Electric light wires and trolley wires, when carelessly strung or improperly insulated, may kill trees (Fig. 67).

Fig. 67.—Elm Tree killed by a Direct Current from an Electric Railroad System.

Fig. 68.—Potato. What are roots, and what stems? Has the plant more than one kind of stem? more than two kinds? Explain.

CHAPTER X THE STEM—ITS GENERAL STRUCTURE

There are two main types of stem structure in flowering plants, the differences being based on the arrangement of bundles or strands of tissue. These types are endogenous and exogenous (page 20). It will require patient laboratory work to understand what these types and structures are.

Endogenous, or Monocotyledonous Stems.—Examples of endogenous stems are all the grasses, cane-brake, sugar-cane, smilax or green-brier, palms, banana, canna, bamboo, lilies, yucca, asparagus, all the cereal grains. For our study, a cornstalk may be used as a type.

Fig. 69.—Cross-section of Cornstalk, showing the scattered fibro-vascular bundles. Slightly enlarged.

A piece of cornstalk, either green or dead, should be in the hand of each pupil while studying this lesson. Fig. 69 will also be of use. Is there a swelling at the nodes? Which part of the internode comes nearest to being perfectly round? There is a grooved channel running along one side of the internode: how is it placed with reference to the leaf? with reference to the groove in the internode below it? What do you find in each groove at its lower end? (In a dried stalk only traces of this are usually seen.) Does any bud on a cornstalk besides the one at the top ever develop? Where do suckers come from? Where does the ear grow?

Cut a cross-section of the stalk between the nodes (Fig. 69). Does it have a distinct bark? The interior consists of soft “pith” and tough woody parts. The wood is found in strands or fibers. Which is more abundant? Do the fibers have any definite arrangement? Which strands are largest? Smallest? The firm smooth rind (which cannot properly be called a bark) consists of small wood strands packed closely together. Grass stems are hollow cylinders; and the cornstalk, because of the lightness of its contents, is also practically a cylinder. Stems of this kind are admirably adapted for providing a strong support to leaves and fruit. This is in accordance with the well-known law that a hollow cylinder is much stronger than a solid cylinder of the same weight of material. Cut a thin slice of the inner soft part and hold it up to the light. Can you make out a number of tiny compartments or cells? These cells consist of a tissue called parenchyma, the tissue from which when young all the other tissues arise and differentiate (Parenchyma = parent + chyma, or tissue). The numerous walls of these cells may serve to brace the outer wall of the cylinder; but their chief function in the young stalk is to give origin to other cells. When alive they are filled with cell sap and protoplasm.

Fig. 70.—Diagram to show the Course of Fibro-vascular Bundles in Monocotyledons.

Trace the woody strands through the nodes. Do they ascend vertically? Do they curve toward the rind at certain places? Compare their course with the strands shown in Fig. 70. The woody strands consist chiefly of tough fibrous cells that give rigidity and strength to the plant, and of long tubular interrupted canals that serve to convey sap upward from the root and to convey food downward from the leaves to the stem and roots.

Monocotyledons, as shown by fossils, existed before dicotyledons appeared, and it is thought that the latter were developed from ancestors of the former. It will be interesting to trace the relationship in stem structure. It will first be necessary to learn something of the structure of the wood strand.

Fig. 71.—Diagram of Wood Strands or Fibro-vascular Bundles in a Root, showing the wood (x) and bast (p) separated.

Wood Strand in Monocotyledons and Dicotyledons.—Each wood strand (or fibro-vascular bundle) consists of two parts—the bast and the wood proper. The wood is on the side of the strand toward the center of the stem and contains large tubular canals that take the watery sap upward from the roots. The bast is on the side toward the bark and contains fine tubes through which diffuses the dense sap containing digested food from the leaves. In the root (Fig. 71) the bast and the wood are separate, so that there are two kinds of strands.

Fig. 72.—Part of Cross-section of Rootstock Of Asparagus, showing a few fibro-vascular bundles. An endogenous stem.

In monocotyledons, as already said, the strands (or bundles) are usually scattered in the stem with no definite arrangement (Figs. 72, 73). In dicotyledons the strands, or bundles, are arranged in a ring. As the dicotyledonous seed germinates, five bundles are usually formed in its hypocotyl (Fig. 74); soon five more are interposed between them, and the multiplication continues, in tough plants, until the bundles touch (Fig. 74, right). The inner parts thus form a ring of wood and the outer parts form the inner bark or bast. A new ring of wood or bast is formed on stems of dicotyledons each year and the age of a cut stem is easily determined.

a

b

Fig. 73.—The Scattered Bundles or Strands, in monocotyledons at a, and the bundles in a circle in dicotyledons at b.

Fig. 74.—Dicotyledonous Stem of One Year at Left with Five Bundles, and a Two-year Stem at Right.

o, the pith; c, the wood part; b, the bast part; a, one year’s growth.

Fig. 75.—Fibro-vascular Bundle of Indian Corn, much magnified.

A, annular vessel; A′, annular or spiral vessel; TT′, thick-walled vessels; W, tracheids or woody tissue; F, sheath of fibrous tissue surrounding the bundle; FT, fundamental tissue or pith; S, sieve tissue; P, sieve plate; C, companion cell; l, intercellular space, formed by tearing down of adjacent cells; W′, wood parenchyma.

Fig. 76.—The Dicotyledonous Bundle or Wood Strand. Upper figure is of moonseed:

c, cambium; d, ducts; 1, end of first year’s growth; 2, end of second year’s growth; bast part at left and wood part at right. Lower figure (from Wettstein) is sunflower; h, wood-cells; g, vessels; c, cambium; p, fundamental tissue or parenchyma; b, bast; bp, bast parenchyma; s, sieve-tubes.

When cross-sections of monocotyledonous and dicotyledonous bundles are examined under the microscope, it is readily seen why dicotyledonous bundles form rings of wood and monocotyledonous cannot (Figs. 75 and 76). The dicotyledonous bundle (Fig. 76) has, running across it, a layer of brick-shaped cells called cambium, which cells are a specialized form of the parenchyma cells and retain the power of growing and multiplying. The bundles containing cambium are called open bundles. There is no cambium in monocotyledonous bundles (Fig. 75) and the bundles are called closed bundles. Monocotyledonous stems soon cease to grow in diameter. The stem of a palm tree is almost as large at the top as at the base. As dicotyledonous plants grow, the stems become thicker each year, for the delicate active cambium layer forms new cells from early spring until midsummer or autumn, adding to the wood within and to the bark without. As the growth in spring is very rapid, the first wood-cells formed are much larger than the last wood-cells formed by the slow growth of the late season, and the spring wood is less dense and lighter colored than the summer wood; hence the time between two years’ growth is readily made out (Figs. 77 and 78). Because of the rapid growth of the cambium in spring and its consequent soft walls and fluid contents, the bark of trees “peels” readily at that season.

Fig. 77.—White Pine Stem, 5 years old. The outermost layer is bark.

Medullary Rays.—The first year’s growth in dicotyledons forms a woody ring which almost incloses the pith, and this is left as a small cylinder which does not grow larger, even if the tree should live a century. It is not quite inclosed, however, for the narrow layers of soft cells separating the bundles remain between them (Fig. 78), forming radiating lines called medullary rays or pith rays.

Fig. 78.—Arrangement of Tissues in Two-year-old Stem of Moonseed.

p, pith; f, parenchyma. The fibro-vascular bundles, or wood strands, are very prominent, with thin medullary rays between.

The Several Plant Cells and their Functions.—In the wood there are some parenchyma cells that are still with thin walls, but have lost the power of division. They are now storage cells. There are also wood fibers which are thick-walled and rigid (h, Fig. 76), and serve to support the sap-canals or wood vessels (or tracheids) that are formed by the absorption of the end walls of upright rows of cells; the canals pass from the roots to the twigs and even to ribs of the leaves and serve to transport the root water. They are recognized (Fig. 79) by the peculiar thickening of the wall on the inner surface of the tubes, occurring in the form of spirals. Sometimes the whole wall is thickened except in spots called pits (g, Fig. 76). These thin spots (Fig. 80) allow the sap to pass to other cells or to neighboring vessels.

Fig. 79.—Markings in Cell Walls of Wood Fibers.

sp, spiral; an, annular; sc, scalariform.

Fig. 80.—Pits in the Cell Wall.

Longitudinal section of wall at b, showing pit borders at o, o.

The cambium, as we have seen, consists of cells whose function is growth. These cells are thin-walled and filled with protoplasm. During the growing season they are continually adding to the wood within and the bark without; hence the layer moves outward as it deposits the new woody layer within.

Fig. 81.—Sieve-tubes, s, s;

p shows a top view of a sieve-plate, with a companion cell. c, at the side; o shows sieve-plates in the side of the cell. In s, s the protoplasm is shrunken from the walls by reagents.

Fig. 82.—Thick-walled Bast Cells.

The bark consists of inner or fibrous bark or new bast (these fibers in flax become linen), the green or middle bark which functions somewhat as the leaves, and the corky or outer bark. The common word “bark” is seen therefore not to represent a homogeneous or simple structure, but rather a collection of several kinds of tissue, all separating from the wood beneath by means of cambium. The new bast contains (1) the sieve-tubes (Fig. 81) which transport the sap containing organic substances, as sugar and proteids, from the leaves to the parts needing it (s, Fig. 76). These tubes have been formed like the wood vessels, but they have sieve-plates to allow the dense organic-laden sap to pass with sufficient readiness for purposes of rapid distribution. (2) There are also thick-walled bast fibers (Fig. 82) in the bast that serve for support. (3) There is also some parenchyma (parent tissue) in the new bast; it is now in part a storage tissue. Sometimes the walls of parenchyma cells in the cortex thicken at the corners and form brace cells (Fig. 83) (collenchyma) for support; sometimes the whole wall is thickened, forming grit cells or stone cells (Fig. 84; examples in tough parts of pear, or in stone of fruits). Some parts serve for secretions (milk, rosin, etc.) and are called latex tubes.

Fig. 83.—Collenchyma in Wild Jewelweed or Touch-me-not (Impatiens).

Fig. 84.—Grit Cells.

The outer bark of old shoots consists of corky cells that protect from mechanical injury, and that contain a fatty substance (suberin) impermeable to water and of service to keep in moisture. There is sometimes a cork cambium (or phellogen) in the bark that serves to extend the bark and keep it from splitting, thus increasing its power to protect.

Transport of the “Sap.”—We shall soon learn that the common word “sap” does not represent a single or simple substance. We may roughly distinguish two kinds of more or less fluid contents: (1) the root water, sometimes called mineral sap, that is taken in by the root, containing its freight of such inorganic substances as potassium, calcium, iron, and the rest; this root water rises, we have found, in the wood vessels,—that is, in the young or “sapwood” (p. 96); (2) the elaborated or organized materials passing back and forth, especially from the leaves, to build up tissues in all parts of the plant, some of it going down to the roots and root-hairs; this organic material is transported, as we have learned, in the sieve-tubes of the inner bast,—that is, in the “inner bark.” Removing the bark from a trunk in a girdle will not stop the upward rise of the root water so long as the wood remains alive; but it will stop the passage of the elaborated or food-stored materials to parts below and thus starve those parts; and if the girdle does not heal over by the deposit of new bark, the tree will in time starve to death. It will now be seen that the common practice of placing wires or hoops about trees to hold them in position or to prevent branches from falling is irrational, because such wires interpose barriers over which the fluids cannot pass; in time, as the trunk increases in diameter, the wire girdles the tree. It is much better to bolt the parts together by rods extending through the branches (Fig. 85). These bolts should fit very tight in their holes. Why?

Fig. 85.—The Wrong Way to brace a Tree. (See Fig. 118).

Wood.—The main stem or trunk, and sometimes the larger branches, are the sources of lumber and timber. Different kinds of wood have value for their special qualities. The business of raising wood, for all purposes, is known as forestry. The forest is to be considered as a crop, and the crop must be harvested, as much as corn or rice is harvested. Man is often able to grow a more productive forest than nature does.

Resistance to decay gives value to wood used for shingles (cypress, heart of yellow pine) and for fence posts (mulberry, cedar, post oak, bois d’arc, mesquite).

Hardness and strength are qualities of great value in building. Live oak is used in ships. Red oak, rock maple, and yellow pine are used for floors. The best flooring is sawn with the straight edges of the annual rings upward; tangential sawn flooring may splinter. Chestnut is common in some parts of the country, being used for ceiling and inexpensive finishing and furniture. Locust and bois d’arc (osage orange) are used for hubs of wheels; bois d’arc makes a remarkably durable pavement for streets. Ebony is a tropical wood used for flutes, black piano keys, and fancy articles. Ash is straight and elastic; it is used for handles for light implements. Hickory is very strong as well as elastic, and is superior to ash for handles, spokes, and other uses where strength is wanted. Hickory is never sawn into lumber, but is split or turned. The “second growth,” which sprouts from stumps, is most useful, as it splits readily. Fast-growing hickory in rich land is most valuable. The supply of useful hickory is being rapidly exhausted.

Softness is often important. White pine and sweet gum because of their softness and lightness are useful in box-making. “Georgia” or southern pine is harder and stronger than white pine; it is much used for floors, ceilings, and some kinds of cabinet work. White pine is used for window-sash, doors, and molding, and cheaper grades for flooring. Hemlock is the prevailing lumber in the east for the framework and clapboarding of buildings. Redwood and Douglas spruce are common building materials on the Pacific coast. Cypress is soft and resists decay and is superior to white pine for sash, doors, and posts on the outside of houses. Cedar is readily carved and has a unique use in the making of chests for clothes, as its odor repels moths and other insects. Willow is useful for baskets and light furniture. Basswood or linden is used for light ceiling and sometimes for cheap floors. Whitewood (incorrectly called poplar) is employed for wagon bodies and often for house finishing. It often resembles curly maple.

Beauty of grain and polish gives wood value for furniture, pianos, and the like. Mahogany and white oak are most beautiful, although red oak is also used. Oak logs which are first quartered and then sawn radially expose the beautiful silver grain (medullary rays). Fig. 86 shows one mode of quartering. The log is quartered on the lines a, a, b, b; then succeeding boards are cut from each quarter at 1, 2, 3, etc. The nearer the heart the better the “grain”: why? Ordinary boards are sawn tangentially, as c, c. Curly pine, curly walnut, and bird’s-eye maple are woods that owe their beauty of grain to wavy lines or buried knots. Merely a stump of curly walnut is worth several hundred dollars. Such wood is sliced very thin for veneering and glued over other woods in making pianos and other pieces. If the cause of wavy grain could be found out and such wood grown at will, the discovery would be very useful. Maple is much used for furniture. Birch may be colored so as very closely to represent mahogany, and it is useful for desks.

Fig. 86.—The Making of Ordinary Boards, and One Way of Making “Quartered” Boards.

Special Products of Trees.—Cork from the bark of the cork oak in Spain, latex from the rubber and sap from the sugar maple trees, turpentine from pine, tannin from oak bark, Peruvian bark from cinchona, are all useful products.

Fig. 87.—The Many-stemmed Thickets of Mangrove of Southernmost Seacoasts, many of the trunks being formed of aërial roots.

CHAPTER XI LEAVES—FORM AND POSITION

Leaves may be studied from four points of view,—with reference (1) to their kinds and shapes; (2) their position, or arrangement on the plant; (3) their anatomy, or structure; (4) their function, or the work they perform. This chapter is concerned with the first two categories.

Fig. 88.—A Simple Netted-veined Leaf.

Fig. 89.—A Simple Parallel-veined Leaf.

Fig. 90.—Compound or Branched Leaf of Brake (a common fern).

Kinds.—Leaves are simple or unbranched (Figs. 88, 89), and compound or branched (Fig. 90). The method of compounding or branching follows the mode of veining. The veining, or venation, is of two general kinds: in some plants the main veins diverge, and there is a conspicuous network of smaller veins; such leaves are netted-veined. They are characteristic of the dicotyledons. In other plants the main veins are parallel, or nearly so, and there is no conspicuous network; these are parallel-veined leaves (Figs. 89, 102). These leaves are the rule in monocotyledonous plants. The venation of netted-veined leaves is pinnate or feather-like when the veins arise from the side of a continuous midrib (Fig. 91); palmate or digitate (hand-like) when the veins arise from the apex of the petiole (Figs. 88, 92). If leaves were divided between the main veins, the former would be pinnately and the latter digitately compound.

Fig. 91.—Complete Leaves of Willow.

Fig. 92.—Digitate-veined Peltate Leaf of Nasturtium.

Fig. 93.—Pinnately Compound Leaf of Ash.

It is customary to speak of a leaf as compound only when the parts or branches are completely separate blades, as when the division extends to the midrib (Figs. 90, 93, 94, 95). The parts or branches are known as leaflets. Sometimes the leaflets themselves are compound, and the whole leaf is then said to be bi-compound or twice-compound (Fig. 90). Some leaves are three-compound, four-compound, or five-compound. Decompound is a general term to express any degree of compounding beyond twice-compound.

Fig. 94.—Digitately Compound Leaf of Raspberry.

Fig. 95.—Poison Ivy. Leaf and Fruit.

Fig. 96.—Lobed Leaf of Sugar Maple.

Leaves that are not divided as far as to the midrib are said to be:

lobed, if the openings or sinuses are not more than half the depth of the blade (Fig. 96);

cleft, if the sinuses are deeper than the middle;

parted, if the sinuses reach two thirds or more to the midrib (Fig. 97);

divided, if sinuses reach nearly or quite to the midrib.

The parts are called lobes, divisions, or segments, rather than leaflets. The leaf may be pinnately or digitately lobed, parted, cleft, or divided. A pinnately parted or cleft leaf is sometimes said to be pinnatifid.

Fig. 97.—Digitately Parted Leaves of Begonia.

Leaves may have one or all of three parts—blade, or expanded part; petiole, or stalk; stipules, or appendages at the base of the petiole. A leaf that has all three of these parts is said to be complete (Figs. 91, 106). The stipules are often green and leaflike and perform the function of foliage, as in the pea and Japanese quince (the latter common in yards).

Fig. 98.—Oblong-ovate Sessile Leaves of Tea.

Leaves and leaflets that have no stalks are said to be sessile (Figs. 98, 103), i.e. sitting. Find several examples. The same is said of flowers and fruits. The blade of a sessile leaf may partly or wholly surround the stem, when it is said to be clasping. Examples: aster (Fig. 99), corn. In some cases the leaf runs down the stem, forming a wing; such leaves are said to be decurrent (Fig. 100). When opposite sessile leaves are joined by their bases, they are said to be connate (Fig. 101).

Fig. 99.—Clasping Leaf of a Wild Aster.

Leaflets may have one or all of these three parts, but the stalks of leaflets are called petiolules and the stipules of leaflets are called stipels. The leaf of the garden bean has leaflets, petiolules, and stipels.

Fig. 100.—Decurrent Leaves of Mullein.

The blade is usually attached to the petiole by its lower edge. In pinnate-veined leaves, the petiole seems to continue through the leaf as a midrib (Fig. 91). In some plants, however, the petiole joins the blade inside or beyond the margin (Fig. 92). Such leaves are said to be peltate or shield-shaped. This mode of attachment is particularly common in floating leaves (e.g. the water lilies). Peltate leaves are usually digitate-veined.

Fig. 101.—Two Pairs of Connate Leaves of Honeysuckle.

How to Tell a Leaf.—It is often difficult to distinguish compound leaves from leafy branches, and leaflets from leaves. As a rule leaves can be distinguished by the following tests: (1) Leaves are temporary structures, sooner or later falling. (2) Usually buds are borne in their axils. (3) Leaves are usually borne at joints or nodes. (4) They arise on wood of the current year’s growth. (5) They have a more or less definite arrangement. When leaves fall, the twig that bore them remains; when leaflets fall, the main petiole or stalk that bore them also falls.

Fig. 102.—Linear-acuminate Leaf of Grass.

Fig. 103.—Short-oblong Leaves of Box.

Shapes.—Leaves and leaflets are infinitely variable in shape. Names have been given to some of the more definite or regular shapes. These names are a part of the language of botany. The names represent ideal or typical shapes; there are no two leaves alike and very few that perfectly conform to the definitions. The shapes are likened to those of familiar objects or of geometrical figures. Some of the commoner shapes are as follows (name original examples in each class):

Linear, several times longer than broad, with the sides nearly or quite parallel. Spruces and most grasses are examples (Fig. 102). In linear leaves, the main veins are usually parallel to the midrib.

Oblong, twice or thrice as long as broad, with the sides parallel for most of their length. Fig. 103 shows the short-oblong leaves of the box, a plant that is used for permanent edgings in gardens.

Elliptic differs from the oblong in having the sides gradually tapering to either end from the middle. The European beech (Fig. 104) has elliptic leaves. (This tree is often planted in this country.)

Lanceolate, four to six times longer than broad, widest below the middle, and tapering to either end. Some of the narrow-leaved willows are examples. Most of the willows and the peach have oblong-lanceolate leaves.

Spatulate, a narrow leaf that is broadest toward the apex. The top is usually rounded.

Fig. 104.—Elliptic Leaf of Purple Beech.

Fig. 105.—Ovate Serrate Leaf of Hibiscus.

Fig. 106.—Leaf of Apple, showing blade, petiole, and small narrow stipules.

Ovate, shaped somewhat like the longitudinal section of an egg: about twice as long as broad, tapering from near the base to the apex. This is one of the commonest leaf forms (Figs. 105, 106).

Obovate, ovate inverted,—the wide part towards the apex. Leaves of mullein and leaflets of horse-chestnut and false indigo are obovate. This form is commonest in leaflets of digitate leaves: why?

Reniform, kidney-shaped. This form is sometimes seen in wild plants, particularly in root-leaves. Leaves of wild ginger are nearly reniform.

Orbicular, circular in general outline. Very few leaves are perfectly circular, but there are many that are nearer circular than any other shape (Fig. 107).

Fig. 107.—Orbicular Lobed Leaves.

Fig. 108.—Truncate Leaf of Tulip Tree.

The shape of many leaves is described in combinations of these terms: as ovate-lanceolate, lanceolate-oblong.

The shape of the base and apex of the leaf or leaflet is often characteristic. The base may be rounded (Fig. 104), tapering (Fig. 93), cordate or heart-shaped (Fig. 105), truncate or squared as if cut off. The apex may be blunt or obtuse, acute or sharp, acuminate or long-pointed, truncate (Fig. 108). Name examples.

The shape of the margin is also characteristic of each kind of leaf. The margin is entire when it is not indented or cut in any way (Figs. 99, 103). When not entire, it may be undulate or wavy (Fig. 92), serrate or saw-toothed (Fig. 105), dentate or more coarsely notched (Fig. 95), crenate or round-toothed, lobed, and the like. Give examples.

Leaves often differ greatly in form on the same plant. Observe the different shapes of leaves on the young growths of mulberries (Fig. 2) and wild grapes; also on vigorous squash and pumpkin vines. In some cases there may be simple and compound leaves on the same plant. This is marked in the so-called Boston ivy or ampelopsis (Fig. 109), a vine that is used to cover brick and stone buildings. Different degrees of compounding, even in the same leaf, may often be found in honey locust and Kentucky coffee tree. Remarkable differences in forms are seen by comparing seed-leaves with mature leaves of any plant (Fig. 30).

Fig. 109.—Different Forms of Leaves from one Plant of Ampelopsis.

The Leaf and its Environment.—The form and shape of the leaf often have direct relation to the place in which the leaf grows. Floating leaves are usually expanded and flat, and the petiole varies in length with the depth of the water. Submerged leaves are usually linear or thread-like, or are cut into very narrow divisions: thereby more surface is exposed, and possibly the leaves are less injured by moving water. Compare the sizes of the leaves on the ends of branches with those at the base of the branches or in the interior of the tree top. In dense foliage masses, the petioles of the lowermost or undermost leaves tend to elongate—to push the leaf to the light.

On the approach of winter the leaf usually ceases to work, and dies. It may drop, when it is said to be deciduous; or it may remain on the plant, when it is said to be persistent. If persistent leaves remain green during the winter, the plant is said to be evergreen. Give examples in each class. Most leaves fall by breaking off at the lower end of the petiole with a distinct joint or articulation. There are many leaves, however, that wither and hang on the plant until torn off by the wind; of such are the leaves of grasses, sedges, lilies, orchids, and other plants of the monocotyledons. Most leaves of this character are parallel-veined.

Leaves also die and fall from lack of light. Observe the yellow and weak leaves in a dense tree top or in any thicket. Why do the lower leaves die on house plants? Note the carpet of needles under the pines. All evergreens shed their leaves after a time. Counting back from the tip of a pine or spruce shoot, determine how many years the leaves persist. In some spruces a few leaves may be found on branches ten or more years old.

Arrangement of Leaves.—Most leaves have a regular position or arrangement on the stem. This position or direction is determined largely by exposure to sunlight. In temperate climates they usually hang in such a way that they receive the greatest amount of light. One leaf shades the other to the least possible degree. If the plant were placed in a new position with reference to light, the leaves would make an effort to turn their blades.

When leaves are opposite the pairs usually alternate. That is, if one pair stands north and south, the next pair stands east and west. See the box-elder shoot, on the left in Fig. 110. One pair does not shade the pair beneath. The leaves are in four vertical ranks.

There are several kinds of alternate arrangement. In the elm shoot, in Fig. 110, the third bud is vertically above the first. This is true no matter which bud is taken as the starting point. Draw a thread around the stem until the two buds are joined. Set a pin at each bud. Observe that two buds are passed (not counting the last) and that the thread makes one circuit of the stem. Representing the number of buds by a denominator, and the number of circuits by a numerator, we have the fraction ¹⁄₂, which expresses the part of the circle that lies between any two buds. That is, the buds are one half of 360 degrees apart, or 180 degrees. Looking endwise at the stem, the leaves are seen to be 2-ranked. Note that in the apple shoot (Fig. 110, right) the thread makes two circuits and five buds are passed: two-fifths represents the divergence between the buds. The leaves are 5-ranked.

Fig. 110.—Phyllotaxy of Box Elder, Elm, Apple.

Every plant has its own arrangement of leaves. For opposite leaves, see maple, box elder, ash, lilac, honeysuckle, mint, fuchsia. For 2-ranked arrangement, see all grasses, Indian corn, basswood, elm. For 3-ranked arrangement, see all sedges. For 5-ranked (which is one of the commonest), see apple, cherry, pear, peach, plum, poplar, willow. For 8-ranked, see holly, osage orange, some willows. More complicated arrangements occur in bulbs, house leeks, and other condensed parts. The buds or “eyes” on a potato tuber, which is an underground stem (why?), show a spiral arrangement (Fig. 111). The arrangement of leaves on the stem is known as phyllotaxy (literally, “leaf arrangement”). Make out the phyllotaxy on six different plants nearest the schoolhouse door.

In some plants, several leaves occur at one level, being arranged in a circle around the stem. Such leaves are said to be verticillate, or whorled. Leaves arranged in this way are usually narrow: why?

Fig. 111.—Phyllotaxy of the Potato Tuber. Work it out on a fresh long tuber.

Although a definite arrangement of leaves is the rule in most plants, it is subject to modification. On shoots that receive the light only from one side or that grow in difficult positions, the arrangement may not be definite. Examine shoots that grow on the under side of dense tree tops or in other partially lighted positions.

Fig. 112.—Cowpea. Describe the leaves. For what is the plant used?

CHAPTER XII LEAVES—STRUCTURE OR ANATOMY

Besides the framework, or system of veins found in blades of all leaves, there is a soft cellular tissue called mesophyll, or leaf parenchyma, and an epidermis or skin that covers the entire outside part.

Fig. 113.—Section of a Leaf, showing the air spaces.

Breathing-pore or stoma at a. The palisade cells which chiefly contain the chlorophyll are at b. Epidermal cells at c.

Mesophyll.—The mesophyll is not all alike or homogeneous. The upper layer is composed of elongated cells placed perpendicular to the surface of the leaf. These are called palisade cells. These cells are usually filled with green bodies called chlorophyll grains. The grain contains a great number of chlorophyll drops imbedded in the protoplasm. Below the palisade cells is the spongy parenchyma, composed of cells more or less spherical in shape, irregularly arranged, and provided with many intercellular air cavities (Fig. 113). In leaves of some plants exposed to strong light there may be more than one layer of palisade cells, as in the India-rubber plant and oleander. Ivy when grown in bright light will develop two such layers of cells, but in shaded places it may be found with only one. Such plants as iris and compass plant, which have both surfaces of the leaf equally exposed to sunlight, usually have a palisade layer beneath each epidermis.

Epidermis.—The outer or epidermal cells of leaves do not bear chlorophyll, but are usually so transparent that the green mesophyll can be seen through them. They often become very thick-walled, and are in most plants devoid of all protoplasm except a thin layer lining the walls, the cavities being filled with cell sap. This sap is sometimes colored, as in the under epidermis of begonia leaves. It is not common to find more than one layer of epidermal cells forming each surface of a leaf. The epidermis serves to retain moisture in the leaf and as a general protective covering. In desert plants the epidermis, as a rule, is very thick and has a dense cuticle, thereby preventing loss of water.

There are various outgrowths of the epidermis. Hairs are the chief of these. They may be (1) simple, as on primula, geranium, nægelia; (2) once branched, as on wallflower; (3) compound, as on verbascum or mullein; (4) disk-like, as on shepherdia; (5) stellate, or star-shaped, as in certain crucifers. In some cases the hairs are glandular, as in Chinese primrose of the greenhouses (Primula Sinensis) and certain hairs of pumpkin flowers. The hairs often protect the breathing pores, or stomates, from dust and water.

Stomates (sometimes called breathing-pores) are small openings or pores in the epidermis of leaves and soft stems that allow the passage of air and other gases and vapors (stomate or stoma, singular; stomates or stomata, plural). They are placed near the large intercellular spaces of the mesophyll, usually in positions least affected by direct sunlight. Fig. 114 shows the structure. There are two guard-cells at the mouth of each stomate, which may in most cases open or close the passage as the conditions of the atmosphere may require. The guard-cells contain chlorophyll. In Fig. 115 is shown a case in which there are compound guard-cells, that of ivy. On the margins of certain leaves, as of fuchsia, impatiens, cabbage, are openings known as water-pores.

 

Fig. 114.—Diagram of Stomate of Iris (Osterhout).

Fig. 115.—Stomate of Ivy, showing compound guard-cells.

 

Stomates are very numerous, as will be seen from the numbers showing the pores to each square inch of leaf surface:

The arrangement of stomates on the leaf differs with each kind of plant. Fig. 116 shows stomates and also the outlines of contiguous epidermal cells.

Fig. 116.—Stomates of Geranium Leaf.

The function or work of the stomates is to regulate the passage of gases into and out of the plant. The directly active organs or parts are guard-cells, on either side the opening. One method of opening is as follows: The thicker walls of the guard-cells (Fig. 114) absorb water from adjacent cells, these thick walls buckle or bend and part from each other at their middles on either side of the opening, causing the stomate to open, when the air gases may be taken in and the leaf gases may pass out. When moisture is reduced in the leaf tissue, the guard cells part with some of their contents, the thick walls straighten, and the faces of the two opposite ones come together, thus closing the stomate and preventing any water vapor from passing out. When a leaf is actively at work making new organic compounds, the stomates are usually open; when unfavorable conditions arise, they are usually closed. They also commonly close at night, when growth (or the utilizing of the new materials) is most likely to be active. It is sometimes safer to fumigate greenhouses and window gardens at night, for the noxious vapors are less likely to enter the leaf. Dust may clog or cover the stomates. Rains benefit plants by washing the leaves as well as by providing moisture to the roots.

Fig. 117.—Lenticels on Young Shoot of Red Osier (Cornus).

Lenticels.—On the young woody twigs of many plants (marked in osiers, cherry, birch) there are small corky spots or elevations known as lenticels (Fig. 117). They mark the location of some loose cork cells that function as stomates, for green shoots, as well as leaves, take in and discharge gases; that is, soft green twigs function as leaves. Under some of these twig stomates, corky material may form and the opening is torn and enlarged: the lenticels are successors to the stomates. The stomates lie in the epidermis, but as the twig ages the epidermis perishes and the bark becomes the external layer. Gases continue to pass in and out through the lenticels, until the branch becomes heavily covered with thick, corky bark. With the growth of the twig, the lenticel scars enlarge lengthwise or crosswise or assume other shapes, often becoming characteristic markings.

Fibro-vascular Bundles.—We have studied the fibro-vascular bundles of stems (Chap. X). These stem bundles continue into the leaves, ramifying into the veins, carrying the soil water inwards and bringing, by diffusion, the elaborated food out through the sieve-cells. Cut across a petiole and notice the hard spots or areas in it; strip these parts lengthwise of the petiole: what are they?

Fall of the Leaf.—In most common deciduous plants, when the season’s work for the leaf is ended, the nutritious matter may be withdrawn, and a layer of corky cells is completed over the surface of the stem where the leaf is attached. The leaf soon falls. It often falls even before it is killed by frost. Deciduous leaves begin to show the surface line of articulation in the early growing season. This articulation may be observed at any time during the summer. The area of the twig once covered by the petioles is called the leaf-scar after the leaf has fallen. In Chap. XV are shown a number of leaf-scars. In the plane tree (sycamore or buttonwood), the leaf-scar is in the form of a ring surrounding the bud, for the bud is covered by the hollowed end of the petiole; the leaf of sumac is similar. Examine with a hand lens leaf-scars of several woody plants. Note the number of bundle-scars in each leaf-scar. Sections may be cut through a leaf-scar and examined with the microscope. Note the character of cells that cover the leaf-scar surface.

CHAPTER XIII LEAVES—FUNCTION OR WORK

We have discussed (in Chap. VIII) the work or function of roots and also (in Chap. X) the function of stems. We are now ready to complete the view of the main vital activities of plants by considering the function of the green parts (leaves and young shoots).

Sources of Food.—The ordinary green plant has but two sources from which to secure food,—the air and the soil. When a plant is thoroughly dried in an oven, the water passes off; this water came from the soil. The remaining part is called the dry substance or dry matter. If the dry matter is burned in an ordinary fire, only the ash remains; this ash came from the soil. The part that passed off as gas in the burning contained the elements that came from the air; it also contained some of those that came from the soil—all those (as nitrogen, hydrogen, chlorine) that are transformed into gases by the heat of a common fire. The part that comes from the soil (the ash) is small in amount, being considerably less than 10 per cent and sometimes less than 1 per cent. Water is the most abundant single constituent or substance of plants. In a corn plant of the roasting-ear stage, about 80 per cent of the substance is water. A fresh turnip is over 90 per cent water. Fresh wood of the apple tree contains about 45 per cent of water.

Carbon.—Carbon enters abundantly into the composition of all plants. Note what happens when a plant is burned without free access of air, or smothered, as in a charcoal pit. A mass of charcoal remains, almost as large as the body of the plant. Charcoal is almost pure carbon, the ash present being so small in proportion to the large amount of carbon that we look on the ash as an impurity. Nearly half of the dry substance of a tree is carbon. Carbon goes off as a gas when the plant is burned in air. It does not go off alone, but in combination with oxygen in the form of carbon dioxid gas, CO₂.

The green plant secures its carbon from the air. In other words, much of the solid matter of the plant comes from one of the gases of the air. By volume, carbon dioxid forms only a very small fraction of 1 per cent of the air. It would be very disastrous to animal life, however, if this percentage were much increased, for it excludes the life-giving oxygen. Carbon dioxid is often called “foul gas.” It may accumulate in old wells, and an experienced person will not descend into such wells until they have been tested with a torch. If the air in the well will not support combustion,—that is, if the torch is extinguished,—it usually means that carbon dioxid has drained into the place. The air of a closed schoolroom often contains far too much of this gas, along with little solid particles of waste matters. Carbon dioxid is often known as carbonic acid gas.

Appropriation of the Carbon.—The carbon dioxid of the air readily diffuses itself into the leaves and other green parts of the plant. The leaf is delicate in texture, and when very young the air can diffuse directly into the tissues. The stomates, however, are the special inlets adapted for the admission of gases into the leaves and other green parts. Through these stomates, or diffusion-pores, the outside air enters into the air-spaces of the plant, and is finally absorbed by the little cells containing the living matter.

Chlorophyll (“leaf green”) is the agent that secures the energy by means of which carbon dioxid is utilized. This material is contained in the leaf cells in the form of grains (p. 86); the grains themselves are protoplasm, only the coloring matter being chlorophyll. The chlorophyll bodies or grains are often most abundant near the upper surface of the leaf, where they can secure the greatest amount of light. Without this green coloring matter, there would be no reason for the large flat surfaces which the leaves possess, and no reason for the fact that the leaves are borne most abundantly at the ends of branches, where the light is most available. Plants with colored leaves, as coleus, have chlorophyll, but it is masked by other coloring matter. This other coloring matter is usually soluble in hot water: boil a coleus leaf and notice that it becomes green and the water becomes colored.

Plants grown in darkness are yellow and slender, and do not reach maturity. Compare the potato sprouts that have grown from a tuber lying in the dark cellar with those that have grown normally in the bright light. The shoots have become slender and are devoid of chlorophyll; and when the food that is stored in the tuber is exhausted, these shoots will have lived useless lives. A plant that has been grown in darkness from the seed will soon die, although for a time the little seedling will grow very tall and slender: why? Light favors the production of chlorophyll, and the chlorophyll is the agent in the making of the organic carbon compounds. Sometimes chlorophyll is found in buds and seeds, but in most cases these places are not perfectly dark. Notice how potato tubers develop chlorophyll, or become green, when exposed to light.

Photosynthesis.—Carbon dioxid diffuses into the leaf; during sunlight it is used, and oxygen is given off. How the carbon dioxid which is thus absorbed may be used in making an organic food is a complex question, and need not be studied here; but it may be stated that carbon dioxid and water are the constituents. Complex compounds are built up out of simpler ones.

Chlorophyll absorbs certain light rays, and the energy thus directly or indirectly obtained is used by the living matter in uniting the carbon dioxid absorbed from the air with some of the water brought up from the roots. The ultimate result usually is starch. The process is obscure, but sugar is generally one step; and our first definite knowledge of the product begins when starch is deposited in the leaves. The process of using the carbon dioxid of the air has been known as carbon assimilation, but the term now most used is photosynthesis (from two Greek words, meaning light and to put together).

Starch and Sugar.—All starch is composed of carbon, hydrogen, and oxygen (C₆H₁₀O₅)ₙ. The sugars and the substance of cell walls are very similar to it in composition. All these substances are called carbohydrates. In making fruit sugar from the carbon and oxygen of carbon dioxid and from the hydrogen and oxygen of the water, there is a surplus of oxygen (6 parts CO₂ + 6 parts H₂O = C₆H₁₂O₆ + 6 O₂). It is this oxygen that is given off into the air during sunlight.

Digestion.—Starch is in the form of insoluble granules. When such food material is carried from one part of the plant to another for purposes of growth or storage, it is made soluble before it can be transported. When this starchy material is transferred from place to place, it is usually changed into sugar by the action of a diastase. This is a process of digestion. It is much like the change of starchy foodstuffs to sugary foods by the saliva.

Distribution of the Digested Food.—After being changed to the soluble form, this material is ready to be used in growth, either in the leaf, in the stem, or in the roots. With other more complex products it is then distributed throughout all of the growing parts of the plant; and when passing down to the root, it seems to pass more readily through the inner bark, in plants which have a definite bark. This gradual downward diffusion through the inner bark of materials suitable for growth is the process referred to when the “descent of sap” is mentioned. Starch and other products are often stored in one growing season to be used in the next season. If a tree is constricted or strangled by a wire around its trunk (Fig. 118), the digested food cannot readily pass down and it is stored above the girdle, causing an enlargement.

Fig. 118.—Trunk Girdled by a Wire. See Fig. 85.

Assimilation.—The food from the air and that from the soil unite in the living tissues. The “sap” that passes upwards from the roots in the growing season is made up largely of the soil water and the salts which have been absorbed in the diluted solutions (p. 67). This upward-moving water is conducted largely through certain tubular canals of the young wood. These cells are never continuous tubes from root to leaf; but the water passes readily from one cell or canal to another in its upward course.

The upward-moving water gradually passes to the growing parts, and everywhere in the living tissues, it is of course in the most intimate contact with the soluble carbohydrates and products of photosynthesis. In the building up or reconstructive and other processes it is therefore available. We may properly conceive of certain of the simpler organic molecules as passing through a series of changes, gradually increasing in complexity. There will be formed substances containing nitrogen in addition to carbon, hydrogen, and oxygen. Others will contain also sulfur and phosphorus, and the various processes may be thought of as culminating in protoplasm. Protoplasm is the living matter in plants. It is in the cells, and is usually semifluid. Starch is not living matter. The complex process of building up the protoplasm is called assimilation.

Respiration.—Plants need oxygen for respiration, as animals do. We have seen that plants need the carbon dioxid of the air. To most plants the nitrogen of the air is inert, and serves only to dilute the other elements; but the oxygen is necessary for all life. We know that all animals need this oxygen in order to breathe or respire. In fact, they have become accustomed to it in just the proportions found in the air; and this is now best for them. When animals breathe the air once, they make it foul, because they use some of the oxygen and give off carbon dioxid. Likewise, all living parts of the plant must have a constant supply of oxygen. Roots also need it, for they respire. Air goes in and out of the soil by diffusion, and as the soil is heated and cooled, causing the air to expand and contract.

The oxygen passes into the air-spaces and is absorbed by the moist cell membranes. In the living cells it makes possible the formation of simpler compounds by which energy is released. This energy enables the plant to work and grow, and the final products of this action are carbon dioxid and water. As a result of the use of this oxygen by night and by day, plants give off carbon dioxid. Plants respire; but since they are stationary, and more or less inactive, they do not need as much oxygen as animals, and they do not give off so much carbon dioxid. A few plants in a sleeping room need not disturb one more than a family of mice. It should be noted, however, that germinating seeds respire vigorously, hence they consume much oxygen; and opening buds and flowers are likewise active.

Transpiration.—Much more water is absorbed by the roots than is used in growth, and this surplus water passes from the leaves into the atmosphere by an evaporation process known as transpiration. Transpiration takes place more abundantly from the under surfaces of leaves, and through the pores or stomates. A sunflower plant of the height of a man, during an active period of growth, gives off a quart of water per day. A large oak tree may transpire 150 gallons per day during the summer. For every ounce of dry matter produced, it is estimated that 15 to 25 pounds of water usually passes through the plant.

When the roots fail to supply to the plant sufficient water to equalize that transpired by the leaves, the plant wilts. Transpiration from the leaves and delicate shoots is increased by all of the conditions which increase evaporation, such as higher temperature, dry air, or wind. The stomata open and close, tending to regulate transpiration as the varying conditions of the atmosphere affect the moisture content of the plant. However, in periods of drought or of very hot weather, and especially during a hot wind, the closing of these stomates cannot sufficiently prevent evaporation. The roots may be very active and yet fail to absorb sufficient moisture to equalize that given off by the leaves. The plant shows the effect (how?). On a hot dry day, note how the leaves of corn “roll” towards afternoon. Note how fresh and vigorous the same leaves appear early the following morning. Any injury to the roots, such as a bruise, or exposure to heat, drought, or cold may cause the plant to wilt.

Water is forced up by root pressure or sap pressure. (Exercise 99.) Some of the dew on the grass in the morning may be the water forced up by the roots; some of it is the condensed vapor of the air.

The wilting of a plant is due to the loss of water from the cells. The cell walls are soft, and collapse. A toy balloon will not stand alone until it is inflated with air or liquid. In the woody parts of the plant the cell walls may be stiff enough to support themselves, even though the cell is empty. Measure the contraction due to wilting and drying by tracing a fresh leaf on page of notebook, and then tracing the same leaf after it has been dried between papers. The softer the leaf, the greater will be the contraction.

Storage.—We have said that starch may be stored in twigs to be used the following year. The very early flowers on fruit trees, especially those that come before the leaves, and those that come from bulbs, as crocuses and tulips, are supported by the starch or other food that was organized the year before. Some plants have very special storage reservoirs, as the potato, in this case being a thickened stem although growing underground. (Why a thickened stem? p. 84.) It is well to make the starch test on winter twigs and on all kinds of thickened parts, as tubers and bulbs.

Carnivorous Plants.—Certain plants capture insects and other very small animals and utilize them to some extent as food. Such are the sundew, that has on the leaves sticky hairs that close over the insect; the Venus’s flytrap of the Southern states, in which the halves of the leaves close over the prey like the jaws of a steel trap; and the various kinds of pitcher plants that collect insects and other organic matter in deep, water-filled, flask-like leaf pouches (Fig. 119).

The sundew and Venus’s flytrap are sensitive to contact. Other plants are sensitive to the touch without being insectivorous. The common cultivated sensitive plant is an example. This is readily grown from seeds (sold by seedsmen) in a warm place. Related wild plants in the south are sensitive. The utility of this sensitiveness is not understood.

Fig. 119.—The Common Pitcher Plant (Sarracenia purpurea) of the North, showing the tubular leaves and the odd, long-stalked flowers.

Parts that Simulate Leaves.—We have learned that leaves are endlessly modified to suit the conditions in which the plant is placed. The most marked modifications are in adaptation to light. On the other hand, other organs often perform the functions of leaves. Green shoots function as leaves. These shoots may look like leaves, in which case they are called cladophylla. The foliage of common asparagus is made up of fine branches: the real morphological leaves are the minute dry functionless scales at the bases of these branchlets. (What reason is there for calling them leaves?) The broad “leaves” of the florist’s smilax are cladophylla: where are the leaves on this plant? In most of the cacti, the entire plant body performs the functions of leaves until the parts become cork-bound.

Leaves are sometimes modified to perform other functions than the vital processes: they may be tendrils, as the terminal leaflets of pea and sweet pea; or spines, as in barberry. Not all spines and thorns, however, represent modified leaves: some of them (as of hawthorns, osage orange, honey locust) are branches.

Fig. 120.—Excluding Light and CO₂ from Part of a Leaf

Fig. 121.—The Result.

 

Fig. 122.—To show the Escape of Oxygen.

Fig. 123.—To illustrate a Product of Respiration.

Fig. 124.—Respiration of Thick Roots.

 

Fig. 125.—To illustrate Transpiration.

Fig. 126.—To illustrate Transpiration.

Fig. 127.—Loss of Water.

Fig. 128.—To show Sap Pressure.

Fig. 129.—Before and after Pruning.

Fig. 130.—An Apple Tree, with suggestions as to pruning when it is set in the orchard. At a is shown a pruned top.

CHAPTER XIV DEPENDENT PLANTS

Thus far we have spoken of plants with roots and foliage and that depend on themselves. They collect the raw materials and make them over into assimilable food. They are independent. Plants without green foliage cannot make food; they must have it made for them or they die. They are dependent. A sprout from a potato tuber in a dark cellar cannot collect and elaborate carbon dioxid. It lives on the food stored in the tuber.

Fig. 131.—A Mushroom, example of a saprophytic plant. This is the edible cultivated mushroom.

All plants with naturally white or blanched parts are dependent. Their leaves do not develop. They live on organic matter—that which has been made by a plant or elaborated by an animal. The dodder, Indian pipe, beech drop, coral root among flower-bearing plants, also mushrooms and other fungi (Figs. 131, 132) are examples. The dodder is common in swales, being conspicuous late in the season from its thread-like yellow or orange stems spreading over the herbage of other plants. One kind attacks alfalfa and is a bad pest. The seeds germinate in the spring, but as soon as the twining stem attaches itself to another plant, the dodder dies away at the base and becomes wholly dependent. It produces flowers in clusters and seeds itself freely (Fig. 133).

Fig. 132.—A Parasitic Fungus, magnified. The mycelium, or vegetative part, is shown by the dotted-shaded parts ramifying in the leaf tissue. The rounded haustoria projecting into the cells are also shown. The long fruiting parts of the fungus hang from the under surface of the leaf.

Parasites and Saprophytes.—A plant that is dependent on a living plant or animal is a parasite, and the plant or animal on which it lives is the host. The dodder is a true parasite; so are the rusts, mildews, and other fungi that attack leaves and shoots and injure them.

The threads of a parasitic fungus usually creep through the intercellular spaces in the leaf or stem and send suckers (or haustoria) into the cells (Fig. 132). The threads (or the hyphæ) clog the air-spaces of the leaf and often plug the stomates, and they also appropriate and disorganize the cell fluids; thus they injure or kill their host. The mass of hyphæ of a fungus is called mycelium. Some of the hyphæ finally grow out of the leaf and produce spores or reproductive cells that answer the purpose of seeds in distributing the plant (b, Fig. 132).

Fig. 133.—Dodder in Fruit.

A plant that lives on dead or decaying matter is a saprophyte. Mushrooms (Fig. 131) are examples; they live on the decaying matter in the soil. Mold on bread and cheese is an example. Lay a piece of moist bread on a plate and invert a tumbler over it. In a few days it will be moldy. The spores were in the air, or perhaps they had already fallen on the bread but had not had opportunity to grow. Most green plants are unable to make any direct use of the humus or vegetable mold in the soil, for they are not saprophytic. The shelf-fungi (Fig. 134) are saprophytes. They are common on logs and trees. Some of them are perhaps partially parasitic, extending the mycelium into the wood of the living tree and causing it to become black-hearted (Fig. 134).

Fig. 134.—Tinder Fungus (Polyporus igniarius) on beech log. The external part of the fungus is shown below; the heart-rot injury above.

Some parasites spring from the ground, as other plants do, but they are parasitic on the roots of their hosts. Some parasites may be partially parasitic and partially saprophytic. Many (perhaps most) of these ground saprophytes are aided in securing their food by soil fungi, which spread their delicate threads over the root-like branches of the plant and act as intermediaries between the food and the saprophyte. These fungus-covered roots are known as mycorrhizas (meaning “fungus root”). Mycorrhizas are not peculiar to saprophytes. They are found on many wholly independent plants, as, for example, the heaths, oaks, apples, and pines. It is probable that the fungous threads perform some of the offices of root-hairs to the host. On the other hand, the fungus obtains some nourishment from the host. The association seems to be mutual.

Fig. 135.—Bacteria of Several Forms, much magnified.

Saprophytes break down or decompose organic substances. Chief of these saprophytes are many microscopic organisms known as bacteria (Fig. 135). These innumerable organisms are immersed in water or in dead animals and plants, and in all manner of moist organic products. By breaking down organic combinations, they produce decay. Largely through their agency, and that of many true but microscopic fungi, all things pass into soil and gas. Thus are the bodies of plants and animals removed and the continuing round of life is maintained.

Fig. 136.—American Mistletoe growing on a Walnut Branch.

Some parasites are green-leaved. Such is the mistletoe (Fig. 136). They anchor themselves on the host and absorb its juices, but they also appropriate and use the carbon dioxid of the air. In some small groups of bacteria a process of organic synthesis has been shown to take place.

Epiphytes.—To be distinguished from the dependent plants are those that grow on other plants without taking food from them. These are green-leaved plants whose roots burrow in the bark of the host plant and perhaps derive some food from it, but which subsist chiefly on materials that they secure from air dust, rain water, and the air. These plants are epiphytes (meaning “upon plants”) or air plants.

Epiphytes abound in the tropics. Certain orchids are among the best known examples (Fig. 37). The Spanish moss or tillandsia of the South is another. Mosses and lichens that grow on trees and fences may also be called epiphytes. In the struggle for existence, the plants probably have been driven to these special places in which to find opportunity to grow. Plants grow where they must, not where they will.

CHAPTER XV WINTER AND DORMANT BUDS

A bud is a growing point, terminating an axis either long or short, or being the starting point of an axis. All branches spring from buds. In the growing season the bud is active; later in the season it ceases to increase the axis in length, and as winter approaches the growing point becomes more or less thickened and covered by protecting scales, in preparation for the long resting season. This resting, dormant, or winter body is what is commonly spoken of as a “bud.” A winter bud may be defined as an inactive covered growing point, waiting for spring.

Structurally, a dormant bud is a shortened axis or branch, bearing miniature leaves or flowers or both, and protected by a covering. Cut in two, lengthwise, a bud of the horse-chestnut or other plant that has large buds. With a pin separate the tiny leaves. Count them. Examine the big bud of the rhubarb as it lies under the ground in late winter or early spring; or the crown buds of asparagus, hepatica, or other early spring plants. Dissect large buds of the apple and pear (Figs. 137, 138).

 

Fig. 137.—Bud of Apricot, showing the miniature leaves.

Fig. 138.—Bud of Pear, showing both leaves and flowers. The latter are the little knobs in the center.

 

The bud is protected by firm and dry scales. These scales are modified leaves. The scales fit close. Often the bud is protected by varnish (see horse-chestnut and the balsam poplars). Most winter buds are more or less woolly. Examine them under a lens. As we might expect, bud coverings are most prominent in cold and dry climates. Sprinkle water on velvet or flannel, and note the result and give a reason.

All winter buds give rise to branches, not to leaves alone; that is, the leaves are borne on the lengthening axis. Sometimes the axis, or branch, remains very short,—so short that it may not be noticed. Sometimes it grows several feet long.

Whether the branch grows large or not depends on the chance it has,—position on the plant, soil, rainfall, and many other factors. The new shoot is the unfolding and enlarging of the tiny axis and leaves that we saw in the bud. If the conditions are congenial, the shoot may form more leaves than were tucked away in the bud. The length of the shoot usually depends more on the lengths of the internodes than on the number of leaves.

Fig. 139.—Leaf-scars.—Ailanthus.

Where Buds are.—Buds are borne in the axils of the leaves,—in the acute angle that the leaf makes with the stem. When the leaf is growing in the summer, a bud is forming above it. When the leaf falls, the bud remains, and a scar marks the place of the leaf. Fig. 139 shows the large leaf-scars of ailanthus. Observe those on the horse-chestnut, maple, apple, pear, basswood, or any other tree or bush.

Sometimes two or more buds are borne in one axil; the extra ones are accessory or supernumerary buds. Observe them in the Tartarian honeysuckle (common in yards), walnut, butternut, red maple, honey locust, and sometimes in the apricot and peach.

If the bud is at the end of a shoot, however short the shoot, it is called a terminal bud. It continues the growth of the axis in a direct line. Very often three or more buds are clustered at the tip (Fig. 140); and in this case there may be more buds than leaf scars. Only one of them, however, is strictly terminal.

Fig. 140.—Terminal Bud between two other Buds.—Currant.

A bud in the axil of a leaf is an axillary or lateral bud. Note that there is normally at least one bud in the axil of every leaf on a tree or shrub in late summer and fall. The axillary buds, if they grow, are the starting points of new shoots the following season. If a leaf is pulled off early in summer, what will become of the young bud in its axil? Try this.

Fig. 141.—A Gigantic Bud.—Cabbage.

Bulbs and cabbage heads may be likened to buds; that is, they are condensed stems, with scales or modified leaves densely overlapping and forming a rounded body (Fig. 141). They differ from true buds, however, in the fact that they are condensations of whole main stems rather than embryo stems borne in the axils of leaves. But bulblets (as of tiger lily) may be scarcely distinguishable from buds on the one hand and from bulbs on the other. Cut a cabbage head in two, lengthwise, and see what it is like.

The buds that appear on roots are unusual or abnormal,—they occur only occasionally and in no definite order. Buds appearing in unusual places on any part of the plant are called adventitious buds. Such usually are the buds that arise when a large limb is cut off, and from which suckers or water sprouts arise.

 

Fig. 142.—Fruit-bud of Pear.

Fig. 143.—The opening of the Pear Fruit-bud.

 

 

Fig. 144.—Opening Pear Leaf-bud.

Fig. 145.—Opening of the Pear-bud.

 

How Buds Open.—When the bud swells, the scales are pushed apart, the little axis elongates and pushes out. In most plants the outside scales fall very soon, leaving a little ring of scars. With terminal buds, this ring marks the end of the year’s growth: how? Notice peach, apple, plum, willow, and other plants. In some others, all the scales grow for a time, as in the pear (Figs. 142, 143, 144). In other plants the inner bud scales become green and almost leaf-like. See the maple and hickory.

Sometimes Flowers come out of the Buds.—Leaves may or may not accompany the flowers. We saw the embryo flowers in Fig. 138. The bud is shown again in Fig. 142. In Fig. 143 it is opening. In Fig. 145 it is more advanced, and the woolly unformed flowers are appearing. In Fig. 146 the growth is more advanced.

 

Fig. 146.—A single Flower in the Pear cluster, as seen at 7 A.M. on the day of its opening. At 10 o’clock it will be fully expanded.

Fig. 147.—The opening of the Flower-bud of Apricot.

 

Fig. 148.—Apricot Flower-bud, enlarged.

Buds that contain or produce only leaves are leaf-buds. Those which contain only flowers are flower-buds or fruit-buds. The latter occur on peach, almond, apricot, and many very early spring-flowering plants. The single flower is emerging from the apricot bud in Fig. 147. A longitudinal section of this bud, enlarged, is shown in Fig. 148. Those that contain both leaves and flowers are mixed buds, as in pear, apple, and most late spring-flowering plants.

Fig. 149.—Fruit-buds and Leaf-buds of Pear.

Fruit buds are usually thicker or stouter than leaf-buds. They are borne in different positions on different plants. In some plants (apple, pear) they are on the ends of short branches or spurs; in others (peach, red maple) they are along the sides of the last year’s growths. In Fig. 149 are shown three fruit-buds and one leaf-bud on E, and leaf-buds on A. See also Figs. 150, 151, 152, 153, and explain.

Fig. 150.—Fruit-buds of Apple on Spurs: a dormant bud at the top.

 

Fig. 151.—Cluster of Fruit-buds of sweet Cherry, with one pointed leaf-bud in center.

Fig. 152.—Two Fruit-buds of Peach with a leaf-bud between.

 

Fig. 153.—Opening of Leaf-buds and Flower-buds of Apple.

“The burst of spring” means in large part the opening of the buds. Everything was made ready the fall before. The embryo shoots and flowers were tucked away, and the food was stored. The warm rain falls, and the shutters open and the sleepers wake: the frogs peep and the birds come.

Arrangement of Buds.—We have found that leaves are usually arranged in a definite order; buds are borne in the axils of leaves: therefore buds must exhibit phyllotaxy. Moreover, branches grow from buds: branches, therefore, should show a definite arrangement; usually, however, they do not show this arrangement because not all the buds grow and not all the branches live. (See Chaps. II and III.) It is apparent, however, that the mode of arrangement of buds determines to some extent the form of the tree: compare bud arrangement in pine or fir with that in maple or apple.

Fig. 154.—Oak Spray. How are the leaves borne with reference to the annual growths?

The uppermost buds on any twig, if they are well matured, are usually the larger and stronger and they are the most likely to grow the next spring; therefore, branches tend to be arranged in tiers (particularly well marked in spruces and firs). See Fig. 154 and explain it.

Winter Buds show what has been the Effect of Sunlight.—Buds are borne in the axils of the leaves, and the size or vigor of the leaf determines to a large extent the size of the bud. Notice that, in most instances, the largest buds are nearest the tip (Fig. 157). If the largest ones are not near the tip, there is some special reason for it. Can you state it? Examine the shoots on trees and bushes.

 

Fig. 155.—An Apple Twig.

Fig. 156.—Same twig before leaves fell.

 

Fig. 157.—Buds of the Hickory.

CHAPTER XVI BUD PROPAGATION

We have learned (in Chap. VI) that plants propagate by means of seeds. They also propagate by means of bud parts,—as rootstocks (rhizomes), roots, runners, layers, bulbs. The pupil should determine how any plant in which he is interested naturally propagates itself (or spreads its kind). Determine this for raspberry, blackberry, strawberry, June-grass or other grass, nut-grass, water lily, May apple or mandrake, burdock, Irish potato, sweet potato, buckwheat, cotton, pea, corn, sugar-cane, wheat, rice.

Plants may be artificially propagated by similar means, as by layers, cuttings, and grafts. The last two we may discuss here.

Cuttings in General.—A bit of a plant stuck into the ground stands a chance of growing; and this bit is a cutting. Plants have preferences, however, as to the kind of a bit which shall be used, but there is no way of telling what this preference is except by trying. In some instances this preference has not been discovered, and we say that the plant cannot be propagated by cuttings.

Most plants prefer that the cutting be made of the soft or growing parts (called “wood” by gardeners), of which the “slips” of geranium and coleus are examples. Others grow equally well from cuttings of the hard or mature parts or wood, as currant and grape; and in some instances this mature wood may be of roots, as in the blackberry. In some cases cuttings are made of tubers, as in the Irish potato (Fig. 60). Pupils should make cuttings now and then. If they can do nothing more, they can make cuttings of potato, as the farmer does; and they can plant them in a box in the window.

 

Fig. 158.—Geranium Cutting.

Fig. 159.—Rose Cutting.

 

The Softwood Cutting.—The softwood cutting is made from tissue that is still growing, or at least from that which is not dormant. It comprises one or two joints, with a leaf attached (Figs. 158, 159). It must not be allowed to wilt. Therefore, it must be protected from direct sunlight and dry air until it is well established; and if it has many leaves, some of them should be removed, or at least cut in two, in order to reduce the evaporating surface. The soil should be uniformly moist. The pictures show the depth to which the cuttings are planted.

For most plants, the proper age or maturity of wood for the making of cuttings may be determined by giving the twig a quick bend: if it snaps and hangs by the bark, it is in proper condition; if it bends without breaking, it is too young and soft or too old; if it splinters, it is too old and woody. The tips of strong upright shoots usually make the best cuttings. Preferably, each cutting should have a joint or node near its base; and if the internodes are very short it may comprise two or three joints.

The stem of the cutting is inserted one third or more its length in clean sand or gravel, and the earth is pressed firmly about it. A newspaper may be laid over the bed to exclude the light—if the sun strikes it—and to prevent too rapid evaporation. The soil should be moist clear through, not on top only.

Fig. 160.—Cutting-box.

Loose sandy or gravelly soil is used. Sand used by masons is good material in which to start most cuttings; or fine gravel—sifted of most of its earthy matter—may be used. Soils are avoided which contain much decaying organic matter, for these soils are breeding places of fungi, which attack the soft cutting and cause it to “damp off,” or to die at or near the surface of the ground. If the cuttings are to be grown in a window, put three or four inches of the earth in a shallow box or a pan. A soap box cut in two lengthwise, so that it makes a box four or five inches deep—as a gardener’s flat—is excellent (Fig. 160). Cuttings of common plants, as geranium, coleus, fuchsia, carnation, are kept at a living-room temperature. As long as the cuttings look bright and green, they are in good condition. It may be a month before roots form. When roots have formed, the plants begin to make new leaves at the tip. Then they may be transplanted into other boxes or into pots. The verbena in Fig. 161 is just ready for transplanting.

Fig. 161.—Verbena Cutting ready for Transplanting.

It is not always easy to find growing shoots from which to make the cuttings. The best practice, in that case, is to cut back an old plant, then keep it warm and well watered, and thereby force it to throw out new shoots. The old geranium plant from the window garden, or the one taken up from the lawn bed, may be treated this way (see Fig. 162). The best plants of geranium and coleus and most window plants are those which are not more than one year old. The geranium and fuchsia cuttings which are made in January, February, or March will give compact blooming plants for the next winter; and thereafter new ones should take their places (Fig. 163).

Fig. 162.—Old Geranium Plant cut back to make it throw out Shoots from which Cuttings can be made.

Fig. 163.—Early Winter Geranium, from a spring cutting.

The Hardwood Cutting.—Best results with cuttings of mature wood are secured when the cuttings are made in the fall and then buried until spring in sand in the cellar. These cuttings are usually six to ten inches long. They are not idle while they rest. The lower end calluses or heals, and the roots form more readily when the cutting is planted in the spring. But if the proper season has passed, take cuttings at any time in winter, plant them in a deep box in the window, and watch. They will need no shading or special care. Grape, currant, gooseberry, willow, and poplar readily take root from the hardwood. Fig. 164 shows a currant cutting. It has only one bud above the ground.

Fig. 164.—Currant Cutting.

The Graft.—When the cutting is inserted in a plant rather than in the soil, it is a graft; and the graft may grow. In this case the cutting grows fast to the other plant, and the two become one. When the cutting is inserted in a plant, it is no longer called a cutting, but a cion; and the plant in which it is inserted is called the stock. Fruit trees are grafted in order that a certain variety or kind may be perpetuated, as a Baldwin or Ben Davis variety of apple, Seckel or Bartlett pear, Navel or St. Michael orange.

Plants have preferences as to the stocks on which they will grow; but we can find out what their choice is only by making the experiment. The pear grows well on the quince, but the quince does not thrive on the pear. The pear grows on some of the hawthorns, but it is an unwilling subject on the apple. Tomato plants will grow on potato plants and potato plants on tomato plants. When the potato is the root, both tomatoes and potatoes may be produced, although the crop will be very small; when the tomato is the root, neither potatoes nor tomatoes will be produced. Chestnut will grow on some kinds of oak. In general, one species or kind is grafted on the same species, as apple on apple, pear on pear, orange on orange.

The forming, growing tissue of the stem (on the plants we have been discussing) is the cambium (Chap. X), lying on the outside of the woody cylinder beneath the bark. In order that union may take place, the cambium of the cion and of the stock must come together. Therefore the cion is set in the side of the stock. There are many ways of shaping the cion and of preparing the stock to receive it. These ways are dictated largely by the relative sizes of cion and stock, although many of them are matters of personal preference. The underlying principles are two: securing close contact between the cambiums of cion and stock; covering the wounded surfaces to prevent evaporation and to protect the parts from disease.

On large stocks the commonest form of grafting is the cleft-graft. The stock is cut off and split; and in one or both sides a wedge-shaped cion is firmly inserted. Fig. 165 shows the cion; Fig. 166, the cions set in the stock; Fig. 167, the stock waxed. It will be seen that the lower bud—that lying in the wedge—is covered by the wax; but being nearest the food supply and least exposed to weather, it is the most likely to grow: it will push through the wax.

Cleft-grafting is practiced in spring, as growth begins. The cions are cut previously, when perfectly dormant, and from the tree which it is desired to propagate. The cions are kept in sand or moss in the cellar. Limbs of various sizes may be cleft-grafted,—from one half inch up to four inches in diameter; but a diameter of one to one and one half inches is the most convenient size. All the leading or main branches of a tree top may be grafted. If the remaining parts of the top are gradually cut away and the cions grow well, the entire top will be changed over to the new variety.

Fig. 165.—Cion Of Apple.

 

Fig. 166.—The Cion Inserted.

Fig. 167.—The Parts Waxed.

 

Another form of grafting is known as budding. In this case a single bud is used, and it is slipped underneath the bark of the stock and securely tied (not waxed) with soft material, as bass bark, corn shuck, yarn, or raffia (the last a commercial palm fiber). Budding is performed when the bark of the stock will slip or peel (so that the bud can be inserted), and when the bud is mature enough to grow. Usually budding is performed in late summer or early fall, when the winter buds are well formed; or it may be practiced in spring with buds cut in winter. In ordinary summer budding (which is the usual mode) the “bud” or cion forms a union with the stock, and then lies dormant till the following spring, as if it were still on its own twig. Budding is mostly restricted to young trees in the nursery. In the spring following the budding, the stock is cut off just above the bud, so that only the shoot from the bud grows to make the future tree. This prevailing form of budding (shield-budding) is shown in Fig. 168.

Fig. 168.—Budding. The “bud”; the opening to receive it; the bud tied.

CHAPTER XVII HOW PLANTS CLIMB

We have found that plants struggle or contend for a place in which to live. Some of them become adapted to grow in the forest shade, others to grow on other plants, as epiphytes, others to climb to the light. Observe how woods grapes, and other forest climbers, spread their foliage on the very top of the forest tree, while their long flexile trunks may be bare.

There are several ways by which plants climb, but most climbers may be classified into four groups: (1) scramblers, (2) root climbers, (3) tendril climbers, (4) twiners.

Scramblers.—Some plants rise to light and air by resting their long and weak stems on the tops of bushes and quick-growing herbs. Their stems may be elevated in part by the growing twigs of the plants on which they recline. Such plants are scramblers. Usually they are provided with prickles or bristles. In most weedy swamp thickets, scrambling plants may be found. Briers, some roses, bed-straw or galium, bittersweet (Solanum Dulcamara, not the Celastrus), the tear-thumb polygonums, and other plants are familiar examples of scramblers.

Root Climbers.—Some plants climb by means of true roots. These roots seek the dark places and therefore enter the chinks in walls and bark. The trumpet creeper is a familiar example (Fig. 36). The true or English ivy, which is often grown to cover buildings, is another instance. Still another is the poison ivy. Roots are distinguished from stem tendrils by their irregular or indefinite position as well as by their mode of growth.

Fig. 169.—Tendril, to show where the coil is changed.

Tendril climbers.—A slender coiling part that serves to hold a climbing plant to a support is known as a tendril. The free end swings or curves until it strikes some object, when it attaches itself and then coils and draws the plant close to the support. The spring of the coil also allows the plant to move in the wind, thereby enabling the plant to maintain its hold. Slowly pull a well-matured tendril from its support, and note how strongly it holds on. Watch the tendrils in a wind-storm. Usually the tendril attaches to the support by coiling about it, but the Virginia creeper and Boston ivy (Fig. 170) attach to walls by means of disks on the ends of the tendrils.

Fig. 170.—Tendril of Boston Ivy.

Since both ends of the tendril are fixed, when it finds a support, the coiling would tend to twist it in two. It will be found, however, that the tendril coils in different directions in different parts of its length. In Fig. 169, showing an old and stretched-out tendril, the change of direction in the coil occurred at a. In long tendrils of cucumbers and melons there may be several changes of direction.

Tendrils may represent either branches or leaves. In the Virginia creeper and grape they are branches; they stand opposite the leaves in the position of fruit clusters, and sometimes one branch of a fruit cluster is a tendril. These tendrils are therefore homologous with fruit-clusters, and fruit-clusters are branches.

In some plants tendrils are leaflets (Chap. XI). Examples are the sweet pea and common garden pea. In Fig. 171, observe the leaf with its two great stipules, petiole, six normal leaflets, and two or three pairs of leaflet tendrils and a terminal leaflet tendril. The cobea, a common garden climber, has a similar arrangement. In some cases tendrils are stipules, as probably in the green briers (smilax).

The petiole or midrib may act as a tendril, as in various kinds of clematis. In Fig. 172, the common wild clematis or “old man vine,” this mode is seen.

Fig. 171.—Leaves of Pea,—very large stipules, opposite leaflets, and leaflets represented by tendrils.

Twiners.—The entire plant or shoot may wind about a support. Such a plant is a twiner. Examples are bean, hop, morning-glory, moonflower, false bittersweet or waxwork (Celastrus), some honeysuckles, wistaria, Dutchman’s pipe, dodder. The free tip of the twining branch sweeps about in curves, much as the tendril does, until it finds support or becomes old and rigid.

Each kind of plant usually coils in only one direction. Most plants coil against the sun, or from the observer’s left across his front to his right as he faces the plant. Examples are bean, morning-glory. The hop twines from the observer’s right to his left, or with the sun.

Fig. 172.—Clematis climbing by Leaf-tendril.

CHAPTER XVIII THE FLOWER—ITS PARTS AND FORMS

The function of the flower is to produce seed. It is probable that all its varied forms and colors contribute to this supreme end. These forms and colors please the human fancy and add to the joy of living, but the flower exists for the good of the plant, not for the good of man. The parts of the flower are of two general kinds—those that are directly concerned in the production of seeds, and those that act as covering and protecting organs. The former parts are known as the essential organs; the latter as the floral envelopes.

Envelopes.—The floral envelopes usually bear a close resemblance to leaves. These envelopes are very commonly of two series or kinds—the outer and the inner. The outer series, known as the calyx, is usually smaller and green. It usually comprises the outer cover of the flower bud. The calyx is the lowest whorl in Fig. 173.

Fig. 173.—Flower Of A Buttercup in Section.

The inner series, known as the corolla, is usually colored and more special or irregular in shape than the calyx. It is the showy part of the flower, as a rule. The corolla is the second or large whorl in Fig. 173.

The calyx may be composed of several leaves. Each leaf is a sepal. If it is of one piece, it may be lobed or divided, in which case the divisions are called calyx-lobes.

In like manner, the corolla may be composed of petals, or it may be of one piece and variously lobed. A calyx of one piece, no matter how deeply lobed, is gamosepalous. A corolla of one piece is gamopetalous. When these series are of separate pieces, as in Fig. 173, the flower is said to be polysepalous and polypetalous. Sometimes both series are of separate parts, and sometimes only one of them is so formed.

The floral envelopes are homologous with leaves. Sepals and petals, at least when more than three or five, are in more than one whorl, and one whorl stands below another so that the parts overlap. They are borne on the expanded or thickened end of the flower stalk; this end is the torus. In Fig. 173 all the parts are seen as attached to the torus. This part is sometimes called the receptacle, but this word is a common-language term of several meanings, whereas torus has no other meaning. Sometimes one part is attached to another part, as in the fuchsia (Fig. 174), in which the petals are borne on the calyx-tube.

Fig. 174.—Flower Of Fuchsia in Section.

Subtending Parts.—Sometimes there are leaf-like parts just below the calyx, looking like a second calyx. Such parts accompany the carnation flower. These parts are bracts (bracts are small specialized leaves); and they form an involucre. We must be careful that we do not mistake them for true flower parts. Sometimes the bracts are large and petal-like, as in the great white blooms of the flowering dogwood: here the real flowers are several, small and greenish, forming a small cluster in the center.

Essential Organs.—The essential organs are of two series. The outer series is composed of the stamens. The inner series is composed of the pistils.

Stamens bear the pollen, which is made up of grains or spores, each spore usually being a single plant cell. The stamen is of two parts, as is readily seen in Figs. 173, 174,—the enlarged terminal part or anther, and the stalk or filament. The filament is often so short as to seem to be absent, and the anther is then said to be sessile. The anther bears the pollen spores. It is made up of two or four parts (known as sporangia or spore-cases), which burst and discharge the pollen. When the pollen is shed, the stamen dies.

The pistil has three parts: the lowest, or seed-bearing part, which is the ovary; the stigma at the upper extremity, which is a flattened or expanded surface, and usually roughened or sticky; the stalk-like part or style, connecting the ovary and stigma. Sometimes the style is apparently wanting, and the stigma is said to be sessile on the ovary. These parts are shown in the fuchsia (Fig. 174). The ovary or seed vessel is at a. A long style, bearing a large stigma, projects from the flower. See also Figs. 175 and 176.

Fig. 175.—The Structure of a Plum Blossom.

se, sepals; p, petals; sta, stamens; o, ovary; s, style; st, stigma. The pistil consists of the ovary, style, and stigma. It contains the seed part. The stamens are tipped with anthers, in which the pollen is borne. The ovary, o, ripens into the fruit.

Stamens and pistils probably are homologous with leaves. A pistil is sometimes conceived to represent anciently a leaf as if rolled into a tube; and an anther, a leaf of which the edges may have been turned in on the midrib.

Fig. 176.—Simple Pistils of Buttercup, one in longitudinal section.

The pistil may be of one part or compartment, or of many parts. The different units or parts of which it is composed are carpels. Each carpel is homologous with a leaf. Each carpel bears one or more seeds. A pistil of one carpel is simple; of two or more carpels, compound. Usually the structure of the pistil may be determined by cutting horizontally across the lower or seed-bearing part, as Figs. 177, 178 explain. A flower may contain a simple pistil (one carpel), as the pea (Fig. 177); several simple pistils (several separate carpels), as the buttercup (Fig. 176); or a compound pistil with carpels united, as the Saint John’s wort (Fig. 178) and apple. How many carpels in an apple? A peach? An okra pod? A bean pod? The seed cavity in each carpel is called a locule (Latin locus, a place). In these locules the seeds are borne.

 

Fig. 177.--Pistil of Garden Pea, the stamens being pulled down in order to disclose it; also a section showing the single compartment (compare Fig. 188).

Fig. 178.—Compound Pistil of a St. John’s Wort. It has 5 carpels.

 

Conformation of the Flower.—A flower that has calyx, corolla, stamens, and pistils is said to be complete (Fig. 173); all others are incomplete. In some flowers both the floral envelopes are wanting: such are naked. When one of the floral envelope series is wanting, the remaining series is said to be calyx, and the flower is therefore apetalous (without petals). The knotweed (Fig. 179), smartweed, buckwheat, elm are examples.

Fig. 179.—Knotweed, a very common but inconspicuous plant along hard walks and roads. Two flowers, enlarged, are shown at the right. These flowers are very small and borne in the axils of the leaves.

Some flowers lack the pistils: these are staminate, whether the envelopes are missing or not. Others lack the stamens: these are pistillate. Others have neither stamens nor pistils: these are sterile (snowball and hydrangea). Those that have both stamens and pistils are perfect, whether or not the envelopes are missing. Those that lack either stamens or pistils are imperfect or diclinous. Staminate and pistillate flowers are imperfect or diclinous.

Fig. 180.—Staminate Catkins Of Oak. The pistillate flowers are in the leaf axils, and not shown in this picture.

Fig. 181.—Begonia Flowers.

Staminate at A; pistillate below, with the winged ovary at B.

When staminate and pistillate flowers are borne on the same plant, e.g. oak (Fig. 180), corn, beech, chestnut, hazel, walnut, hickory, pine, begonia (Fig. 181), watermelon, gourd, pumpkin, the plant is monœcious (“in one house”). When they are on different plants, e.g. poplar, cottonwood, bois d’arc, willow (Fig. 182), the plant is diœcious (“in two houses”). Some varieties of strawberry, grape, and mulberry are partly diœcious. Is the rose either monœcious or diœcious?

Fig. 182.—Catkins of a Willow.

A staminate flower is shown at s, and a pistillate flower at p. The staminate and pistillate are on different plants.

 

Fig. 183.—Flower Of Garden Nasturtium.

Separate petal at a. The calyx is produced into a spur.

Fig. 184.—The Five Petals Of the Pansy, detached to show the form.

 

Fig. 185.—Flower Of Catnip.

Flowers in which the parts of each series are alike are said to be regular (as in Figs. 173, 174, 175). Those in which some parts are unlike other parts of the same series are irregular. Their regularity may be in calyx, as in nasturtium (Fig. 183); in corolla (Figs. 184, 185); in the stamens (compare nasturtium, catnip, Fig. 185, sage); in the pistils. Irregularity is most frequent in the corolla.

Various Forms of Corolla.—The corolla often assumes very definite or distinct forms, especially when gamopetalous. It may have a long tube with a wide-flaring limb, when it is said to be funnelform, as in morning-glory and pumpkin. If the tube is very narrow and the limb stands at right angles to it, the corolla is salverform, as in phlox. If the tube is very short and the limb wide-spreading and nearly circular in outline, the corolla is rotate or wheel-shaped, as in potato.

A gamopetalous corolla or gamosepalous calyx is often cleft in such way as to make two prominent parts. Such parts are said to be lipped or labiate. Each of the lips or lobes may be notched or toothed. In 5-membered flowers, the lower lip is usually 3-lobed and the upper one 2-lobed. Labiate flowers are characteristic of the mint family (Fig. 185), and the family therefore is called the Labiatæ. (Literally, labiate means merely “lipped,” without specifying the number of lips or lobes; but it is commonly used to designate 2-lipped flowers.) Strongly 2-parted polypetalous flowers may be said to be labiate; but the term is oftenest used for gamopetalous corollas.

Labiate gamopetalous flowers that are closed in the throat (or entrance to the tube) are said to be grinning or personate (personate means masked, or person-like). Snap-dragon is a typical example; also toadflax or butter-and-eggs (Fig. 186), and many related plants. Personate flowers usually have definite relations to insect pollination. Observe how an insect forces his head into the closed throat of the toadflax.

Fig. 186.—Personate Flower Of Toadflax.

The peculiar flowers of the pea tribes are explained in Figs. 187, 188.

Fig. 187.—Flowers of the Common Bean, with one flower opened (a) to show the structure.

Fig. 188.—Diagram of Alfalfa Flower in Section:

C, calyx; D, standard; W, wing; K, keel; T, stamen-tube; F, filament of tenth stamen; X, stigma; Y, style; O, ovary; the dotted lines at E show position of stamen tube, when pushed upward by insects. Enlarged.

Spathe Flowers.—In many plants, very simple (often naked) flowers are borne in dense, more or less fleshy spikes, and the spike is inclosed in or attended by a leaf, sometimes corolla-like, known as a spathe. The spike of flowers is technically known as a spadix. This type of flower is characteristic of the great arum family, which is chiefly tropical. The commonest wild representatives in the North are Jack-in-the-pulpit, or Indian turnip, and skunk cabbage. In the former the flowers are all diclinous and naked. In the skunk cabbage all the flowers are perfect and have four sepals. The common calla is a good example of this type of inflorescence.

Fig. 189.—Head of Sunflower.

Compositous Flowers.—The head (anthodium) or so-called “flower” of sunflower (Fig. 189), thistle, aster, dandelion, daisy, chrysanthemum, goldenrod, is composed of several or many little flowers, or florets. These florets are inclosed in a more or less dense and usually green involucre. In the thistle (Fig. 190) this involucre is prickly. A longitudinal section discloses the florets, all attached at bottom to a common torus, and densely packed in the involucre. The pink tips of these florets constitute the showy part of the head.

Fig. 190.—Longitudinal Section of Thistle Head; also a Floret of Thistle.

Each floret of the thistle (Fig. 190) is a complete flower. At a is the ovary. At b is a much-divided plumy calyx, known as the pappus. The corolla is long-tubed, rising above the pappus, and is enlarged and 5-lobed at the top, c. The style projects at e. The five anthers are united about the style in a ring at d. Such anthers are said to be syngenesious. These are the various parts of the florets of the Compositæ. In some cases the pappus is in the form of barbs, bristles, or scales, and sometimes it is wanting. The pappus, as we shall see later, assists in distributing the seed. Often the florets are not all alike. The corolla of those in the outer circles may be developed into a long, straplike, or tubular part, and the head then has the appearance of being one flower with a border of petals. Of such is the sunflower (Fig. 189), aster, bachelor’s button or cornflower, and field daisy (Fig. 211). These long corolla-limbs are called rays. In some cultivated composites, all the florets may develop rays, as in the dahlia and chrysanthemum. In some species, as dandelion, all the florets naturally have rays. Syngenesious arrangement of anthers is the most characteristic single feature of the composites.

Fig. 191.—Petals Arising from the Staminal Column of Hollyhock, and accessory petals in the corolla-whorl.

Double Flowers.—Under the stimulus of cultivation and increased food supply, flowers tend to become double. True doubling arises in two ways, morphologically: (1) stamens or pistils may produce petals (Fig. 191); (2) adventitious or accessory petals may arise in the circle of petals. Both of these categories may be present in the same flower. In the full double hollyhock the petals derived from the staminal column are shorter and make a rosette in the center of the flower. In Fig. 192 is shown the doubling of a daffodil by the modification of stamens. Other modifications of flowers are sometimes known as doubling. For example, double dahlias, chrysanthemums, and sunflowers are forms in which the disk flowers have developed rays. The snowball is another case. In the wild snowball the external flowers of the cluster are large and sterile. In the cultivated plant all the flowers have become large and sterile. Hydrangea is a similar case.

Fig. 192.—Narcissus Or Daffodil. Single flower at the right.

CHAPTER XIX THE FLOWER—FERTILIZATION AND POLLINATION

Fertilization.—Seeds result from the union of two elements or parts. One of these elements is a cell-nucleus of the pollen-grain. The other element is the cell-nucleus of an egg-cell, borne in the ovary. The pollen-grain falls on the stigma (Fig. 193). It absorbs the juices exuded by the stigma, and grows by sending out a tube (Fig. 194). This tube grows downward through the style, absorbing food as it goes, and finally reaches the egg-cell in the interior of an ovule in the ovary (Fig. 195), and fertilization, or union of a nucleus of the pollen and the nucleus of the egg-cell in the ovule, takes place. The ovule and embryo within then develops into a seed. The growth of the pollen-tube is often spoken of as germination of the pollen, but it is not germination in the sense in which the word is used when speaking of seeds.

Fig. 193.—B, Pollen escaping from anther; A, pollen germinating on a stigma. Enlarged.

Fig. 194.—A Pollen-grain and the Growing Tube.

Better seeds—that is, those that produce stronger and more fruitful plants—often result when the pollen comes from another flower. Fertilization effected between different flowers is cross-fertilization; that resulting from the application of pollen to pistils in the same flower is close-fertilization or self-fertilization. It will be seen that the cross-fertilization relationship may be of many degrees—between two flowers in the same cluster, between those in different clusters on the same branch, between those on different plants. Usually fertilization takes place only between plants of the same species or kind.

Fig. 195.—Diagram to represent Fertilization.

s, stigma; st, style; ov, ovary; o, ovule; p, pollen-grain; pt, pollen-tube; e, egg-cell; m, micropyle.

In many cases there is, in effect, an apparent selection of pollen when pollen from two or more sources is applied to the stigma. Sometimes the foreign pollen, if from the same kind of plant, grows, and fertilization results, while pollen from the same flower is less promptly effective. If, however, no foreign pollen is present, the pollen from the same flower may finally serve the same purpose.

In order that the pollen may grow, the stigma must be ripe. At this stage the stigma is usually moist and sometimes sticky. A ripe stigma is said to be receptive. The stigma may remain receptive for several hours or even days, depending on the kind of plant, the weather, and how soon pollen is received. Watch a certain flower every day to see the anther locules open and the stigma ripen. When fertilization takes place, the stigma dies. Observe, also, how soon the petals wither after the stigma has received pollen.

Pollination.—The transfer of the pollen from anther to stigma is known as pollination. The pollen may fall of its own weight on the adjacent stigma, or it may be carried from flower to flower by wind, insects, or other agents. There may be self-pollination or cross-pollination, and of course it must always precede fertilization.

Usually the pollen is discharged by the bursting of the anthers. The commonest method of discharge is through a slit on either side of the anther (Fig. 193). Sometimes it discharges through a pore at the apex, as in azalea (Fig. 196), rhododendron, huckleberry, wintergreen. In some plants a part of the anther wall raises or falls as a lid, as in barberry (Fig. 197), blue cohosh, May apple. The opening of an anther (as also of a seed-pod) is known as dehiscence (de, from; hisco, to gape). When an anther or seed pod opens, it is said to dehisce.

 

Fig. 196.—Anther of Azalea, opening by terminal pores.

Fig. 197.—Barberry Stamen, with anther opening by lids.

 

Most flowers are so constructed as to increase the chances of cross-pollination. We have seen that the stigma may have the power of choosing foreign pollen. The commonest means of necessitating cross-pollination is the different times of maturing of stamens and pistils in the same flower. In most cases the stamens mature first: the flower is then proterandrous. When the pistils mature first, the flower is proterogynous. (Aner, andr, is a Greek root often used, in combinations, for stamen, and gyne for pistil.) The difference in time of ripening may be an hour or two, or it may be a day. The ripening of the stamens and pistils at different times is known as dichogamy, and flowers of such character are said to be dichogamous. There is little chance for dichogamous flowers to pollinate themselves. Many flowers are imperfectly dichogamous—some of the anthers mature simultaneously with the pistils, so that there is chance for self-pollination in case foreign pollen does not arrive. Even when the stigma receives pollen from its own flower, cross-fertilization may result. The hollyhock is proterandrous. Fig. 198 shows a flower recently expanded. The center is occupied by the column of stamens. In Fig. 199, showing an older flower, the long styles are conspicuous.

Fig. 198.—Flower of Hollyhock; proterandrous.

Fig. 199.—Older Flower of Hollyhock.

Some flowers are so constructed as to prohibit self-pollination. Very irregular flowers are usually of this kind. With some of them, the petals form a sac to inclose the anthers and the pollen cannot be shed on the stigma but is retained until a bee forces the sac open; the pollen is rubbed on the hairs of the bee and transported. Regular flowers usually depend mostly on dichogamy and the selective power of the pistil to insure crossing. Flowers that are very irregular and provided with nectar and strong perfume are usually pollinated by insects. Gaudy colors probably attract insects in many cases, but perfume appears to be a greater attraction.

 

Fig. 200.—Flower of Larkspur.

Fig. 201.—Envelopes of a Larkspur. There are five wide sepals, the upper one being spurred. There are four small petals.

 

The insect visits the flower for the nectar (for the making of honey) and may unknowingly carry the pollen. Spurs and sacs in the flower are nectaries (Fig. 200), but in spurless flowers the nectar is usually secreted in the bottom of the flower cup. This compels the insect to pass by the anther and rub against the pollen before it reaches the nectar. Sometimes the anther is a long lever poised on the middle point and the insect bumps against one end and lifts it, thus bringing the other end of the lever with the pollen sacs down on its back. Flowers that are pollinated by insects are said to be entomophilous (“insect loving”). Fig. 200 shows a larkspur. The envelopes are separated in Fig. 201. The long spur at once suggests insect pollination. The spur is a sepal. Two hollow petals project into this spur, apparently serving to guide the bee’s tongue. The two smaller petals, in front, are peculiarly colored and perhaps serve the bee in locating the nectary. The stamens ensheath the pistils (Fig. 202). As the insect stands on the flower and thrusts its head into the center, the envelopes are pushed downward and outward and the pistil and stamens come in contact with its abdomen. Since the flower is proterandrous, the pollen that the pistils receive from the bee’s abdomen must come from another flower. Note a somewhat similar arrangement in the toadflax or butter-and-eggs.

Fig. 202.—Stamens of Larkspur, surrounding the pistils.

In some cases (Fig. 203) the stamens are longer than the pistil in one flower and shorter in another. If the insect visits such flowers, it gets pollen on its head from the long-stamen flower, and deposits this pollen on the stigma in the long-pistil flower. Such flowers are dimorphous (of two forms). If pollen from its own flower and from another flower both fall on the stigma, the probabilities are that the stigma will choose the foreign pollen.

Fig. 203.—Dimorphic Flowers of Primrose.

Many flowers are pollinated by the wind. They are said to be anemophilous (“wind loving”). Such flowers produce great quantities of pollen, for much of it is wasted. They usually have broad stigmas, which expose large surfaces to the wind. They are usually lacking in gaudy colors and in perfume. Grasses and pine trees are typical examples of anemophilous plants.

Fig. 204.—Flowers of Black Walnut: two pistillate flowers at A, and staminate catkins at B.

In many cases cross-pollination is insured because the stamens and pistils are in different flowers (diclinous). Monœcious and diœcious plants may be pollinated by wind or insects, or other agents (Fig. 204). They are usually wind-pollinated, although willows are often, if not mostly, insect-pollinated. The Indian corn is a monœcious plant. The staminate flowers are in a terminal panicle (tassel). The pistillate flowers are in a dense spike (ear), inclosed in a sheath or husk. Each “silk” is a style. Each pistillate flower produces a kernel of corn. Sometimes a few pistillate flowers are borne in the tassel and a few staminate flowers on the tip of the ear. Is self-fertilization possible with the corn? Why does a “volunteer” stalk standing alone in a garden have only a few grains on the ear? What is the direction of the prevailing wind in summer? If only two or three rows of corn are planted in a garden where prevailing winds occur, in which direction would they better run?

Fig. 205.—Common Blue Violet. The familiar flowers are shown, natural size. The corolla is spurred. Late in the season, cleistogamous flowers are often borne on the surface of the ground. A small one is shown at a. A nearly mature pod is shown at b. Both a and b are one third natural size.

Although most flowers are of such character as to insure or increase the chances of cross-pollination, there are some that absolutely forbid crossing. These flowers are usually borne beneath or on the ground, and they lack showy colors and perfumes. They are known as cleistogamous flowers (meaning “hidden flowers”). The plant has normal showy flowers that may be insect-pollinated, and in addition is provided with these simplified flowers. Only a few plants bear cleistogamous flowers. Hog-peanut, common blue violet, fringed wintergreen, and dalibarda are the best subjects in the Northern states. Fig. 205 shows a cleistogamous flower of the blue violet at a. Above the true roots, slender stems bear these flowers, that are provided with a calyx, and a curving corolla which does not open. Inside are the stamens and pistils. Late in the season the cleistogamous flowers may be found just underneath the mold. They never rise above ground. The following summer one may find a seedling plant, in some kinds of plants, with the remains of the old cleistogamous flower still adhering to the root. Cleistogamous flowers usually appear after the showy flowers have passed. They seem to insure a crop of seed by a method that expends little of the plant’s energy. The pupil will be interested to work out the fruiting of the peanut (Fig. 206). Unbaked fresh peanuts grow readily and can easily be raised in the North in a warm sandy garden.

Fig. 206.—Pods of Peanuts ripening underground.

Fig. 207.—Struggle for Existence among the Apple Flowers.

 

Fig. 208.—A Paper Bag, with string inserted.

Fig. 209.—The Bag tied over a Flower.

 

Fig. 210.—Fig. The fig is a hollow torus with flowers borne on the inside, and pollinated by insects that enter at the apex.

CHAPTER XX FLOWER-CLUSTERS

Origin of the Flower-cluster.—We have seen that branches arise from the axils of leaves. Sometimes the leaves may be reduced to bracts and yet branches are borne in their axils. Some of the branches grow into long limbs; others become short spurs; others bear flowers. In fact, a flower is itself a specialized branch.

Flowers are usually borne near the top of the plant. Often they are produced in great numbers. It results, therefore, that flower branches usually stand close together, forming a cluster. The shape and arrangement of the flower-cluster differ with the kind of plant, since each plant has its own mode of branching.

Fig. 211.—Terminal Flowers of the Whiteweed (in some places called ox-eye daisy).

Certain definite or well-marked types of flower-clusters have received names. Some of these names we shall discuss, but the flower-clusters that perfectly match the definitions are the exception rather than the rule. The determining of the kinds of flower-clusters is one of the most perplexing subjects in descriptive botany. We may classify the subject around three ideas: solitary flowers, centrifugal or determinate clusters, centripetal or indeterminate clusters.

Solitary Flowers.—In many cases flowers are borne singly; they are separated from other flowers by leaves. They are then said to be solitary. The solitary flower may be either at the end of the main shoot or axis (Fig. 211), when it is said to be terminal; or from the side of the shoot (Fig. 212), when it is said to be lateral or axillary.

Fig. 212.—Lateral Flower Of an Abutilon. A greenhouse plant.

Centripetal Clusters.—If the flower-bearing axils were rather close together, an open or leafy flower-cluster might result. If the plant continues to grow from the tip, the older flowers are left farther and farther behind. If the cluster were so short as to be flat or convex on top, the outermost flowers would be the older. A flower-cluster in which the lower or outer flowers open first is said to be a centripetal cluster. It is sometimes said to be an indeterminate cluster, since it is the result of a type of growth which may go on more or less continuously from the apex.

The simplest form of a definite centripetal cluster is a raceme, which is an open elongated cluster in which the flowers are borne singly on very short branches and open from below (that is, from the older part of the shoot) upwards (Fig. 213). The raceme may be terminal to the main branch; or it may be lateral to it, as in Fig. 214. Racemes often bear the flowers on one side of the stem, thus forming a single row.

When a centripetal flower-cluster is long and dense and the flowers are sessile or nearly so, it is called a spike (Fig. 215). Common examples of spikes are plantain, mignonette, mullein.

Fig. 213.—Raceme of Currant. Terminal or lateral?

Fig. 214.—Lateral Racemes (in fruit) of Barberry.

Fig. 215.—Spike of Plantain.

 

A very short and dense spike is a head. Clover (Fig. 216) is a good example. The sunflower and related plants bear many small flowers in a very dense and often flat head. Note that in the sunflower (Fig. 189) the outside or exterior flowers open first. Another special form of spike is the catkin, which usually has scaly bracts, the whole cluster being deciduous after flowering or fruiting, and the flowers (in typical cases) having only stamens or pistils. Examples are the “pussies” of willows (Fig. 182) and flower-clusters of oak (Fig. 180), walnuts (Fig. 204), poplars.

 

Fig. 216.—Head of Clover Blossoms.

Fig. 217.—Corymb of Candy-tuft.

 

When a loose, elongated centripetal flower-cluster has some primary branches simple, and others irregularly branched, it is called a panicle. It is a branching raceme. Because of the earlier growth of the lower branches, the panicle is usually broadest at the base or conical in outline. True panicles are not very common.

When an indeterminate flower-cluster is short, so that the top is convex or flat, it is a corymb (Fig. 217). The outermost flowers open first. Centripetal flower-clusters are sometimes said to be corymbose in mode.

When the branches of an indeterminate cluster arise from a common point, like the frame of an umbrella, the cluster is an umbel (Fig. 218). Typical umbels occur in carrot, parsnip, caraway and other plants of the parsley family: the family is known as the Umbelliferæ, or umbel-bearing family. In the carrot and many other Umbelliferæ, there are small or secondary umbels, called umbellets, at the end of each of the main branches. (In the center of the wild carrot umbel one often finds a single, blackish, often aborted flower, comprising a 1-flowered umbellet.)

Fig. 218.—Remains of a Last Year’s Umbel of Wild Carrot.

Centrifugal or Determinate Clusters.—When the terminal or central flower opens first, the cluster is said to be centrifugal. The growth of the shoot or cluster is determinate, since the length is definitely determined or stopped by the terminal flower. Fig. 219 shows a determinate or centrifugal mode of flower bearing.

Dense centrifugal clusters are usually flattish on top because of the cessation of growth in the main or central axis. These compact flower-clusters are known as cymes. Centrifugal clusters are sometimes said to be cymose in mode. Apples, pears (Fig. 220), and elders bear flowers in cymes. Some cyme-forms are like umbels in general appearance. A head-like cymose cluster is a glomerule; it blooms from the top downwards rather than from the base upwards.

Mixed Clusters.—Often the cluster is mixed, being determinate in one part and indeterminate in another part of the same cluster. The main cluster may be indeterminate, but the branches determinate. The cluster has the appearance of a panicle, and is usually so called, but it is really a thyrse. Lilac is a familiar example of a thyrse. In some cases the main cluster is determinate and the branches are indeterminate, as in hydrangea and elder.

 

Fig. 219.—Determinate or Cymose Arrangement.—Wild geranium.

Fig. 220.—Cyme of Pear. Often imperfect.

 

Inflorescence.—The mode or method of flower arrangement is known as the inflorescence. That is, the inflorescence is cymose, corymbose, paniculate, spicate, solitary, determinate, indeterminate. By custom, however, the word “inflorescence” has come to be used for the flower-cluster itself in works on descriptive botany. Thus a cyme or a panicle may be called an inflorescence. It will be seen that even solitary flowers follow either indeterminate or determinate methods of branching.

Fig. 221.—Forms of Centripetal Flower-clusters.

1, raceme; 2, spike; 3, umbel; 4, head or anthodium; 5, corymb.

Fig. 222.—Centripetal Inflorescence, continued.

6, spadix; 7, compound umbel; 8, catkin.

Fig. 223.—Centrifugal Inflorescence.

1, cyme; 2, scirpioid raceme (or half cyme).

The flower-stem.—The stem of a solitary flower is known as a peduncle; also the general stem of a flower-cluster. The stem of the individual flower in a cluster is a pedicel. In the so-called stemless plants the peduncle may arise directly from the ground, or crown of the plant, as in dandelion, hyacinth, garden daisy; this kind of peduncle is called a scape. A scape may bear one or many flowers. It has no foliage leaves, but it may have bracts.

CHAPTER XXI FRUITS

The ripened ovary, with its attachments, is known as the fruit. It contains the seeds. If the pistil is simple, or of one carpel, the fruit also will have one compartment. If the pistil is compound, or of more than one carpel, the fruit usually has an equal number of compartments. The compartments in pistil and fruit are known as locules (from Latin locus, meaning “a place”).

Fig. 224.—Dentaria, or Tooth-wort, in fruit.

The simplest kind of fruit is a ripened 1-loculed ovary. The first stage in complexity is a ripened 2- or many-loculed ovary. Very complex forms may arise by the attachment of other parts to the ovary. Sometimes the style persists and becomes a beak (mustard pods, dentaria, Fig. 224), or a tail as in clematis; or the calyx may be attached to the ovary; or the ovary may be embedded in the receptacle, and ovary and receptacle together constitute the fruit: or an involucre may become a part of the fruit, as possibly in the walnut and hickory (Fig. 225), and cup of the acorn (Fig. 226). The chestnut and the beech bear a prickly involucre, but the nuts, or true fruits, are not grown fast to it, and the involucre can scarcely be called a part of the fruit. A ripened ovary is a pericarp. A pericarp to which other parts adhere has been called an accessory or reënforced fruit. (Page 169.)

 

Fig. 225.—Hickory-nut. The nut is the fruit, contained in a husk.

Fig. 226.—Live-oak Acorn. The fruit is the “seed” part; the involucre is the “cup.”

 

 

Fig. 227.—Key of Sugar Maple.

Fig. 228.—Key of Common American Elm.

 

Some fruits are dehiscent, or split open at maturity and liberate the seeds; others are indehiscent, or do not open. A dehiscent pericarp is called a pod. The parts into which such a pod breaks or splits are known as valves. In indehiscent fruits the seed is liberated by the decay of the envelope, or by the rupturing of the envelope by the germinating seed. Indehiscent winged pericarps are known as samaras or key fruits. Maple (Fig. 227), elm (Fig. 228), and ash (Fig. 93) are examples.

Pericarps.—The simplest pericarp is a dry, one-seeded, indehiscent body. It is known as an akene. A head of akenes is shown in Fig. 229, and the structure is explained in Fig. 230. Akenes may be seen in buttercup, hepatica, anemone, smartweed, buckwheat.

 

Fig. 229.—Akenes Of Buttercup.

Fig. 230.—akenes Of Buttercup, one in longitudinal section.

 

A 1-loculed pericarp which dehisces along the front edge (that is, the inner edge, next the center of the flower) is a follicle. The fruit of the larkspur (Fig. 231) is a follicle. There are usually five of these fruits (sometimes three or four) in each larkspur flower, each pistil ripening into a follicle. If these pistils were united, a single compound pistil would be formed. Columbine, peony, ninebark, milkweed, also have follicles.

Fig. 231.—Follicle of Larkspur.

Fig. 233.—Capsule of Castor-oil Bean after Dehiscence.

Fig. 232.—A Bean Pod.

 

A 1-loculed pericarp that dehisces on both edges is a legume. Peas and beans are typical examples (Fig. 232); in fact, this character gives name to the pea family,—Leguminosæ. Often the valves of the legume twist forcibly and expel the seeds, throwing them some distance. The word “pod” is sometimes restricted to legumes, but it is better to use it generically for all dehiscent pericarps.

A compound pod—dehiscing pericarp of two or more carpels—is a capsule (Figs. 233, 234, 236, 237). Some capsules are of one locule, but they may have been compound when young (in the ovary stage) and the partitions may have vanished. Sometimes one or more of the carpels are uniformly crowded out by the exclusive growth of other carpels (Fig. 235). The seeds or parts which are crowded out are said to be aborted.

 

Fig. 234.—Capsule of Morning Glory.

Fig. 235.—Three-carpeled Fruit of Horse-chestnut. Two locules are closing by abortion of the ovules.

 

There are several ways in which capsules dehisce or open. When they break along the partitions (or septa), the mode is known as septicidal dehiscence (Fig. 236); In septicidal dehiscence the fruit separates into parts representing the original carpels. These carpels may still be entire, and they then dehisce individually, usually along the inner edge as if they were follicles. When the compartments split in the middle, between the partitions, the mode is loculicidal dehiscence (Fig. 237). In some cases the dehiscence is at the top, when it is said to be apical (although several modes of dehiscence are here included). When the whole top comes off, as in purslane and garden portulaca (Fig. 238), the pod is known as a pyxis. In some cases apical dehiscence is by means of a hole or clefts.

 

Fig. 236.—St. John’s Wort. Septicidal.

Fig. 237.—Loculicidal Pod of Day-lily.

 

The peculiar capsule of the mustard family, or Cruciferæ, is known as a silique when it is distinctly longer than broad (Fig. 224), and a silicle when its breadth nearly equals or exceeds its length. A cruciferous capsule is 2-carpeled, with a thin partition, each locule containing seeds in two rows. The two valves detach from below upwards. Cabbage, turnip, mustard, water-cress, radish, rape, shepherd’s purse, sweet alyssum, wallflower, honesty, are examples.

 

Fig. 238.—Pyxis of Portulaca or Rose-moss.

Fig. 239.—Berries of Gooseberry. Remains of calyx at c.

 

Fig. 240.—Berry of the Ground Cherry or Husk Tomato, contained in the inflated calyx.

Fig. 241.—Orange; example of a berry.

The pericarp may be fleshy and indehiscent. A pulpy pericarp with several or many seeds is a berry (Figs. 239, 240, 241). To the horticulturist a berry is a small, soft, edible fruit, without particular reference to its structure. The botanical and horticultural conceptions of a berry are, therefore, unlike. In the botanical sense, gooseberries, currants, grapes, tomatoes, potato-balls, and even eggplant fruits and oranges (Fig. 241) are berries; strawberries, raspberries, blackberries are not.

Fig. 242.—Plum; example of a drupe.

A fleshy pericarp containing one relatively large seed or stone is a drupe. Examples are plum (Fig. 242), peach, cherry, apricot, olive. The walls of the pit in the plum, peach, and cherry are formed from the inner coats of the ovary, and the flesh from the outer coats. Drupes are also known as stone-fruits.

Fig. 243.—Fruit of Raspberry.

Fig. 244.—Aggregate Fruit of Mulberry; and a separate fruit.

Fruits that are formed by the subsequent union of separate pistils are aggregate fruits. The carpels in aggregate fruits are usually more or less fleshy. In the raspberry and blackberry flower, the pistils are essentially distinct, but as the pistils ripen they cohere and form one body (Figs. 243, 244). Each of the carpels or pistils in the raspberry and blackberry is a little drupe, or drupelet. In the raspberry the entire fruit separates from the torus, leaving the torus on the plant. In the blackberry and dewberry the fruit adheres to the torus, and the two are removed together when the fruit is picked.

Accessory Fruits.—When the pericarp and some other part grow together, the fruit is said to be accessory or reënforced. An example is the strawberry (Fig. 245). The edible part is a greatly enlarged torus, and the pericarps are akenes embedded in it. These akenes are commonly called seeds.

Fig. 245.—Strawberry; fleshy torus in which akenes are embedded.

Various kinds of reënforced fruits have received special names. One of these is the hip, characteristic of roses. In this case, the torus is deep and hollow, like an urn, and the separate akenes are borne inside it. The mouth of the receptacle may close, and the walls sometimes become fleshy; the fruit may then be mistaken for a berry. The fruit of the pear, apple, and quince is known as a pome. In this case the five united carpels are completely buried in the hollow torus, and the torus makes most of the edible part of the ripe fruit, while the pistils are represented by the core (Fig. 246). Observe the sepals on the top of the torus (apex of the fruit) in Fig. 246. Note the outlines of the embedded pericarp in Fig. 247.

 

Fig. 246.—Section of an Apple.

Fig. 247.—Cross-section of an Apple.

 

Gymnospermous Fruits.—In pine, spruces, and their kin, there is no fruit in the sense in which the word is used in the preceding pages, because there is no ovary. The ovules are naked or uncovered, in the axils of the scales of the young cone, and they have neither style nor stigma. The pollen falls directly on the mouth of the ovule. The ovule ripens into a seed, which is usually winged. Because the ovule is not borne in a sac or ovary, these plants are called gymnosperms (Greek for “naked seeds”). All the true cone-bearing plants are of this class; also certain other plants, as red cedar, juniper, yew. The plants are monœcious or sometimes diœcious. The staminate flowers are mere naked stamens borne beneath scales, in small yellow catkins which soon fall. The pistillate flowers are naked ovules beneath scales on cones that persist (Fig. 29). Gymnospermous seeds may have several cotyledons.

Fig. 248.—Pecan Fruit.

CHAPTER XXII DISPERSAL OF SEEDS

It is to the plant’s advantage to have its seeds distributed as widely as possible. It has a better chance of surviving in the struggle for existence. It gets away from competition. Many seeds and fruits are of such character as to increase their chances of wide dispersal. The commonest means of dissemination may be classed under four heads: explosive fruits; transportation by wind; transportation by birds; burs.

 

Fig. 249.—Explosion of the Balsam Pod.

Fig. 250.—Explosive Fruits of Oxalis.

An exploding pod is shown at c. The dehiscence is shown at b. The structure of the pod is seen at a.

 

Explosive Fruits.—Some pods open with explosive force and discharge the seeds. Even bean and everlasting peas do this. More marked examples are the locust, witch hazel, garden balsam (Fig. 249), wild jewel-weed or impatiens (touch-me-not), violet, crane’s-bill or wild geranium, bull nettle, morning glory, and the oxalis (Fig. 250). The oxalis is common in several species in the wild and cultivation. One of them is known as wood sorrel. Fig. 250 shows the common yellow oxalis. The pod opens loculicidally. The elastic tissue suddenly contracts when dehiscence takes place, and the seeds are thrown violently. The squirting cucumber is easily grown in a garden (procure seeds of seedsmen), and the fruits discharge the seeds with great force, throwing them many feet.

Wind Travelers.—Wind-transported seeds are of two general kinds: those that are provided with wings, as the flat seeds of catalpa (Fig. 251) and cone-bearing trees and the samaras of ash, elm, tulip-tree, ailanthus, and maple; and those which have feathery buoys or parachutes to enable them to float in the air. Of the latter kind are the fruits of many composites, in which the pappus is copious and soft. Dandelion and thistle are examples. The silk of the milkweed and probably the hairs on the cotton seed have a similar office, and also the wool of the cat-tail. Recall the cottony seeds of the willow and poplar.

Fig. 251.—Winged Seeds of Catalpa.

Dispersal by Birds.—Seeds of berries and of other small fleshy fruits are carried far and wide by birds. The pulp is digested, but the seeds are not injured. Note how the cherries, raspberries, blackberries, June-berries, and others spring up in the fence rows, where the birds rest. Some berries and drupes persist far into winter, when they supply food to cedar birds, robins, and the winter birds. Red cedar is distributed by birds. Many of these pulpy fruits are agreeable as human food, and some of them have been greatly enlarged or “improved” by the arts of the cultivator. The seeds are usually indigestible.

Burs.—Many seeds and fruits bear spines, hooks, and hairs, which adhere to the coats of animals and to clothing. The burdock has an involucre with hooked scales, containing the fruits inside. The clotbur is also an involucre. Both are compositous plants, allied to thistles, but the whole head, rather than the separate fruits, is transported. In some compositous fruits the pappus takes the form of hooks and spines, as in the “Spanish bayonets” and “pitchforks.” Fruits of various kinds are known as “stick tights,” as of the agrimony and hound’s-tongue. Those who walk in the woods in late summer and fall are aware that plants have means of disseminating themselves (Fig. 252). If it is impossible to identify the burs which one finds on clothing, the seeds may be planted and specimens of the plant may then be grown.

Fig. 252.—Stealing a Ride.

Fig. 253.—The Fruits of the Cat-tail are loosened by Wind and Weather.

CHAPTER XXIII PHENOGAMS AND CRYPTOGAMS

The plants thus far studied produce flowers; and the flowers produce seeds by means of which the plant is propagated. There are other plants, however, that produce no seeds, and these plants (including bacteria) are probably more numerous than the seed-bearing plants. These plants propagate by means of spores, which are generative cells, usually simple, containing no embryo. These spores are very small, and sometimes are not visible to the naked eye.

 

Fig. 254.—Christmas Fern.—Dryopteris acrostichoides; known also as Aspidium.

Fig. 255.—Fruiting Frond of Christmas Fern.

Sori at a. One sorus with its indusium at b.

 

Prominent among the spore-propagated plants are ferns. The common Christmas fern (so called because it remains green during winter) is shown in Fig. 254. The plant has no trunk. The leaves spring directly from the ground. The leaves of ferns are called fronds. They vary in shape, as other leaves do. Some of the fronds in Fig. 254 are seen to be narrower at the top. If these are examined more closely (Fig. 255), it will be seen that the leaflets are contracted and are densely covered beneath with brown bodies. These bodies are collections of sporangia or spore-cases.

 

Fig. 256.—Common Polypode Fern. Polypodium vulgare.

Fig. 257.—Sori and Sporangium of Polypode. A chain of cells lies along the top of the sporangium, which springs back elastically on drying, thus disseminating the spores.

 

 

Fig. 258.—The Brake Fruits underneath the Revolute Edges of the Leaf.

Fig. 259.—Fruiting Pinnules of Maidenhair Fern.

 

The sporangia are collected into little groups, known as sori (singular, sorus) or fruit-dots. Each sorus is covered with a thin scale or shield, known as an indusium. This indusium separates from the frond at its edges, and the sporangia are exposed. Not all ferns have indusia. The polypode (Figs. 256, 257) does not; the sori are naked. In the brake (Fig. 258) and maidenhair (Fig. 259) the edge of the frond turns over and forms an indusium. The nephrolepis or sword fern of greenhouses is allied to the polypode. The sori are in a single row on either side the midrib (Fig. 260). The indusium is circular or kidney-shaped and open at one edge or finally all around. The Boston fern, Washington fern, Pierson fern, and others, are horticultural forms of the common sword fern. In some ferns (Fig. 261) an entire frond becomes contracted to cover the sporangia.

 

Fig. 260.—Part of Frond of Sword Fern. To the pupil: Is this illustration right side up?

Fig. 261.—Fertile and Sterile Fronds of the Sensitive Fern.

 

The sporangium or spore-case of a fern is a more or less globular body and usually with a stalk (Fig. 257). It contains the spores. When ripe it bursts and the spores are set free.

In a moist, warm place the spores germinate. They produce a small, flat, thin, green, more or less heart-shaped membrane (Fig. 262). This is the prothallus. Sometimes the prothallus is an inch or more across, but oftener it is less than a dime in size. Although easily seen, it is commonly unknown except to botanists. Prothalli may often be found in greenhouses where ferns are grown. Look on the moist stone or brick walls, or on the firm soil of undisturbed pots and beds; or spores may be sown in a damp, warm place.

Fig. 262.—Prothallus of a Fern. Enlarged.

Archegonia at a; antheridia at b.

On the under side of the prothallus two kinds of organs are borne. These are the archegonium (containing egg-cells) and the antheridium (containing sperm-cells). These organs are minute specialized parts of the prothallus. Their positions on a particular prothallus are shown at a and b in Fig. 262, but in some ferns they are on separate prothalli (plant diœcious). The sperm-cells escape from the antheridium and in the water that collects on the prothallus are carried to the archegonium, where fertilization of the egg takes place. From the fertilized egg-cell a plant grows, becoming a “fern.” In most cases the prothallus soon dies. The prothallus is the gametophyte (from Greek, signifying the fertilized plant).

The fern plant, arising from the fertilized egg in the archegonium, becomes a perennial plant, each year producing spores from its fronds (called the sporophyte); but these spores—which are merely detached special kinds of cells—produce the prothallic phase of the fern plant, from which new individuals arise. A fern is fertilized but once in its lifetime. The “fern” bears the spore, the spore gives rise to the prothallus, and the egg-cell of the prothallus (when fertilized) gives rise to the fern.

A similar alternation of generations runs all through the vegetable kingdom, although there are some groups of plants in which it is very obscure or apparently wanting. It is very marked in ferns and mosses. In algæ (including the seaweeds) the gametophyte is the “plant,” as the non-botanist knows it, and the sporophyte is inconspicuous. There is a general tendency, in the evolution of the vegetable kingdom, for the gametophyte to lose its relative importance and for the sporophyte to become larger and more highly developed. In the seed-bearing plants the sporophyte generation is the only one seen by the non-botanist. The gametophyte stage is of short duration and the parts are small; it is confined to the time of fertilization.

The sporophyte of seed plants, or the “plant” as we know it, produces two kinds of spores—one kind becoming pollen-grains and the other kind embryo-sacs. The pollen-spores are borne in sporangia, which are united into what are called anthers. The embryo-sac, which contains the egg-cell, is borne in a sporangium known as an ovule. A gametophytic stage is present in both pollen and embryo sac: fertilization takes place, and a sporophyte arises. Soon this sporophyte becomes dormant, and is then known as an embryo. The embryo is packed away within tight-fitting coats, and the entire body is the seed. When the conditions are right the seed grows, and the sporophyte grows into herb, bush, or tree. The utility of the alternation of generations is not understood.

The spores of ferns are borne on leaves; the spores of seed-bearing plants are also borne amongst a mass of specially developed conspicuous leaves known as flowers, therefore these plants have been known as the flowering plants. Some of the leaves are developed as envelopes (calyx, corolla), and others as spore-bearing parts, or sporophylls (stamens, pistils). But the spores of the lower plants, as of ferns and mosses, may also be borne in specially developed foliage, so that the line of demarcation between flowering plants and flowerless plants is not so definite as was once supposed. The one definite distinction between these two classes of plants is the fact that one class produces seeds and the other does not. The seed-plants are now often called spermaphytes, but there is no single coördinate term to set off those which do not bear seeds. It is quite as well, for popular purposes, to use the terms phenogams for the seed-bearing plants and cryptogams for the others. These terms have been objected to in recent years because their etymology does not express literal facts (phenogam signifying “showy flowers,” and cryptogam “hidden flowers”), but the terms represent distinct ideas in classification. The cryptogams include three great series of plants—the Thallophytes or algæ, lichens, and fungi; the Bryophytes or mosslike plants; the Pteridophytes or fernlike plants.

Fig. 263.—Diagram to explain the Terminology of the Frond.

CHAPTER XXIV STUDIES IN CRYPTOGAMS — Bacteria — Algæ — Fungi — Lichens — Liverworts — Mosses (Bryophyta) — Equisetums, or Horsetails (Pteridophyta) — Isoëtes (Pteridophyta) — Club-Mosses (Pteridophyta)

The pupil who has acquired skill in the use of the compound microscope may desire to make more extended excursions into the cryptogamous orders. The following plants have been chosen as examples in various groups. Ferns are sufficiently discussed in the preceding chapter.

Fig. 264.—Filament of Oscillatoria, showing one dead cell where the strand will break.

Fig. 265.—Strand of Spirogyra, showing the chlorophyll bands. There is a nucleus at a. How many cells, or parts of cells, are shown in this figure?

 

Fig. 266.—Conjugation of Spirogyra. Ripe zygospores on the left; a, connecting tubes.

Fig. 267.—Strand, or Filament of Zygnema, freed from its gelatinous covering.

 

Fig. 268.—Thread of Vaucheria with Oögonia and Antheridia.

 

Fig. 269.—Fucus. Fruiting branches at s, s. On the stem are two air-bladders.

Fig. 270.—Nitella.

 

 

Fig. 271.—Mucor mucedo, showing habit.

Fig. 272.—Spores of Mucor, some germinating.

 

 

Fig. 273.—Mucor, showing formation of zygospore on the right; germinating zygospore on the left.

Fig. 274.—Mucor.

a, sporangium; b, sporangium bursting; c, columella.

 

Fig. 275.—Yeast Plants.

Fig. 276.—Colonies of Willow Mildew.

 

Fig. 277.—Summer-spores of Willow Mildew.

Fig. 278.—Perithecium of Willow Mildew.

 

Fig. 279.—Section through Perithecium of Willow Mildew.

Fig. 280.—Sori containing Teleutospores of Wheat Rust.

Fig. 281.—Teleutospore of Wheat Rust.

 

Fig. 282.—Germinating Teleutospore of Wheat Rust.

Fig. 283.—Leaf of Barberry with Cluster-cups.

 

Fig. 284.—Section through a Cluster-cup on Barberry Leaf.

Fig. 285.—How a Parasitic Fungus works. Anthracnose on a bean pod entering the bean beneath. (Whetzel.)

Fig. 286.—Part of Gill of the Cultivated Mushroom.

tr, trama tissue; sh, hymenium; b, basidium; st, sterigma; sp, spore. (Atkinson.)

Lichens are so common everywhere that the attention of the student is sure to be drawn to them. They grow on rocks, trunks of trees (Fig. 287), old fences, and on the earth. They are thin, usually gray ragged objects, apparently lifeless. Their study is too difficult for beginners, but a few words of explanation may be useful.

Fig. 287.—Lichen on an Oak Trunk. (A species of Physcia.)

 

Fig. 288.

Fig. 289.

 

Plants of Marchantia.

Fig. 290.—Section of Thallus of Marchantia. Stomate at a.

Fig. 291.—Section through Antheridiophore of Marchantia, showing antheridia. One antheridium more magnified.

 

Fig. 292.—Archegonium of Marchantia.

Fig. 293.—Archegoniophore, with Sporogonia, of Marchantia.

 

Fig. 294.—Spores and Elaters of Marchantia.

Fig. 295.—Polytrichum commune. f,f, fertile plants, one on the left in fruit, m, antheridial plant.

Fig. 296.—Section of Leaf of Polytrichum commune.

Fig. 297.—Section through a Receptacle of Polytrichum commune, showing paraphyses and antheridia.

Fig. 298.—Funaria hygroscopica.

Fig. 299.—Equisetum arvense.

st, sterile shoot; f, fertile shoot showing the spike at a; b, sporophyll, with sporangia; s, spore.

Fig. 300.—Isoëtes, showing habit of plant at a; b, base of leaf, showing sporangium, velum, and ligule.

 

Fig. 301.—A Lycopodium with Sporangia in the Axils of the Foliage Leaves. (Lycopodium lucidulum).

Fig. 302.—A Club-moss (Lycopodium complanatum).

 

ANIMAL BIOLOGY — CHAPTER I THE PRINCIPLES OF BIOLOGY

Biology (Greek, bios, life; logos, discourse) means the science of life. It treats of animals and plants. That branch of biology which treats of animals is called zoology (Gr. zoon, animal; logos, discourse). The biological science of botany (Gr. botane, plant or herb) treats of plants.

Living things are distinguished from the not living by a series of processes, or changes (feeding, growth, development, multiplication, etc.), which together constitute what is called life. These processes are called functions. Both plants and animals have certain parts called organs which have each a definite work, or function; hence animals and plants are said to be organized. For example, men and most animals have a certain organ (the mouth) for taking in nourishment; another (the food tube), for its digestion.

Because of its organization, each animal or plant is said to be an organism. Living things constitute the organic kingdom. Things without life and not formed by life constitute the inorganic, or mineral, kingdom. Mark I for inorganic and O for organic after the proper words in this list: granite, sugar, lumber, gold, shellac, sand, coal, paper, glass, starch, copper, gelatine, cloth, air, potatoes, alcohol, oil, clay. Which of these things are used for food by animals? Conclusion?

Energy in the Organic World.—We see animals exerting energy; that is, we see them moving about and doing work. Plants are never seen acting that way; yet they need energy in order to form their tissues, grow, and raise themselves in the air.

Source of Plant Energy.—We notice that green plants thrive only in the light, while animal growth is largely independent of light. In fact, in the salt mines of Poland there are churches and villages below the ground, and children are born, become adults, and live all their lives below ground, without seeing the sun. (That these people are not very strong is doubtless due more to want of fresh air and other causes than want of sunlight.)

Fig. 1.—Surfaces of a Leaf, magnified.

Fig. 2.—A Leaf storing Energy in Sunlight.

The need of plants for sunlight shows that they must obtain something from the sun. This has been found to be energy. This enables them to lift their stems in growth, and form the various structures called tissues which make up their stems and leaves. (See Part I, Chap. XIII.) It is noticed that they take in food and water from the soil through their roots. Experiments also show that green plants take in through pores (Fig. 1), on the under side of their leaves, a gas composed of carbon and oxygen, and called carbon dioxid. The energy in the sunlight enables the plant to separate out the carbon of the carbon dioxid and build mineral and water and carbon into organic substances. The oxygen of the carbon dioxid is set free and returns to the air (Fig. 2). Starch, sugar, oil, and woody fiber are examples of substances thus formed. Can you think of any fuel not due to plants?

How Animals obtain Energy.—You have noticed that starch, oil, etc., will burn, or oxidize, that is, unite with the oxygen of the air; thus the sun’s energy, stored in these substances, is changed back to heat and motion. The oxidation of oil or sugar may occur in a furnace; it may also occur in the living substance of the active animal.

 

Fig. 3.—Colorless plants, as Mushrooms, give off no oxygen.

A green leaf, even after it is cut, gives off oxygen (O) if kept in the sun.

 

Fortunately for the animals the plants oxidize very little of the substances built up by them, since they do not move about nor need to keep themselves warm. We notice that animals are constantly using plant substances for food, and constantly drawing the air into their bodies. If the sunlight had not enabled the green plant to store up these substances and set free the oxygen (Fig. 3), animals would have no food to eat nor air to breathe; hence we may say that the sunlight is indirectly the source of the life and energy of animals. Mushrooms and other plants without green matter cannot set oxygen free (Fig. 3).

Experiment to show the Cause of Burning, or Oxidation.—Obtain a large glass bottle (a pickle jar), a short candle, and some matches. Light the candle and put it on a table near the edge, and cover it with the glass jar. The flame slowly smothers and goes out. Why is this? Is the air now in the jar different from that which was in it before the candle was lighted? Some change must have taken place or the candle would continue to burn. To try whether the candle will burn again under the jar without changing the air, slide the jar to the edge of the table and let the candle drop out. Light the candle and slip it up into the jar again, the jar being held with its mouth a little over the edge of the table to receive the candle (Fig. 5). The flame goes out at once. Evidently the air in the jar is not the same as the air outside. Take up the jar and wave it to and fro a few times, so as to remove the old air and admit fresh air. The candle now burns in it with as bright a flame as at first. So we conclude that the candle will not continue to burn unless there is a constant supply of fresh air. The gas formed by the burning is carbon dioxid. It is the gas from which plants extract carbon. (See Plant Biology, Chap. V.) One test for the presence of this gas is that it forms a white, chalky cloud in lime water; another is that it smothers a fire.

Experiment to show that Animals give off Carbon Dioxid.—Place a cardboard over the mouth of a bottle containing pure air. Take a long straw, the hollow stem of a weed, a glass tube, or a sheet of stiff paper rolled into a tube, and pass the tube into the bottle through a hole in the cardboard. Without drawing in a deep breath, send one long breath into the bottle through the tube, emptying the lungs by the breath as nearly as possible (Fig. 4). Next invert the bottle on the table as in the former experiment, afterward withdrawing the cardboard. Move the bottle to the edge of the table and pass the lighted candle up into it (Fig. 5). Does the flame go out as quickly as in the former experiment?

If you breathe through a tube into clear lime water, the water turns milky. The effect of the breath on the candle and on the lime water shows that carbon dioxid is continually leaving our bodies in the breath.

 

Fig. 4.—Breathing into a bottle.[1]

Fig. 5.—Testing the air in the bottle.[1]

 

Oxidation and Deoxidation.—The union of oxygen with carbon and other substances, which occurs in fires and in the bodies of animals, is called oxidation. The separation of the oxygen from carbon such as occurs in the leaves of plants is called deoxidation. The first process sets energy free, the other process stores it up. Animals give off carbon dioxid from their lungs or gills, and plants give off oxygen from their leaves. But plants need some energy in growing, so oxidation also occurs in plants, but to a far less extent than in animals. At night, because of the absence of sunlight, no deoxidation is taking place in the plant, but oxidation and growth continue; so at night the plant actually breathes out some carbon dioxid. The deepest part of the lungs contains the most carbon dioxid. Why was it necessary to empty the lungs as nearly as possible in the experiment with the candle? Why would first drawing a deep breath interfere with the experiment? Why does closing the draught of a stove, thus shutting off part of the air, lessen the burning? Why does a “firefly” shine brighter at each breath? Why is the pulse and breathing faster in a fever? Very slow in a trance?

The key for understanding any animal is to find how it gets food and oxygen, and how it uses the energy thus obtained to grow, move, avoid its enemies, and get more food. Because it moves, it needs senses to guide it.

The key for understanding a plant is to find how it gets food and sunlight for its growth. It makes little provision against enemies; its food is in reach, so it needs no senses to guide it. The plant is built on the plan of having the nutritive activities near the surface (e.g. absorption by roots; gas exchange in leaves). The animal is built on the plan of having its nutritive activities on the inside (e.g. digestion; breathing).

Cell and Protoplasm.—Both plants and animals are composed of small parts called cells. Cells are usually microscopic in size. They have various shapes, as spherical, flat, cylindrical, fiber-like, star-shaped. The living substance of cells is called protoplasm. It is a stiff, gluey fluid, albuminous in its nature. Every cell has a denser spot or kernel called a nucleus, and in the nucleus is a still smaller speck called a nucleolus. Most cells are denser and tougher on the outside, and are said to have a cell wall, but many cells are naked, or without a wall. Hence the indispensable part of a cell is not the wall but the nucleus, and a cell may be defined as a bit of protoplasm containing a nucleus. This definition includes naked cells as well as cells with walls.

One-celled Animals.—There are countless millions of animals and plants the existence of which was not suspected until the invention of the microscope several centuries ago. They are one-celled, and hence microscopic in size. It is believed that the large animals and plants are descended from one-celled animals and plants. In fact, each individual plant or animal begins life as a single cell, called an egg cell, and forms its organs by the subdivision of the egg cell into many cells. An egg cell is shown in Fig. 6, and the first stages in the development of an egg cell are shown in Fig. 7.

Fig. 6.—Egg cell of mammal with yolk.

Fig. 7.—Egg cell subdivides into many cells forming a sphere (morula) containing a liquid. A dimple forms and deepens to form the next stage (gastrula).

The animals to be studied in the first chapter are one-celled animals. To understand them we must learn how they eat, breathe, feel, and move. They are called Protozoans (Greek protos, first; zoon, life). All other animals are composed of many cells and are called Metazoans (Greek meta, beyond or after). The cells composing the mucous membrane in man are shown in Fig. 8. The cellular structure of the leaf of a many-celled plant is illustrated in Fig. 1. (See also Chap. I, Human Biology.)

Fig. 8.—Mucous Membrane formed of one layer of cells. A few cells secrete mucus.

Method of Classifying Animals.—The various animals display differences more or less marked. The question arises, are not some of them more closely related than others? We conclude that they are, since the difference between some animals is very slight, while the difference between others is quite marked.

To show the different steps in classifying an animal, we will take an example,—the cow. Even little children learn to recognize a cow, although individual cows differ somewhat in form, size, color, etc. The varieties of cows, such as short-horn, Jersey, etc., all form one species of animals, having the scientific name taurus. Let us include in a larger group the animals closest akin to a cow. We see a cat, a bison, and a dog; rejecting the cat and the dog, we see that the bison has horns, hoofs, and other similarities. We include it with the cow in a genus called Bos, calling the cow Bos taurus, and the bison, Bos bison. The sacred cow of India (Bos indicus) is so like the cow and buffalo as also to belong in the genus Bos. Why is not the camel, which, like Bos bison, has a hump, placed in the genus Bos? (Fig. 390.)

The Old World buffaloes,—most abundant in Africa and India,—the antelopes, sheep, goats, and several other genera are placed with the genus Bos in a family called the hollow horns.

This family, because of its even number of toes and the habit of chewing the cud, resembles the camel family, the deer family, and several other families. These are all placed together in the next higher systematic unit called an order, in this case, the order of ruminants.

The ruminants, because they are covered with hair and nourish the young with milk, are in every essential respect related to the one-toed horses, the beasts of prey, the apes, etc. Hence they are all placed in a more inclusive division of animals, the class called mammals.

All mammals have the skeleton, or support of the body, on the inside, the axis of which is called the vertebral column. This feature also belongs to the classes of reptiles, amphibians, and fishes. It is therefore consistent to unite these classes by a general idea or conception into a great branch of animals called the vertebrates.

Returning from the general to the particular by successive steps, state the branch, class, order, family, genus, and species to which the cow belongs.

The Eight Branches or Sub-kingdoms.—The simplest classification divides the whole animal kingdom into eight branches, named and characterized as follows, beginning with the lowest: I. Protozoans. One-celled. II. Sponges. Many openings. III. Polyps. Circular; cup-like; having only one opening which is both mouth and vent. IV. Echinoderms. Circular; rough-skinned; two openings. V. Mollusks. No skeleton; usually with external shell. VI. Vermes. Elongate body, no jointed legs. VII. Arthropods. External jointed skeleton; jointed legs. VIII. Vertebrates. Internal jointed skeleton with axis or backbone.

CHAPTER II PROTOZOA (One-celled Animals) — The ameba — The Slipper Animalcule or Paramecium

Fig. 9.—Ameba proteus, much enlarged.

Fig. 10.—Ameba.

cv, contractile vacuole; ec, ectoplasm; en, endoplasm; n, nucleus; ps, pseudopod; ps′, pseudopod forming; ectoplasm protrudes and endoplasm flows into it.

Form and Structure.—The ameba (also spelled amœba) looks so much like a clear drop of jelly that a beginner cannot be certain that he has found one until it moves. It is a speck of protoplasm (Fig. 9), with a clear outer layer, the ectoplasm; and a granular, internal part, the endoplasm. Is there a distinct line between them? (Fig. 10.)

Note the central portion and the slender prolongations or pseudopods (Greek, false feet). Does the endoplasm extend into the pseudopods? (Fig. 10.) Are the pseudopods arranged with any regularity?

Sometimes it is possible to see a denser appearing portion, called the nucleus; also a clear space, the contractile vacuole (Fig. 10).

Movements.—Sometimes while the pseudopods are being extended and contracted, the central portion remains in the same place (this is motion). Usually only one pseudopod is extended, and the body flows into it; this is locomotion (Fig. 11). There is a new foot made for each step.

Fig. 11.—The same ameba seen at different times.

Feeding.—If the ameba crawls near a food particle, the pseudopod is pressed against it, or a depression occurs (Fig. 12), and the particle is soon embedded in the endoplasm. Often a clear space called a food vacuole is noticed around the food particle. This is the water that is taken in with the particle (Fig. 12). The water and the particle are soon absorbed and assimilated by the endoplasm.

Fig. 12.—The Ameba taking food.

Excretion.—If a particle of sand or other indigestible matter is taken in, it is left behind as the ameba moves on. There is a clear space called the contractile vacuole, which slowly contracts and disappears, then reappears and expands (Figs. 9 and 10). This possibly aids in excreting oxidized or useless material.

Circulation in the ameba consists of the movement of its protoplasmic particles. It lacks special organs of circulation.

Feeling.—Jarring the glass slide seems to be felt, for it causes the activity of the ameba to vary. It does not take in for food every particle that it touches. This may be the beginning of taste, based upon mere chemical affinity. The pseudopods aid in feeling.

Reproduction.—Sometimes an ameba is seen dividing into two parts. A narrowing takes place in the middle; the nucleus also divides, a part going to each portion (Fig. 13). The mother ameba finally divides into two daughter amebas. Sex is wanting.

Fig. 13.—Ameba, dividing.

Source of the Ameba’s Energy.—We thus see that the ameba moves without feet, eats without a mouth, digests without a stomach, feels without nerves, and, it should also be stated, breathes without lungs, for oxygen is absorbed from the water by its whole surface. Its movements require energy; this, as in all animals, is furnished by the uniting of oxygen with the food. Carbon dioxid and other waste products are formed by the union; these pass off at the surface of the ameba and taint the water with impurities.

Cysts.—If the water inhabited by a protozoan dries up, it encysts, that is, it forms a tough skin called a cyst. Upon return of better conditions it breaks the cyst and comes out. Encysted protozoans may be blown through the air: this explains their appearance in vessels of water containing suitable food but previously free from protozoans.

Shape and Structure.—The paramecium’s whole body, like the ameba’s, is only one cell. It resembles a slipper in shape, but the pointed end is the hind end, the front end being rounded (Fig. 14). The paramecium is propelled by the rapid beating of numerous fine, threadlike appendages on its surface, called cilia (Latin, eyelashes) (Figs.). The cilia, like the pseudopods of the ameba, are merely prolongations of the cell protoplasm, but they are permanent. The separation between the outer ectoplasm and the interior granular endoplasm is more marked than in the ameba (Fig. 14).

 

Fig. 14.—Paramecium, showing cilia, c.

Two contractile vacuoles, cv; the macronucleus, mg; two micronuclei, mi; the gullet (Œ), a food ball forming and ten food balls in their course from gullet to vent, a.

Fig. 15.

 

Nucleus and Vacuoles.—There is a large nucleus called the macronucleus, and beside it a smaller one called the micronucleus. They are hard to see. About one third of the way from each end is a clear, pulsating space (bb. Fig. 15) called the pulsating vacuole. These spaces contract until they disappear, and then reappear, gradually expanding. Tubes lead from the vacuoles which probably serve to keep the contents of the cell in circulation.

Fig. 16.—Two Paramecia exchanging parts of their nuclei.

Feeding.—A depression, or groove, is seen on one side, this serves as a mouth (Figs.). A tube which serves as a gullet leads from the mouth-groove to the interior of the cell. The mouth-groove is lined with cilia which sweep food particles inward. The particles accumulate in a mass at the inner end of the gullet, become separated from it as a food ball (Fig. 14), and sink into the soft protoplasm of the body. The food balls follow a circular course through the endoplasm, keeping near the ectoplasm.

Reproduction.—This, as in the ameba, is by division, the constriction being in the middle, and part of the nucleus going to each half. Sometimes two individuals come together with their mouth-grooves touching and exchange parts of their nuclei (Fig. 16). They then separate and each divides to form two new individuals.

We thus see that the paramecium, though of only one cell, is a much more complex and advanced animal than the ameba. The tiny paddles, or cilia, the mouth-groove, etc., have their special duties similar to the specialized organs of the many-celled animals to be studied later.

 

Fig. 17.—Vorticella (or bell animalcule), two extended, one withdrawn.

Fig. 18.—Euglena.

 

Fig. 19.—Shell of a Radiolarian.

If time and circumstances allow a prolonged study, several additional facts may be observed by the pupil, e.g. Does the paramecium swim with the same end always foremost, and same side uppermost? Can it move backwards? Avoid obstacles? Change shape in a narrow passage? Does refuse matter leave the body at any particular place? Trace movement of the food particles.

Draw the paramecium.

Which has more permanent parts, the ameba or paramecium? Name two anatomical similarities and three differences; four functional similarities and three differences.

The ameba belongs in the class of protozoans called Rhizopoda “root footed.”

Other classes of Protozoans are the Infusorians, which have many waving cilia (Fig. 17) or one whip-like flagellum (Fig. 18), and the Foraminifers which possess a calcareous shell pierced with holes. Much chalky limestone has been formed of their shells. These and the radiolarians, which have flinty shells (Fig. 19), are often placed in the class rhizopoda. To which class does the paramecium belong?

Protozoans furnish a large amount of food to the higher animals.

CHAPTER III SPONGES

Fig. 21.—Fresh-water Sponge.

Fig. 22.—Section of fresh-water sponge (enlarged).

The small fresh-water sponge (Fig. 21) lacks the more or less vase-like form typical of sponges. It is a rounded mass growing upon a rock or log. As indicated by the arrows, where does water enter the sponge? This may be tested by putting coloring matter in the water near the living sponge. Where does the water come out? (Fig. 22.) Does it pass through ciliated chambers in its course? Is the surface of the sponge rough or smooth? Do any of the skeletal spicules show on the surface? (Fig. 21.) Does the sponge thin out near its edge?

Fig. 23.—Eggs and spicules of fresh-water sponge (enlarged).

The egg of this sponge is shown in Fig. 23. It escapes from the parent sponge through the osculum, or large outlet. As in most sponges, the first stage after the egg is ciliated and free-swimming.

Marine Sponges.—The grantia (Fig. 24) is one of the simplest of marine sponges. What is the shape of grantia? What is its length and diameter? How does the free end differ from the fixed end? Are the spicules projecting from its body few or many?

Fig. 24.—Grantia.

Where is the osculum, or large outlet? With what is this surrounded? The osculum opens from a central cavity called the cloaca. The canals from the pores lead to the cloaca.

Buds are sometimes seen growing out from the sponge near its base. These are young sponges formed asexually. Later they become detached from the parent sponge.

Fig. 25.—Plan of a sponge.

Commercial “Sponge.”—What part of the complete animal remains in the bath sponge? Slow growing sponges grow more at the top and form tall, simple, tubular or vase-like animals. Fast growing sponges grow on all sides at once and form a complicated system of canals, pores, and oscula. Which of these habits of growth do you think belonged to the bath sponge? Is there a large hole in the base of your specimen? If so, this is because the cloaca was reached in trimming the lower part where it was attached to a rock. Test the elasticity of the sponge when dry and when wet by squeezing it. Is it softer when wet or dry? Is it more elastic when wet or dry? How many oscula does your specimen have? How many inhalent pores to a square inch? Using a probe (a wire with knob at end, or small hat pin), try to trace the canals from the pores to the cavities inside.

Do the fibers of the sponge appear to interlace, or join, according to any system? Do you see any fringe-like growths on the surface which show that new tubes are beginning to form? Was the sponge growing faster at the top, on the sides, or near the bottom?

Burn a bit of the sponge; from the odor, what would you judge of its composition? Is the inner cavity more conspicuous in a simple sponge or in a compound sponge like the bath sponge? Is the bath sponge branched or lobed? Compare a number of specimens (Figs. 26, 27, 28) and decide whether the common sponge has a typical shape. What features do their forms possess in common?

Fig. 26.—Bath Sponge.

 

Fig. 27.—Bath Sponge.

Fig. 28.—Bath Sponge.

 

Sponges are divided into three classes, according as their skeletons are flinty (silicious), limy (calcareous), or horny.

Some of the silicious sponges have skeletons that resemble spun glass in their delicacy. Flint is chemically nearly the same as glass. The skeleton shown in Fig. 29 is that of a glass sponge which lives near the Philippine Islands.

Fig. 29.—Skeleton of a glass sponge.

The horny sponges do not have spicules in their skeletons, as the flinty and limy sponges have, but the skeleton is composed of interweaving fibers of spongin, a durable substance of the same chemical nature as silk (Figs. 30 and 31).

 

Fig. 30.—A horny sponge.

Fig. 31.—Section of horny sponge.

 

The limy sponges have skeletons made of numerous spicules of lime. The three-rayed spicule is the commonest form.

The commercial sponge, seen as it grows in the ocean, appears as a roundish mass with a smooth, dark exterior, and having about the consistency of beef liver. Several large openings (oscula), from which the water flows, are visible on the upper surface. Smaller holes (inhalent pores—many of them so small as to be indistinguishable) are on the sides. If the sponge is disturbed, the smaller holes, and perhaps the larger ones, will close.

The outer layer of cells serves as a sort of skin. Since so much of the sponge is in contact with water, most of the cells do their own breathing, or absorption of oxygen and giving off of carbon dioxid. Nutriment is passed on from the surface cells to nourish the rest of the body.

Reproduction.—Egg-cells and sperm-cells are produced by certain cells along the canals. The egg-cell, after it is fertilized by the sperm-cell, begins to divide and form new cells, some of which possess cilia. The embryo sponge passes out at an osculum. By the vibration of the cilia, it swims about for a while. It afterwards settles down with the one end attached to the ocean floor and remains fixed for the rest of its life. The other end develops oscula. Some of the cilia continue to vibrate and create currents which bring food and oxygen.

The cilia in many species are found only in cavities called ciliated chambers. (Figs. 22, 32.) There are no distinct organs in the sponge and there is very little specialization of cells. The ciliated cells and the reproductive cells are the only specialized cells. The sponges were for a long time considered as colonies of separate one-celled animals classed as protozoans. They are, without doubt, many-celled animals. If a living sponge is cut into pieces, each piece will grow and form a complete sponge.

That the sponge is not a colony of one-celled animals, each like an ameba, but is a many-celled animal, will be realized by examining Fig. 32, which shows a bit of sponge highly magnified. A sponge may be conceived as having developed from a one-celled animal as follows: Several one-celled animals happened to live side by side; each possessed a thread-like flagellum (E, Fig. 32) or whip-lash for striking the water. By lashing the water, they caused a stronger current (Fig. 25) than protozoans living singly could cause. Thus they obtained more food and multiplied more rapidly than those living alone. The habit of working together left its impress on the cells and was transmitted by inheritance.

Fig. 32.—Microscopic plan of ciliated chamber. Each cell lining the chamber has a nucleus, a whip-lash, and a collar around base of whip-lash. Question: State two uses of whip-lash.

Cell joined to cell formed a ring; ring joined to ring formed a tube which was still more effective than a ring in lashing the water into a current and bringing fresh food (particles of dead plants and animals) and oxygen.

No animals eat sponges; possibly because spicules, or fibers, are found throughout the flesh, or because the taste and odor is unpleasant enough to protect them. Small animals sometimes crawl into them to hide. One species grows upon shells inhabited by hermit crabs. Moving of the shell from place to place is an advantage to them, while they conceal the crab and thus protect it.

Special Report: Sponge “Fisheries.” (Localities; how sponges are taken, cleaned, dried, shipped, and sold.)

CHAPTER IV POLYPS (CUPLIKE ANIMALS) — The Hydra, or Fresh Water Polyp

Fig. 33.—A Hydra.

Fig. 34.—Forms assumed by Hydra.

Fig. 35.—Hydra (much enlarged).

The conical projection bearing the mouth is called hypostome (Fig. 34). The mouth opens into the digestive cavity. Is this the same as the general body cavity, or does the stomach have a wall distinct from the body cavity? How far down does the body cavity extend? Does it extend up into the tentacles? (Fig. 39.)

Locomotion.—The round surface, or disk, by which the hydra is attached, is called its foot. Can you move on one foot without hopping? The hydra moves by alternately elongating and rounding the foot. Can you discover other ways by which it moves? Does the hydra always stand upon its foot?

Fig. 36.—Nettling Cell.

II. discharged, and I. not discharged.

Lasso Cells.—Upon the tentacles (Fig. 35) are numerous cells provided each with a thread-like process (Fig. 36) which lies coiled within the cell, but which may be thrown out upon a water flea, or other minute animal that comes in reach. The touch of the lasso paralyzes the prey (Fig. 37). These cells are variously called lasso cells, nettling cells, or thread cells. The thread is hollow and is pushed out by the pressure of liquid within. When the pressure is withdrawn the thread goes back as the finger of a glove may be turned back into the glove by turning the finger outside in. When a minute animal, or other particle of food comes in contact with a tentacle, how does the tentacle get the food to the mouth? By bending and bringing the end to the mouth, or by shortening and changing its form, or in both ways? (Fig. 34, C.) Do the neighboring tentacles seem to bend over to assist a tentacle in securing prey? (Fig. 34, C.)

Fig. 37.—Hydra capturing a water flea.

Digestion.—The food particles break up before remaining long in the stomach, and the nutritive part is absorbed by the lining cells, or endoderm (Fig. 39). The indigestible remnants go out through the mouth. The hydra is not provided with a special vent. Why could the vent not be situated at the end opposite the mouth?

Circulation and Respiration.—Does water have free access to the body cavity? Does the hydra have few or nearly all of its cells exposed to the water in which it lives? From its structure, decide whether it can breathe like a sponge or whether special respiratory cells are necessary to supply it with oxygen and give off carbon dioxid. Blood vessels are unnecessary for transferring oxygen and food from cell to cell.

Fig. 38.—Hydras on pondweed.

Reproduction.—Do you see any swellings upon the side of the hydra? (Fig. 34, A.) If the swelling is near the tentacles, it is a spermary; if near the base it is an ovary. A sperm coalesces with or fertilizes the ovum after the ovum is exposed by the breaking of the ovary wall. Sometimes the sperm from one hydra unites with the ovum of another hydra. This is called cross-fertilization. The same term is applied to the process in plants when the male element, or pollen, of the flower unites with the ovules, or female element, of the flower on another plant. The hydra, like most plants and some other animals, is hermaphrodite, that is to say, both sperms and ova are produced by one individual. In the autumn, eggs are produced with hard shells to withstand the cold until spring. Sexual reproduction takes place when food is scarce. Asexual generation (by budding) is common with the hydra when food supply is abundant. After the bud grows to a certain size, the outer layer of cells at the base of the bud constricts and the young hydra is detached.

Compare the sponge and the hydra in the following respects:—many celled, or one celled; obtaining food; breathing; tubes and cavities; openings; reproduction; locomotion. Which ranks higher among the metazoa? The metazoa, or many celled animals, include all animals except which branch?

Fig. 39.—Longitudinal section of hydra (microscopic and diagrammatic).

Habits.—The hydra’s whole body is a hollow bag, the cavity extending even into the tentacles. The tentacles may increase in number as the hydra grows but seldom exceed eight. The hydra has more active motion than locomotion. It seldom moves from its place, but its tentacles are constantly bending, straightening, contracting, and expanding. The body is also usually in motion, bending from one side to another. When the tentacles approach the mouth with captured prey, the mouth (invisible without a hand lens) opens widely, showing five lobes or lips, and the booty is soon tucked within. A hydra can swallow an animal larger in diameter than itself.

The endoderm cells have ameboid motion, that is, they extend pseudopods. They also resemble amebas in the power of intra-cellular digestion; that is, they absorb the harder particles of food and digest them afterwards, rejecting the indigestible portions. Some of these cells have flagella (see Fig. 39) which keep the fluid of the cavity in constant motion.

Sometimes the hydra moves after the manner of a small caterpillar called a “measuring worm,” that is, it takes hold first by the foot, then by the tentacles, looping its body at each step. Sometimes the body goes end over end in slow somersaults.

The length of the extended hydra may reach one half inch. When touched, both tentacles and body contract until it looks to the unaided eye like a round speck of jelly. This shows sensibility, and a few small star-shaped cells are believed to be nerve cells, but the hydra has not a nervous system. Hydras show their liking for light by moving to the side of the vessel or aquarium whence the light comes.

Fig. 40.—Hydroid Colony, with nutritive (P) reproductive (M) and defensive (S) hydranths.

The Branch Polyps (sometimes called Cœlenterata).—The hydra is the only fresh water representative of this great branch of the animal kingdom. This branch is characterized by its members having only one opening to the body. The polyps also include the salt water animals called hydroids, jellyfishes, and coral polyps.

Fig. 41.—“Portuguese Man-o’-War” (compare with Fig. 40). A floating hydroid colony with long, stinging (and sensory) streamers. Troublesome to bathers in Gulf of Mexico. Notice balloon-like float.

Hydroids.—Figure 40 shows a hydroid, or group of hydra-like growths, one of which eats and digests for the group, another defends by nettling cells, another produces eggs. Each hydra-like part of a hydroid is called a hydranth. Sometimes the buds on the hydra remain attached so long that a bud forms upon the first bud. Thus three generations are represented in one organism. Such growths show us that it is not always easy to tell what constitutes an individual animal.

a	b	g	e	h
c	d		f

Fig. 42.—The formation of many free swimming jellyfishes from one fixed hydra-like form. The saucer-like parts (h) turn over after they separate and become like Fig. 43 or 44. Letters show sequence of diagrams.

Hydroids may be conceived to have been developed by the failure of budding hydras to separate from the parent, and by the gradual formation of the habit of living together and assisting each other. When each hydranth of the hydroid devoted itself to a special function of digestion, defense, or reproduction, this group lived longer and prospered; more eggs were formed, and the habits of the group were transmitted to a more numerous progeny than were the habits of a group where members worked more independently of each other.

As the sponge is the first, lowest, and simplest example of the devotion of special cells to special purposes, the hydroid is the first, lowest, and simplest example of the occurrence of organs, that is of special parts of the body (groups of cells) set aside for a special work.

How many mature hydranths are seen in the hydroid shown in Fig. 40? Why are the defensive hydranths on the outside of the colony? Which hydranths have no tentacles? Why not?

Jellyfish.—Alternation of Generations.—Medusa.—With some species of hydroids, a very curious thing happens.—The hydranth that is to produce the eggs falls off and becomes independent of the colony. More surprising still, its appearance changes entirely and instead of being hydra-like, it becomes the large and complex creature called jellyfish (Fig. 43). But the egg of the jellyfish produces a small hydra-like animal which gives rise by budding to a hydroid, and the cycle is complete.

Fig. 43.—A Jellyfish.

Fig. 44.—A Jellyfish (medusa).

The bud (or reproductive hydranth) of the hydroid does not produce a hydroid, but a jellyfish; the egg of the jellyfish does not produce a jellyfish, but a hydroid. This is called by zoologists, alternation of generations. A complete individual is the life from the germination of one egg to the production of another. So that an “individual” consists of a hydroid colony fixed in one place together with all the jellyfish produced from its buds, and which may now be floating miles away from it in the ocean. Bathers in the surf are sometimes touched and stung by the long, streamer-like tentacles of the jellyfish. These, like the tentacles of the hydra, have nettling cells (Fig. 41).

The umbrella-shaped free swimming jellyfish is called a medusa (Fig. 44).

Coral Polyps.—Some of the salt water relatives of the hydra produce buds which remain attached to the parent without, however, becoming different from the parent in any way. The coral polyps and corallines are examples of colonies of this kind, possessing a common stalk which is formed as the process of multiplication goes on. In the case of coral polyps, the separate animals and the flesh connecting them secrete within themselves a hard, limy, supporting structure known as coral. In some species, the coral, or stony part, is so developed that the polyp seems to be inserted in the coral, into which it withdraws itself for partial protection (Fig. 45).

Fig. 45.—Coral Polyps (tentacles, a multiple of six). Notice hypostome.

The corallines secrete a smooth stalk which affords no protection, but they also secrete a coating or sheath which incloses both themselves and the stalk. The coating has apertures through which the polyps protrude in order to feed when no danger is near (Fig. 46). The red “corals” used for jewelry are bits of stalks of corallines. The corallines (Figs. 47, 48) are not so abundant nor so important as the coral polyps (Figs. 45, 49).

Fig. 46.—Red Coralline with crust and polyps (eight tentacles).

Fig. 47.—Sea Fan (a coralline).

Colonies of coral polyps grow in countless numbers in the tropical seas. The coral formed by successive colonies of polyps accumulates and builds up many islands and important additions to continents. The Florida “keys,” or islands, and the southern part of the mainland of Florida were so formed.

Fig. 48.—Organ Pipe “Coral” (a coralline).

The Sea Anemone, like the coral polyp, lives in the sea, but like the fresh water hydra, it deposits no limy support for its body. The anemone is much larger than the hydra and most coral polyps, many species attaining a height of several inches. It does not form colonies. When its arms are drawn in, it looks like a large knob of shiny but opaque jelly. Polyps used to be called zoophytes (plant-animals), because of their flower-like appearance (Figs. 50, 51).

Fig. 49.—Upright Cut through coral polyp × 4.

ms, mouth; mr, gullet; ls, ls, fleshy partitions (mesenteries) extending from outer body wall to gullet (to increase absorbing surface); s, s, shorter partitions; mb, fb, stony support (of lime, called coral); t, tentacles.

Fig. 50.—Sea Anemone.

Fig. 51.—Sea Anemones.

CHAPTER V ECHINODERMS (SPINY ANIMALS) — The Starfish — The Sea Urchin — Other Echinoderms

Fig. 52.—Starfish on a rocky shore.

Fig. 53.—Plan of starfish; III, madreporite.

External Features.—Starfish are usually brown or yellow. Why? (See Fig. 52.) Has it a head or tail? Right and left sides? What is the shape of the disk, or part which bears the five arms or rays? (Fig. 53.) Does the body as a whole have symmetry on two sides of a line (bilateral symmetry), or around a point (radial symmetry)? Do the separate rays have bilateral symmetry? The skeleton consists of limy plates embedded in the tough skin (Fig. 54). Is the skin rough or smooth? Hard or soft? Are the projections (or spines) in the skin long or short? The skin is hardened by the limy plates, except around the mouth, which is at the center of the lower side and surrounded by a membrane. Which is rougher, the mouth side, (oral side) or the opposite (aboral side)? Which side is more nearly flat? The vent is at or near the center of the disk on the aboral surface. It is usually very small and sometimes absent. Why a vent is not of much use will be understood after learning how the starfish takes food.

 

Fig. 54.—Limy Plates in portion of a ray.

Fig. 55.—Starfish (showing Madreporite).

 

An organ peculiar to animals of this branch, and called the madreporic plate, or madreporite, is found on the aboral surface between the bases of two rays (Fig. 55). It is wartlike, and usually white or red. This plate is a sieve; the small openings keep out sand but allow water to filter through.

Fig. 56.—Water tube System of starfish.

m, madreporite; stc, stone canal; ap, ampulla.

Movements: the Water-tube System.—The water, which is filtered through the perforated madreporite, is needed to supply a system of canals (Fig. 56). The madreporite opens into a canal called the stone canal, the wall of which is hardened by the same kind of material as that found in the skin. The stone canal leads to the ring canal which surrounds the mouth (Fig. 56). The ring canal sends radial canals into each ray to supply the double row of tube feet found in the groove at the lower side of each ray (Fig. 57). Because of their arrangement in rows, the feet are also called ambulacral feet (Latin ambulacra, “forest walks”). There is a water holder (ampulla), or muscular water bulb at the base of each tube foot (Fig. 58). These contract and force the water into the tube feet and extend them. The cuplike ends of the tubes cling to the ground by suction. The feet contain delicate muscles by which they contract and shorten. Thus the animal pulls itself slowly along, hundreds of feet acting together. The tube feet, for their own protection, may contract and retire into the groove, the water which extended them being sent back into the ampulla. This system of water vessels (or water-vascular system) of the echinodermata is characteristic of them; i.e. is not found elsewhere in the animal kingdom. The grooves and the plates on each side of them occupy the ambulacral areas. The rows of spines on each side of the grooves are freely movable. (What advantage?) The spines on the aboral surface are not freely movable.

Fig. 57.—Starfish, from below; tube feet extended.

Fig. 58.—Section of one ray and central portion of starfish.

f1, f2, f3, tube feet more or less extended; au, eye spot; k, gills; da, stomach; m, madreporite; st, stone canal; p, ampulla; ei, ovary.

Respiration.—The system of water vessels serves the additional purpose of bringing water containing oxygen into contact with various parts of the body, and the starfish was formerly thought to have no special respiratory organs. However there are holes in the aboral wall through which the folds of the delicate lining membrane protrude. These are now supposed to be gills (k, Fig. 58).

The nervous system is so close to the aboral surface that much of it is visible without dissection. Its chief parts are a nerve ring around the mouth, which sends off a branch along each ray. These branches may be seen by separating the rows of tube feet. They end in a pigmented cell at the end of each ray called the eye-spot.

The food of starfish consists of such animals as crabs, snails, and oysters. When the prey is too large to be taken into the mouth, the starfish turns its stomach inside out over the prey (Fig. 59). After the shells separate, the stomach is applied to the soft digestible parts. After the animal is eaten, the stomach is retracted. This odd way of eating is very economical to its digestive powers, for only that part of the food which can be digested and absorbed is taken into the body. Only the lower part of the stomach is wide and extensible. The upper portion (next to the aboral surface) is not so wide. This portion receives the secretion from five pairs of digestive glands, a pair of which is situated in each ray. Jaws and teeth are absent. (Why?) The vent is sometimes wanting. Why?

Fig. 59.—Starfish eating a sea snail.

b, stomach everted.

Reproduction.—There is a pair of ovaries at the base of each ray of the female starfish (Fig. 58). The spermaries of the male have the same position and form as the ovaries, but they are lighter colored, usually white.[2]

Regeneration after Mutilation.—If a starfish loses one or more rays, they are replaced by growth. Only a very ignorant oyster-man, angry at the depredations of starfish upon his oyster beds, would chop starfish to pieces, as this only serves to multiply them. This power simulates multiplication by division in the simplest animals.

Steps in Advance of Lower Branches.—The starfish and other echinodermata have a more developed nervous system, sensory organs, and digestion, than forms previously studied; most distinctive of all, they have a body cavity distinct from the food cavity. The digestive glands, reproductive glands, and the fluid which serves imperfectly for blood, are in the body cavity. There is no heart or blood vessels. The motions of the stomach and the bending of the rays give motion to this fluid in the body cavity. It cannot be called blood, but it contains white blood corpuscles.

Fig. 60.—Young starfish crawling upon their mother. (Challenger Reports.)

The starfish when first hatched is an actively swimming bilateral animal, but it soon becomes starlike (Fig. 60). The limy plates of the starfish belong neither to the outer nor inner layer (endoderm and ectoderm) of the body wall, but to a third or middle layer (mesoderm); for echinoderms, like the polyps, belong to the three-layered animals. In this its skeleton differs from the shell of a crawfish, which is formed by the hardening of the skin itself.

Protective Coloration.—Starfish are brown or yellow. This makes them inconspicuous on the brown rocks or yellow sands of the seashore. This is an example of protective coloration.

External Features.—What is the shape of the body? What kind of symmetry has it? Do you find the oral (or mouth) surface? The aboral surface? Where is the body flattened? What is the shape of the spines? What is their use? How are the tube feet arranged? Where do the rows begin and end? Would you think a sea urchin placed upside down in water, could right itself less or more readily than a starfish? What advantage in turning would each have that the other would not have? The name sea urchin has no reference to a mischievous boy, but means sea hedgehog (French oursin, hedgehog), the name being suggested by its spines.

Comparison of Starfish and Sea Urchin.—The water system of the sea urchin, consisting of madreporite, tubes, and water bulbs, or ampullæ, is similar to that of the starfish. The tube feet and locomotion are alike. There is no need for well-developed respiratory organs in either animal, as the whole body, inside and out, is bathed in water. The method of reproduction is the same.

Fig. 61.—A Sea Urchin crawling up the glass front wall of an aquarium (showing mouth spines and tube feet).

 

Fig. 62.—A Sea Urchin with spines removed, the limy plates showing the knobs on which the spines grew.

Fig. 63.—Section of Sea Urchin with soft parts removed, showing the jaws which bear the teeth protruding in Fig. 62.

 

The starfish eats animal food. The food of the sea urchin is almost exclusively vegetable, hence it needs teeth (Fig. 62, 63); its food tube is longer than that of a starfish, just as the food tube of a sheep, whose food digests slowly, is much longer than that of a dog.

Fig. 64.—The Sea Otter, an urchin with mouth (o) and vent (A) on same side of body.

The largest species of sea urchins are almost as big as a child’s head, but this size is unusual. The spines are mounted on knobs, and the joint resembles a ball-and-socket joint, and allow as wide range of movement. Some sea urchins live on sandy shores, other species live upon the rocks. The sand dollars are lighter colored. (Why?) They are usually flatter and have lighter, thinner walls, for there is danger of sinking into the sand. The five-holed sand cake or sand dollar has its weight still further diminished by the holes, which also allow it to rise more easily through the water. The flattened lower surface of both starfish and sea urchin causes the body to remain still while the tube feet are stretching forward for another step.

The sea cucumbers, or holothurians, resemble the sea urchin in many respects, but their bodies are elongated, and the limy plates are absent or very minute. The mouth is surrounded by tentacles (Fig. 65).

Fig. 65.—Sea Cucumbers.

Fig. 66.—A Brittle Star.

The brittle stars resemble the starfish in form, but their rays are very slender, more distinct from the disk, and the tube feet are on the edges of the rays, not under them (Fig. 66).

The crinoids are the most ancient of the echinoderms. (Figs. 67, 68.) Their fossils are very abundant in the rocks. They inhabited the geological seas, and it is believed that the other echinoderms descended from them. A few now inhabit the deep seas. Some species are fixed by stems when young, and later break away and become free-swimming, others remain fixed throughout life.

The four classes of the branch echinoderms are Starfish (asteroids), Sea urchins (echinoids), Sea cucumbers (holothurians), and Sea lilies (crinoids).

 

Fig. 67.—Crinoid, arms closed.

Fig. 68.—Disk of Crinoid from above, showing mouth in center and vent near it, at right (arms removed).

 

Comparative Review

Make a table like this as large as the page of the notebook will allow, and fill in without guessing.

CHAPTER VI WORMS

External Features.—Is the body bilateral? Is there a dorsal and ventral surface? Can you show this by a test with a live worm? Do you know of an animal with dorsal and ventral surface, but not bilateral?

Fig. 69.—An Earthworm.

Can you make out a head? A head end? A neck? Touch the head and test whether it can be made to crawl backwards. Which end is more tapering? Is the mouth at the tip of the head end or on the upper or lower surface? How is the vent situated? Its shape? As the worm lies on a horizontal surface, is the body anywhere flattened? Are there any very distinct divisions in the body? Do you see any eyes?

Is red blood visible through the skin? Can you notice any pulsations in a vessel along the back? Do all earthworms have the same number of divisions or rings? Compare the size of the rings or segments. Can it crawl faster on glass or on paper?

 

Fig. 70.—Mouth and Setæ.

Fig. 71.—Earthworm, mouth end above.

 

A magnifying glass will show on most species tiny bristle-like projections called setæ. How are the setæ arranged? (d, Fig. 70.) How many on one ring of the worm? How do they point? Does the worm feel smoother when it is pulled forward or backward between the fingers? Why? Are setæ on the lower surface? Upper surface? The sides? What is the use of the setæ? Are they useful below ground? Does the worm move at a uniform rate? What change in form occurs as the front part of the body is pushed forward? As the hinder part is pulled onward? How far does it go at each movement? At certain seasons a broad band, or ring, appears, covering several segments and making them seem enlarged (Fig. 71). This is the clitellum, or reproductive girdle. Is this girdle nearer the mouth or the tail?

Draw the exterior of an earthworm.

Dorsal and Ventral Surfaces.—The earthworm always crawls with the same surface to the ground; this is called the ventral surface, the opposite surface is the dorsal surface. This is the first animal studied to which these terms are applicable. What are the ventral and dorsal surfaces of a fish, a frog, a bird, a horse, a man?

 

Fig. 72.—Food Tube of earthworm. (Top view.)

Fig. 73.—Food Tube and Blood Vessels of earthworm showing the ring-like hearts. (Side view.)

 

The Food Tube.—The earthworm has no teeth, and the food tube, as might be inferred from the form of the body, is simple and straight. On account of slight variation in size and structure, its parts are named the pharynx (muscular), gullet, crop, gizzard (muscular), and the long intestine extending through the last three fourths of its body (Fig. 72). The functions of the parts of the food tube are indicated by their names.

Fig. 74.

Circulation.—There is a large dorsal blood vessel above the food tube (Fig. 73). From the front portion of this tube arise several large tubular rings or “hearts” which are contractile and serve to keep the blood circulating. They lead to a ventral vessel below the food tube (Fig. 74). The blood is red, but the coloring matter is in the liquid, not in the blood cells.

Nervous System.—Between the ventral blood vessels is a nerve cord composed of two strands (see Fig. 75). There is a slight swelling, or ganglion, on each strand, in each segment (Fig. 75). The strands separate near the front end of the worm, and a branch goes up each side of the gullet and enters the two pear-shaped cerebral ganglia, or “brain” (Fig. 75).

Fig. 75.—Ganglia Near Mouth and part of nerve chain of earthworm.

Food.—The earthworm eats earth containing organic matter, the inorganic part passing through the vent in the form of circular casts found in the morning at the top of the earthworm’s hole. What else does it eat?

The earth worm needs no teeth, as it excretes through the mouth an alkaline fluid which softens and partly digests the food before it is eaten. When this fluid is poured out upon a green leaf, the leaf at once turns brown. The starch in the leaf is also acted upon. The snout aids in pushing the food into the mouth.

Kidneys.—Since oxidation is occurring in its tissues, and impurities are forming, there must be some way of removing impurities from the tissues. The earthworm does not possess one-pair organs like the kidneys of higher animals to serve this purpose, but it has numerous pairs of small tubular organs called nephridia which serve the purpose. Each one is simply a tube with several coils (Fig. 76). There is a pair on the floor of each segment (Fig. 76). Each nephridium has an inner open end within the body cavity, and its outer end opens by a pore on the surface between the setæ (Fig. 78). The nephridia absorb waste water from the liquid in the celom, or body cavity surrounding the food tube, and convey it to the outside.

Fig. 76.—Two pairs of Nephridia.

Respiration.—The skin of the earthworm is moist, and the blood capillaries approach so near to the surface of the body that the oxygen is constantly passing in from the air, and carbon dioxid passing out; hence it is constantly breathing through all parts of its skin. It needs no lungs nor special respiratory organs of any kind.

Fig. 77.—Sperm (sp) and egg glands (es) of earthworm.

Reproduction.—When one individual animal produces both sperm cells and egg cells, it is said to be hermaphrodite. This is true of the earthworm. The egg cell is always fertilized, however, not by the sperm cells of the same worm, but by sperm cells formed by another worm. The openings of these ova or egg glands consist of two pairs of small pores found on the ventral surface of the fourteenth and fifteenth segments in most species (see Fig. 77). There are also two pairs of small receptacles for temporarily holding the foreign sperm cells. One pair of the openings from these receptacles is found (with difficulty) in the wrinkle behind the ninth segment (Fig. 77), and the other pair behind the tenth segment. The sperm glands are in front of the ovaries (Fig. 77), but the sperm ducts are longer than the oviducts, and open behind them (Figs. 77, 78). The worms exchange sperm cells, but not egg cells. The reproductive girdle, or clitellum, already spoken of, forms the case which is to hold the eggs (see Fig. 71). When the sperm cells have been exchanged, and the ova are ready for fertilization, the worm draws itself backward from the collarlike case or clitellum so that it slips over the head. As it passes the fifteenth and sixteenth segments, it collects the ova, and as it passes the ninth and tenth segments, it collects the sperm cells previously received by touching another worm. The elastic, collar-like clitellum closes at the ends after it has slipped over the worm’s head, forming a capsule. The ova are fertilized in this capsule, and some of them hatch into worms in a few days. These devour the eggs which do not hatch. The eggs develop into complete but very small worms before the worms escape from the capsule.

Fig. 78.—Side view showing setæ, nephridia pores, and reproductive openings.

Habits.—The earthworm is omnivorous. It will eat bits of meat as well as leaves and other vegetation. It has also the advantage, when digging its hole, of eating the earth which must be excavated. Every one has noticed the fresh “casts” piled up at the holes in the morning. As the holes are partly filled by rains, the casts are most abundant after rains. The chief enemies of the earthworm are moles and birds. The worms work at night and retire so early in the morning that it takes a very early bird to catch a worm. Perhaps the nearest to an intelligent act the earthworm accomplishes is to conceal the mouth of its hole by plugging it with a pebble or bit of leaf. They hibernate, going below danger of frost in winter. In dry weather they burrow several feet deep.

The muscular coat beneath, and much thicker than the skin, consists of two layers: an outer layer runs around the body just beneath the skin, and an inner, thicker layer of fibers runs lengthwise. The worm crawls by shortening the longitudinal muscles. As the bristles (setæ) point backward, they prevent the front part of the body from slipping back, so the hinder part is drawn forward. Next, the circular muscles contract, and the bristles preventing the hind part from slipping back, the fore portion is pushed forward. Is the worm thicker when the hinder part is being pulled up or when the fore part is being thrust forward? Does the earthworm pull or push itself along, or does it do both? Occasionally it travels backward, e.g. it sometimes goes backward into its hole. Then the bristles are directed forward.

The right and left halves of the body are counterparts of each other, hence the earthworm is bilaterally symmetrical. The lungs and gills of animals must always be kept moist. The worm cannot live long in dry air, for respiration in the skin ceases when it cannot be kept moist, and the worm smothers. Long immersion in water is injurious to them, perhaps because there is far less oxygen in water than in the air.

Darwin wrote a book called “Vegetable Mold and Earthworms.” He estimated that there were fifty thousand earthworms to the acre on farm land in England, and that they bring up eighteen tons of soil in an acre each year. As the acids of the food tube act upon the mineral grains that pass through it, the earthworm renders great aid in forming soil. By burrowing it makes the soil more porous and brings up the subsoil.

Although without eyes, the worm is sensitive to light falling upon its anterior segments. When the light of a lantern suddenly strikes it at night, it crawls quickly to its burrow. Its sense of touch is so keen that it can detect a light puff of breath. Which of the foods kept in a box of damp earth disappeared first? What is indicated as to a sense of taste?

Why is the bilateral type of structure better adapted for development and higher organization than the radiate type of the starfish? The earthworm’s body is a double tube; the hydra’s body is a single tube; which plan is more advantageous, and why? Would any other color do just as well for an earthworm? Why, or why not?

The sandworm (Nereis) lives in the sand of the seashore, and swims in the sea at night (Fig. 79). It is more advanced in structure than the earthworm, as it has a distinct head (Fig. 80), eyes, two teeth, two lips, and several pairs of antennæ, and two rows of muscular projections which serve as feet. It is much used by fishermen for bait. If more easily obtained, it may be studied instead of the earthworm.

Fig. 79.—Sand Worm × ²⁄₃ (Nereis).

There are four classes in the branch Vermes: 1) the earthworms, including sandworms and leeches; 2) the roundworms, including trichina, hairworms, and vinegar eels; 3) flatworms, including tapeworm and liver fluke; 4) rotifers, which are mere specks in size.

Fig. 80.—Head of Sandworm (enlarged).

The tapeworm is a flatworm which has lost most of its organs on account of its parasitic life. Its egg is picked up by an herbivorous animal when grazing. The embryo undergoes only partial development in the body of the herbivorous animal, e.g. an ox. The next stage will not develop until the beef is eaten by a carnivorous animal, to whose food canal it attaches itself and soon develops a long chain of segments called a “tape.” Each segment absorbs fluid food through its body wall. As the segments at the older end mature, each becomes full of germs, and the segments become detached and pass out of the canal, to be dropped and perhaps picked up by an herbivorous animal and repeat the life cycle.

The trichina is more dangerous to human life than the tapeworm. It gets into the food canal in uncooked pork (bologna sausage, for example), multiplies there, migrates into the muscles, causing great pain, and encysts there, remaining until the death of the host. It is believed to get into the bodies of hogs again when they eat rats, which in turn have obtained the cysts from carcasses.

Summary of the Biological Process.—An earthworm is a living machine which does work (digging and crawling; seizing, swallowing, and digesting food; pumping blood; growing and reproducing). To do the work it must have a continual supply of energy. The energy for its work is set free by the protoplasm (in its microscopic cells) undergoing a destructive chemical change (oxidation). The waste products from the breaking down of the protoplasm must be continually removed (excretion). The broken-down protoplasm must be continually replaced if life is to continue (the income must exceed the outgo if the animal is still growing). The microscopic cells construct more protoplasm out of food and oxygen (assimilation) supplied them by the processes of nutrition (eating, digesting, breathing, circulating). This protoplasm in turn oxidizes and releases more energy to do work, and thus the cycle of life proceeds.

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CHAPTER VII CRUSTACEANS — Crawfish

Habitat.—Do you often see crawfish, or crayfish, moving about, even in water where they are known to be abundant? What does your answer suggest as to the time when they are probably most active?

Why do you never see one building its chimney, even where crawfish holes are abundant? Is the chimney always of the same color as the surface soil? Are the crawfish holes only of use for protection? In what kind of spots are crawfish holes always dug? Why? What becomes of crawfish when the pond or creek dries up? How deep are the holes? How large are the lumps of mud of which the chimney is built? How does it get them out of the hole? Why is the mud built into a chimney instead of thrown away? (What would happen to a well with its mouth no higher than the ground?) Why are crawfish scarce in rocky regions, as New England?

How does the color of the crawfish compare with its surroundings? Is its color suited to life in clear or muddy water? Define protective coloration.

Habits.—Does the crawfish walk better in water or out of it? Why? Does it use the legs with the large claws to assist in walking? Do the swimmerets (under the abdomen) move fast or slow? (Observe it from below in a large jar of clear water.) What propels it backward? Forward? Does the crawfish move at a more uniform rate when swimming backward or forward? Why? In which way can it swim more rapidly? Do the big legs with claws offer more resistance to the water while it is swimming backward or forward? How does it hold the tail after the stroke, while it is darting backward through the water? Hold a crawfish with its tail submerged and its head up. Can the tail strike the water with much force? Allow it to grasp a pencil: can it sustain its own weight by its grip?

Feeding.—Offer several kinds of food to a crawfish that has not been alarmed or teased. Does it prefer bread, meat, or vegetables? How does it get the food to its mouth? Does it eat rapidly or slowly? Does it tear the food with the big pincers? Can it gnaw with the small appendages near the mouth?

Breathing.—Does the crawfish breathe with gills or lungs? Place a few drops of ink near the base of the hind legs of a crawfish resting quietly in shallow water. Where is the ink drawn in? Where does it come out? To explain the cause and purpose of this motion, place a crawfish in a large glass jar containing water, and see the vibratory motion of the parts under the front portion of the body. There is a gill paddle, or gill bailer, under the shell on each side of the body that moves at the same rate.

Senses.—Crawfish are best caught with a piece of meat or beef’s liver tied to a string. Do they always lose hold as soon as they are lifted above the water? What do you conclude as to the alertness of their senses? Does the covering of its body suggest the possession of a delicate or dull sense of touch?

Of what motions are the eyes capable? Touch one of the eyes. The result? Can a crawfish see in all directions? To test this, place a crawfish on a table and try whether you can move to a place where you can see the crawfish without seeing its eyes. What are the advantages and disadvantages of having the eyes on stalks?

 

Fig. 81.—Crawfish (dorsal surface).

Fig. 82.

 

Touch the body and the several appendages of the crawfish. Where does it seem most sensitive to touch? Which can reach farther, the antennæ or the big claws? Why are short feelers needed as well as long ones?

Make a loud and sudden noise without jarring the crawfish. Is it affected by sound?

External Anatomy (Figs. 81, 82, 83, 84).—Is the body of the crawfish rounded out (convex) everywhere, or is any part of its surface either flat or rounded in (concave)? What color has the crawfish? Is this color of any use to the crawfish?

Fig. 83.—Lateral view of Crawfish.

Fig. 84.—Fourth Abdominal Segment of Crawfish with swimmeret.

Make out the two distinct regions or divisions of the body (Fig. 81). The anterior (front) region is called the head-chest or cephalothorax, and the posterior (rear) region is called the tail. Which region is larger? Why? Which is flexible? Why?

Is the covering of the body hard or soft? What is the advantage of such a covering? What are its disadvantages? How is the covering modified at the joints to permit motion?

Tail.—How many joints, or segments, on the tail? (Figs. 81, 83.) Does the hard covering of each segment slip under or over the segment behind it when the abdomen is straight? Does this lessen friction while swimming forward?

Is there a pair of swimmerets to each segment of the abdomen? (Figs. 82, 86.) Notice that each swimmeret has a main stalk (protopod), an outer branch (exopod), and an inner branch (endopod) (Fig. 84). Are the stalk and the branches each in one piece or jointed? The middle part of the tail fin is called the telson. By finding the position of the vent, decide whether the food tube goes into the telson (Fig. 82). Should it be called an abdominal segment? Are the side pieces of the tail fin attached to the telson or to the sixth segment? Do these side pieces correspond to swimmerets? Do they likewise have the Y-shaped structure? (Fig. 86.)

If the swimmerets on the first abdominal segment are large, the specimen is a male. If they are small, it is a female. Which sex is shown in Fig. 82? Fig. 86?

Carapace.—The covering of the head chest (cephalothorax) is called the carapace. Has it free edges? The gills are on the sides of the body and are covered by the carapace (Fig. 87). The projection in front is called the rostrum, meaning beak. Does the rostrum project beyond the eyes? There is a transverse groove across the carapace which may be said to divide the head from the abdomen. Where does this groove end at the sides?

Fig. 85.—1, mandible; 2, 3, maxillæ; 4, 5, 6, maxillipeds.

Legs.—How many legs has the crawfish? How many are provided with large claws? Small claws? Is the outer claw hinged in each of the large grasping pincers? The inner claw?

Fig. 86.—Crawfish (ventral surface).

Appendages for Taking Food.—If possible to watch a living crawfish eating, notice whether it places the food directly into the mouth with the large claws. Bend the large claws under and see if they will reach the mouth.

Attached just in front of the legs the crawfish has three pairs of finger-like appendages, called foot jaws (maxillipeds), with which it passes the food from the large pincers to its mouth (Figs. 85, 86). They are in form and use more like fingers than feet. In front of the foot jaws are two pairs of thin jaws (maxillæ) and in front of the thin jaws are a pair of stout jaws (mandibles) (Fig. 85). Do the jaws move sidewise or up and down? Which of the jaws has a jointed finger (palp) attached to it? Do all of the appendages for taking food have both exopod and endopod branches on a basal stalk or protopod? Which of the appendages have a scalloped edge? How would you know from looking at the crawfish that it is not merely a scavenger? Why are there no pincers on the hind feet?

Fig. 87.—Gill cover removed and gills exposed. Mp, gill bailer.

Sense Organs.—Find the antennæ, or long feelers (Figs. 82, 90). Are the antennæ attached above or below the eyes? (Fig. 87.)

Fig. 88.—Lengthwise Section of Male Crawfish.

c, heart; Ac, artery to head; Aa, artery to abdomen; Km, stomach; D, intestine; L, liver; T, spermary; Go, opening of sperm duct; G, brain; N, nerve chain.

Find the pair of antennules, or small feelers. Are their divisions like or unlike each other? Compare the length of the antennules and the antennæ. Compare the flexibility of the antennæ with that of the other appendages.

Observe the position of the eyes (Figs. 81, 88). How long are the eyestalks? Is the stalk flexible or stiff? Touch the eye. Where is the joint which enables the stalk to move? Is the outer covering of the eye hard or soft? A mounted preparation of the transparent covering (cornea) of the eye, seen with lower power of microscope, reveals that the cornea is made up of many divisions, called facets. Each facet is the front of a very small eye, hundreds of which make up the whole eye, which is therefore called a compound eye. The elongated openings to the ear sacs are located each on the upper side of the base of a small feeler just below the eye.

Respiratory System.—The respiratory organs are gills located on each side of the thorax in a space between the carapace and body (Fig. 87). The gills are white, curved, and feathery. Is the front gill the largest or the smallest? The gills overlap each other; which is the outermost gill? On the second maxilla is a thin, doubly curved plate called a gill bailer (Fig. 85). The second maxilla is so placed that the gill bailer comes at the front end of the gill chamber. The bailer paddles continually, bringing the water forward out of the gill. The gills are attached below at the base of the legs. Are the gills thick or thin? How far upward do they go? Does the backward motion in swimming aid or hinder the passage of the water through the gills? Does a crawfish, when at rest on the bottom of a stream, have its head up or down stream? Why?

Openings.—The slitlike vent is on the under side of the telson (Figs. 82, 88). The mouth is on the under side of the thorax behind the mandibles. At the base of the long antennæ are the openings from the green glands, two glands in the head which serve as kidneys (Fig. 89). The openings of the reproductive organs are on the third pair of legs in the female, and the fifth pair of legs in the male (Fig. 88). The eggs are carried on the swimmerets.

Fig. 89.—Level lengthwise section showing

h, heart.
d, green gland.
le, liver.
kie, gills.
kh, gill cavity.
ma, stomach.

(After Huxley.)

Fig. 90.—Section of Crawfish showing stomach s, liver li, and vent a.

 

Fig. 91.—Showing heart and main blood vessels.

Fig. 92.

 

Crustacea.—Because of the limy crust which covers the crawfish and its kindred, they are placed in the class called Crustacea.

 

Fig. 93.—Crab from below.

Fig. 94.—Hermit Crab, using shell of sea snail for a house.

 

Decapods.—All crustacea which have ten feet belong in the order called decap′oda (ten-footed). This order includes the crabs, lobsters, shrimp, etc. The crabs and lobsters are of considerable importance because of use as food. Small boys sometimes catch crawfish, and in some instances are known to cook and eat them for amusement, the only part cooked being the muscular tail. The crab’s tail is small and flat and held under the body (Fig. 93).

Fig. 95.—Development of a Crab.

a, nauplius just after hatching; b, c, d, zoëa; e, megalops; f, adult.

Question: Which stage is most like a crayfish? Compare with metamorphoses of insects.

Since the limy covering to serve the purpose of protection is not soft enough to be alive and growing, it is evident that the crustacea are hampered in their growth by their crusty covering. During the first year the crawfish sheds its covering, or molts three times, and once each year thereafter. It grows very fast for a few days just after molting, while the covering is soft and extensible. Since it is at the mercy of birds, fish, and other enemies while in this soft and defenseless condition, it stays hidden until the covering hardens. Hence it cannot eat much, but probably by the absorption of water the tissues grow; that is, enlarge. In the intervening periods, when growth is impossible, it develops; that is, the tissues and organs change in structure and become stronger. “Soft-shelled crab” is a popular dish, but there is no species by that name, this being only a crab just after molting which has been found by fishermen in spite of its hiding.

KEY TO THE FOUR CLASSES IN BRANCH ARTHROPODS

Fig. 96.—Pill Bug.	Fig. 97.—Lady Beetle.	Fig. 98.—Scorpion.
	Fig. 101.—One Segment of Centipede with one pair of legs.		Fig. 99.—Tick before and after feeding.
	Fig. 102.—One Segment of Thousand Legs with two pairs of legs.		Fig. 103.—Thousand Legs.
Fig. 100.—Centipede.	Fig. 104.—A Spider.		Fig. 105.—King Crab.

Illustrated Study. Classification of Arthropods. Key on p. 61.

CHAPTER VIII INSECTS — The Grasshopper — Anatomy and General Characteristics of the Class Insecta

What is the general shape of its body? (Fig. 106.) Where is the body thickest? Is it bilaterally symmetrical, that is, are the two sides of the body alike? Is the skeleton, or hard part of the body, internal or external? Is the skeleton as stiff and thick as that of a crawfish? What is the length of your specimen? Its color? Why does it have this coloration? In what ways does the grasshopper resemble the crawfish? Differ from it?

Fig. 106.—A Grasshopper.

The Three Regions of the Body.—The body of the grasshopper is divided into three regions,—the head, thorax, and abdomen. Which of these three divisions has no distinct subdivisions? The body of the grasshopper, like that of the earthworm, is made of ringlike segments. Are the segments most distinct in the head, thorax, or abdomen? Which region is longest? Shortest? Strongest? Why? Which region bears the chief sense organs? The appendages for taking food? The locomotory appendages? Which division of the body is most active in breathing?

The Abdomen.—About how many segments or rings in the abdomen? Do all grasshoppers have the same number of rings? (Answer for different species and different individuals of the same species.) The first segment and the last two are incomplete rings. Does the flexibility of the abdomen reside in the rings, or the joints between the rings? Is there merely a thin, soft line between the rings, or is there a fold of the covering? Does one ring slip into the ring before it or behind it when the abdomen is bent?

As the grasshopper breathes, does each ring enlarge and diminish in size? Each ring is divided into two parts by folds. Does the upper half-ring overlap the lower half-ring, or the reverse? With magnifying glass, find a small slit, called a spiracle, or breathing hole, on each side of each ring just above the side groove (Fig. 106). A tube leads from each spiracle. While the air is being taken in, do the two portions of the rings move farther apart? When they are brought together again, what must be the effect? In pumping the air, the abdomen may be said to work like a bellows. Bellows usually have folds to allow motion. Is the comparison correct?

How many times in a minute does the grasshopper take in air? If it is made to hop vigorously around the room and the breathing is again timed, is there any change?

Fig. 107.—A Grasshopper Dissected.

Find the ears on the front wall of the first abdominal ring (Fig. 107). They may be seen by slightly pressing the abdomen so as to widen the chink between it and the thorax. The ears are merely glistening, transparent membranes, oval in form. A nerve leads from the inner surface of each membrane. State any advantage or disadvantage in having the ears located where they are.

Ovipositor.—If the specimen is a female, it has an egg-placer or ovipositor, consisting of four blunt projections at the end of the abdomen (Fig. 107). If it is a male, there are only two appendages. These are above the end of the abdomen, and smaller than the parts of the ovipositor. Females are larger and more abundant than males. In laying the eggs, the four blunt points are brought tightly together and then forced into the ground and opened (Fig. 108). By repeating this, a pit is made almost as deep as the abdomen is long. What sex is shown in Fig. 106? Fig. 107?

Draw a side view of the grasshopper.

Fig. 108.—Grasshopper Laying Eggs. (Riley.)

Thorax.—This, the middle portion of the body, consists of three segments or rings (Fig. 107). Is the division between the rings most apparent above or below? Which two of the three rings are more closely united?

The front ring (prothorax) of the thorax has no rings. Is it larger above or below? Does it look more like a collar or a cape? (Fig. 106.) A spiracle is found on the second ring (mesothorax, or middle thorax) just above the second pair of legs. There is another in the soft skin between the prothorax and mesothorax just under the large cape or collar. The last ring of the thorax is called the metathorax (rear thorax).

How many legs are attached to each ring of the thorax? Can a grasshopper walk? Run? Climb? Jump? Fly? Do any of the legs set forward? (See Fig. 106.) Outward? Backward? Can you give reasons for the position of each pair? (Suggestion: What is the use of each pair?) If an organ is modified so that it is suited to serve some particular purpose or function, it is said to be specialized. Are any of the legs specialized so that they serve for a different purpose than the other legs?

The leg of a grasshopper (as of all insects) is said to have five parts, all the small parts after the first four parts being counted as one part and called the foot. Are all the legs similar, that is, do the short and long joints in all come in the same order? Numbered in order from the body, which joint of the leg is the largest,—the first, second, third, or fourth? Which joint is the shortest? The slenderest? Which joint has a number of sharp points or spines on it? Find by experiment whether these spines are of use in walking (Fig. 106). Jumping? Climbing? In what order are the legs used in walking? How many legs support the body at each step?

Fig. 109.—How a Grasshopper Walks.

Fig. 110.—How a Spider Walks.

 

All animals that have ears have ways of communicating by sounds. Why would it be impossible for the grasshopper to have a voice, even if it had vocal cords in its throat? The male grasshoppers of many species make a chirping, or stridulation, by rubbing the wing against the leg. Look on the inner side (why not outer side?) of the largest joint of the hind leg for a row of small spines visible with the aid of a hand lens (Fig. 111). The sound is produced by the outer wings rubbing against the spines. Have you noticed whether the sound is produced while the insect is still or in motion? Why? The male grasshoppers of some species, instead of having spines, rub the under side of the front wing on the upper side of the hind wing.

Fig. 111.—A, Row of Spines, z, used in chirping.

B, the same more enlarged.

Wings.—To what is the first pair of wings attached? The second pair? Why are the wings not attached to the prothorax? Why are the wings attached so near the dorsal line of the body? Why are the second and third rings of the thorax more solidly joined than the first and second rings?

Compare the first and second pairs of wings in shape, size, color, thickness, and use (Fig. 112). How are the second wings folded so as to go under the first wings? About how many folds in each?

Fig. 112.—Grasshopper in Flight.

Draw a hind wing opened out.

Head.—What is the shape of the head viewed from the front, the side, and above? Make sketches. What can you say of a neck? Is the head movable in all directions?

What is the position of the large eyes? Like the eyes of the crawfish, they are compound, with many facets. But the grasshopper has also three simple eyes, situated one in the middle of the forehead and one just above each antenna. They are too small to be seen without a hand lens. How does the grasshopper’s range of vision compare with that of the crawfish?

Are the antennæ flexible? What is their shape? Position? Are they segmented? Touch an antenna, a wing, a leg, and the abdomen in succession. Which seems to be the most sensitive to touch? The antennæ are for feeling; in some species of insects they are also the organs of hearing.

Fig. 113.

The mouth parts of a grasshopper are highly specialized. They should be compared with the mouth parts of a beetle shown in Fig. 113, since the mouth parts of these two insects correspond closely. If the grasshopper is fed with a blade of fresh grass, the function of each mouth part may be plainly seen. It is almost impossible to understand these functions by studying a dead specimen, but a fresh specimen is much better than a dry one.

Fig. 114.—a, Food Tube of Beetle.

b, gizzard; d, intestine; c, biliary vessels. See Fig. 127.

The upper lip, or labrum, is seen in front. Is it tapering or expanded? In what direction is it movable? The dark pointed biting jaws (mandibles) are next. Are they curved or straight? Sharp or blunt pointed? Notched or smooth? Do they work up and down, or sideways? The holding jaws (maxillæ), each with two jaw fingers (maxillary palpi) are behind the chewing jaws. Why? The lower lip (labium) has a pair of lip fingers (labial palpi) upon it. The brown tongue, usually bathed in saliva, is seen in the lower part of the mouth. Since the grasshopper has no lips, or any way of producing suction, it must lap the dew in drinking. Does it merely break off bits of a grass blade, or does it chew?

The heart, circulation, nervous system, digestive and respiratory organs of the grasshopper agree mainly with the general description of the organs of insects given in the next section.

Microscopic Objects.—These may be bought ready mounted, or may be examined fresh. A portion of the covering of the large eye may be cut off and the dark layer on the inside of the covering scraped off to make it transparent. What is the shape of the facets? Can you make any estimate of their number? A portion of the transparent hind wing may be used, and the “veins” in it studied. A thin bit of an abdominal segment containing a spiracle will show the structure of these important organs.

Fig. 115.—Egg and Molts of a Grasshopper.

Growth of the Grasshopper.—Some species hibernate in sheltered places and lay eggs in the spring, but adult species are scarce at that season. Most species lay the eggs in the fall; these withstand the cold and hatch out in the spring. Those hatched from one set of eggs sometimes stay together for a few days. They eat voraciously, and as they grow, the soft skin becomes hardened by the deposit of horny substance called chitin. This prevents further growth until the insect molts, the skin first splitting above the prothorax. After hatching, there are five successive periods of growth. At which molt do the very short wings first appear? (Fig. 115.) After the last molt the animal is complete, and changes no more in size for the rest of its life. There has been an attempt among writers to restrict the term grasshopper to the long-winged, slender species, and to call the shorter winged, stouter species locusts according to old English usage.

Fig. 116.—Cockroach.

 

Fig. 117.—Praying Mantis, or devil’s horse.

Fig. 118.—Cricket.

 

Fig. 119.—Mole Cricket.

Economic Importance of Grasshoppers.—Great injury is often done to vegetation by grasshoppers; however, the millions of tiny but ravenous eaters hatched in early spring are usually soon thinned out by the birds. The migratory locusts constitute a plague when they appear, and they have done so since ancient times. The Rocky Mountain locusts flying eastward have darkened the sky, and where they settled to the earth ate almost every green thing. In 1874-5 they produced almost a famine in Kansas, Nebraska, and other Western states. The young hatched away from the mountains were not healthy, and died prematurely, and their devastations came to an end. Of course the migrations may occur again. Packard calculates that the farmers of the West lost $200,000,000 because of their ravages in 1874-5.

Fig. 120.—Front Leg of Mole Cricket. × 3.

The cockroaches (Fig. 116), kindred of the grasshoppers, are household pests that have migrated almost everywhere that ships go. The praying mantis (Fig. 117), or devil’s horse, also belongs to this order. It is beneficial, since it destroys other insects. Which of its legs are specialized? The walking stick (Fig. 121) and cricket (Fig. 118), like most members of the order, are vegetarian.

Fig. 121.—Four Walking Stick Insects.

Are grasshoppers more common in fields and meadows, or in wooded places? How many different colors have you seen on grasshoppers? Which colors are most common?

Grasshoppers are very scarce in Europe as they love dry, warm countries. Why do locusts migrate? Give an instance in ancient times.

How long do most grasshoppers live? Does a grasshopper spread its wings before it flies? Does it jump and fly together? Can it select the place for alighting?

The body of an insect (e.g. a wasp, Fig. 122) is divided by means of two marked narrowings into three parts: the head (K), chest (B), and abdomen (H).

Fig. 122.—A Wasp.

The head is a freely movable capsule bearing four pairs of appendages. Hence it is regarded as having been formed by the union of four rings, since the ancestor of the insects is believed to have consisted of similar rings, each ring bearing a pair of unspecialized legs. The early grub or caterpillar stage of insects is believed to resemble somewhat the ancestral form.

The typical mouth parts of an insect (Fig. 123), named in order from above, are (1) an upper lip (labrum, ol) (2) a pair of biting jaws (mandibles, ok) (3) a pair of grasping jaws (maxillæ, A, B), and (4) a lower lip (labium, m, a, b). The grasping jaws bear two pairs of jointed jaw fingers (maxillary palpi, D, C), and the lower lip bears a pair of similar lip fingers (labial palpi, d). The biting jaws move sideways; they usually have several pointed notches which serve as teeth. Why should the grasping jaws be beneath the chewing jaws? Why is it better for the lower lip to have fingers than the upper lip? Why are the fingers (or palpi) jointed? (Watch a grasshopper or beetle eating.) Why does an insect need grasping jaws?

 

Fig. 123.—Mouth Parts of Beetle.

Fig. 124.—External Parts of a Beetle.

 

The chest, or thorax, consists of three rings (Fig. 124) called the front thorax (prothorax), middle thorax (mesothorax) and hind thorax (metathorax), or first, second, and third rings. The first ring bears the first pair of legs, the second ring bears the second pair of legs and the upper or front wings, and the third ring bears the third pair of legs and the under or hind wings.

Fig. 125.—Leg of Insect.

The six feet of insects are characteristic of them, since no other animals have that number, the spider having eight, the crawfish and crabs having ten, the centipedes still more, while the birds and beasts have less than six. Hence the insects are sometimes called the Six-Footed class (Hexapoda). The insects are the only animals that have the body in three divisions. Man, beasts, and birds have only two divisions (head and trunk); worms are not divided.

Define the class insecta by the two facts characteristic of them (i.e. possessed by them alone), viz.: Insects are animals with ____ and ____. Why would it be ambiguous to include “hard outer skeleton” in this definition? To include “bilateral symmetry”? “Segmented body”? The definition of a class must include all the individuals of the class, and exclude all the animals that do not belong to the class.

Fig. 126.—Foot of Fly, with climbing pads.

The abdomen in various species consists of from five to eleven overlapping rings with their foldlike joints between them. Does each ring overlap the ring in front or the one behind it?

The food tube (Fig. 127) begins at the mouth, which usually contains salivary glands (4, Fig. 127). What is the color of the grasshopper’s saliva? The food tube expands first into a croplike enlargement; next to this is the stomach (6, Fig. 127), which resembles the gizzard in birds, as its inner wall is furnished with chitinous teeth (b, Fig. 114). These reduce the food fragments that were imperfectly broken up by the biting jaws before swallowing. Glands comparable to the liver of higher animals open into the food tube where the stomach joins the small intestine. At the junction of the small and large intestine (9) are a number of fine tubes (8) which correspond to kidneys and empty their secretion into the large intestine.

Fig. 127.—Viscera of Grasshopper. Key in text. Compare with Fig. 114.

Fig. 128.—Air Tubes of Insect.

The breathing organs of the insects are peculiar to them (see Fig. 128). They consist of tubes which are kept open by having in their walls continuous spirals of horny material called chitin. Most noticeable are the two large membranous tubes filled with air and situated on each side of the body. Do these tubes extend through the thorax? (Fig. 128.) The air reaches these two main tubes by a number of pairs of short windpipes, or tracheas, which begin at openings (spiracles). In which division are the spiracles most numerous? (Fig. 128.) Which division is without spiracles? Could an insect be drowned, i.e. smothered, by holding its body under water? Could it be drowned by immersing all of it but its head? The motion of the air through the breathing tubes is caused by a bellowslike motion of the abdomen. This is readily observed in grasshoppers, beetles, and wasps. As each ring slips into the ring in front of it, the abdomen is shortened, and the impure air, laden with carbon dioxid, is forced out. As the rings slip out, the abdomen is extended and the fresh air comes in, bringing oxygen.

Fig. 129.—Insect’s Heart (plan).

Fig. 130.—Diagrams of Evolution of Pericardial Sac around insect’s heart from a number of veins (Lankester).

Fig. 131.—Position of Insect’s Heart, food tube, and nerve chain.

The Circulation.—Near the dorsal surface of the abdomen (Fig. 131) extends the long, slender heart (Fig. 129). The heart has divisions separated by valvelike partitions. The blood comes into each of the heart compartments through a pair of openings. The heart contracts from the rear toward the front, driving the blood forward. The blood contains bodies corresponding to the white corpuscles of human blood, but lacks the red corpuscles and the red color. The blood is sent even to the wings. The ribs on the wings consist of blood tubes inclosed in air tubes, so that the blood vessels are surrounded by air, and the purification of the blood is taking place throughout the course of the circulation. Hence the imperfect circulation is no disadvantage. The perfect provision for supplying oxygen explains the remarkable activity of which insects are capable and their great strength, which, considering their size, is unequaled by any other animals.

Fig. 132.—Nervous System of Bee.

The Nervous System.—The heart in backboned animals, e.g. man, is ventral and the chief nerve trunk is dorsal. As already stated, the heart of an insect is dorsal; its chief nerve chain, consisting of a double row of ganglia, is near the ventral surface (Fig. 131). All the ganglia are below the food tube except the first pair in the head, which are above the gullet. This pair may be said to correspond somewhat to the brain of backboned animals; the nerves from the eyes and feelers lead to it. With social insects, as bees and ants, it is large and complex (Fig. 132). In a typical insect they are the largest ganglia.

Fig. 133.—Feeler of a beetle.

The Senses.—The sense of smell of most insects is believed to be located in the feelers. The organ of hearing is variously located in different insects. Where is it in the grasshopper? The organs of sight are highly developed, and consist of two compound eyes on the side of the head and three simple eyes on the top or front of the head between the compound eyes. The simple eye has nerve cells, pigments, and a lens resembling the lens in the eyes of vertebrates (Fig. 134). The compound eye (Fig. 135) has thousands of facets, usually hexagonal, on its surface, the facets being the outer ends of cones which have their inner ends directed toward the center of the eye. It is probable that the large, or compound, eyes of insects only serve to distinguish bright objects from dark objects. The simple eyes afford distinct images of objects within a few inches of the eye. In general, the sight of insects, contrary to what its complex sight organs would lead us to expect, is not at all keen. Yet an insect can fly through a forest without striking a twig or branch. Is it better for the eyes that are immovable in the head to be large or small? Which has comparatively larger eyes, an insect or a beast?

 

Fig. 134.—Diagram of simple eye of insect.

L, lens; N, optic nerve.

Fig. 135.—Compound Eye of Insect.

1, hexagonal facets of crystalline cones. 6, blood vessel in optic nerve.

 

Inherited Habit, or Instinct.—Insects and other animals inherit from their parents their particular form of body and of organs which perform the different functions. For example, they inherit a nervous system with a structure similar to that of their parents, and hence with a tendency to repeat similar impulses and acts. Repeated acts constitute a habit, and an inherited habit is called an instinct. Moths, for example, are used to finding nectar in the night-blooming flowers, most of which are white. The habit of going to white flowers is transmitted in the structure of the nervous system; so we say that moths have an instinct to go to white objects; it is sometimes more obscurely expressed by saying they are attracted or drawn thereby.

Instincts are not Infallible.—They are trustworthy in only one narrow set of conditions. Now that man makes many fires and lights at night, the instinct just mentioned often causes the death of the moth. The instinct to provide for offspring is necessary to the perpetuation of all but the simplest animals. The dirt dauber, or mud wasp, because of inherited habit, or instinct, makes the cell of the right size, lays the egg, and provides food for offspring that the mother will never see. It seals stung and semiparalyzed spiders in the cell with the egg. If you try the experiment of removing the food before the cell is closed, the insect will bring more spiders; if they are removed again, a third supply will be brought; but if taken out the third time, the mud wasp will usually close the cell without food, and when the egg hatches the grub will starve.

The Development of Insects.—The growth and molting of the grasshopper from egg to adult has been studied. All insects do not develop exactly by this plan. Some hatch from the egg in a condition markedly different from the adult. The butterfly’s egg produces a wormlike caterpillar which has no resemblance to the butterfly. After it grows it forms an inclosing case in which it spends a quiet period of development and comes out a butterfly. This change from caterpillar to butterfly is called the metamorphosis. The life of an insect is divided into four stages: (1) egg, (2) larva, (3) pupa, and (4) imago, or perfect insect (Figs. 136, 137, 138).

 

Fig. 136.—Measuring worm, the larva of a moth.

Fig. 137.—Pupa of a mosquito.

 

Fig. 138.—The Four Stages of a Botfly, all enlarged.

a, egg on hair of horse (bitten off and swallowed); b, larva; c, larva with hooks for holding to lining of stomach; d, pupal stage, passed in the earth; e, adult horse fly.

The egg stage is one of development, no nourishment being absorbed. The larval stage is one of voracious feeding and rapid growth. In the pupa stage no food is taken and there is no growth in size, but rapid development takes place. In the perfect stage food is eaten, but no growth in size takes place. In this stage the eggs are produced. When there is very little resemblance between the larva and imago, and the pupa is quiescent, the metamorphosis, or change, is said to be complete. When, as with the grasshopper, no very marked change takes place between the larva and imago (that is to say, during the pupa stage, which is active), the metamorphosis is said to be incomplete. By studying the illustrations and specimens, and by thinking of your past observations of insects, determine which of the insects in the following list have a complete metamorphosis: beetle, house fly, grasshopper, butterfly, cricket, wasp.

TABLE FOR CLASSIFYING INSECTS (class Insecta) INTO ORDERS

Fig. 139.—May Fly. What order (see table)?

Fig. 140.—Silver Scale. (Order?)

Moths and Butterflies.—Order ____? Why ____ (p. 82)?

The presence of scales on the wings is a never-failing test of a moth or butterfly. The wings do not fold at all. They are so large and the legs so weak and delicate that the butterfly keeps its balance with difficulty when walking.

The maxillæ are developed to form the long sucking proboscis. How do they fit together to form a tube? (See Fig. 147.) The proboscis varies from a fraction of an inch in the “miller” to five inches in some tropical moths, which use it to extract nectar from long tubular flowers. When not in use, it is held coiled like a watch spring under the head (Fig. 148). The upper lip (labrum), under lip (labium), and lip fingers (labial palpi) are very small, and the mandibles small or wanting (Fig. 146).

The metamorphosis is complete, the contrast between the caterpillar or larva of the moth and butterfly and the adult form being very great. The caterpillar has the three pairs of jointed legs typical of insects; these are found near the head (Fig. 141). It has also from three to five pairs of fleshy unjointed proplegs, one pair of which is always on the last segment. How many pairs of proplegs has the silkworm caterpillar? (Fig. 143.) The measuring worm, or looper? (Fig. 136.) The pupa has a thin shell. Can you see external signs of the antennæ, wings, and legs in this stage? (Fig. 143.) The pupa is concealed by protective coloration, and is sometimes inclosed in a silken cocoon which was spun by the caterpillar before the last molt. Hairy caterpillars usually produce butterflies, and the naked ones usually produce moths. Hairy ones are uncomfortable for birds to eat. The naked and brightly marked ones (warning coloration) often contain an acrid and distasteful fluid. The injuries from lepidoptera are done in the caterpillar stage. The codling moth (Fig. 141) destroys apples to the value of $6,000,000 annually. The clothes moth (Fig. 171) is a household pest. The tent caterpillar denudes trees of their leaves. The only useful caterpillar is the silkworm (Fig. 143). In Italy and Japan many of the country dwellings have silk rooms where thousands of these caterpillars are fed and tended by women and children. Why is the cabbage butterfly so called? Why can it not eat cabbage? Why does sealing clothes in a paper bag prevent the ravages of the clothes moth?

Flight of Lepidoptera.—Which appears to use more exertion to keep afloat, a bird or a butterfly? Explain why. Of all flying insects which would more probably be found highest up mountains? How does the butterfly suddenly change direction of flight? Does it usually fly in a straight or zigzag course? Advantage of this? Why is zigzag flight unnecessary to moths? Bright colors are protective, as lepidoptera are in greatest danger when at rest on flowers. Are the brightest colors on upper or under side of wings of butterfly? Why? (Think of the colors in a flower.) Why is it better for moths to hold their wings flat out when at rest? Where are moths during the day? How can you test whether the color of the wings is given by the scales?

State how moths and butterflies differ in respect to: body, wings, feelers, habits; abundance of scales.

Insects and Flowers.—We are indebted to insects for the bright colors and sweet honey of flowers. Flowers need insects to carry their pollen to other flowers, as cross-fertilization produces the best seeds. The insects need the nectar of the flowers for food, and the bright colors and sweet odors are the advertisements of the flowers to attract insects. There were no flowers in the world before flying insects were developed. Moths, butterflies, and bees carry most pollen (see Plant Biology, Chap. VI).

Fig. 141.—Codling Moth, from egg to adult. (See Farmers’ Bulletin, p. 95.)

Fig. 142.—Cabbage Butterfly, male and female, larva and pupa.

Fig. 143.—Life History of Silkworm.

Fig. 144.—Scales from Butterflies’ Wings, as seen under microscope.

 

Fig. 145.—Scales on Moth’s Wing.

Fig. 146.—Head of Butterfly.

 

Fig. 147.—Section of Proboscis of butterfly showing lapping joint and dovetail joint.

Fig. 148.—Head of Butterfly (side view).

Fig. 149.

 

Fig. 150.

Fig. 151.

 

Fig. 152.

Fig. 153.

 

Fig. 157.

Fig. 158.—Anatomy of bee.

Fig. 159.—Ichneumon fly.

Fig. 160.

Fig. 161.—Wasp using pebble.

From Peckham’s “Solitary Wasps,” Houghton, Mifflin & Co.

Illustrated Study of Beetles.

Fig. 162.—Diving beetle (Dysticus), with larva, a.

Fig. 163.—Weevil.

 

Fig. 164.

Fig. 165.

 

Fig. 166.—Click beetle.

Fig. 167.—May Beetle.

Fig. 168.

Fig. 169.—Colorado beetle (potato bug).

Fig. 170.—Life history of ant lion.

 

Fig. 171.

Fig. 172.—Metamorphosis of house fly (enlarged).

 

Fig. 173.—Metamorphosis of flea.

 

Fig. 174.—Louse.

Fig. 175.—Bed bug. × 5.

 

Fig. 176.—Life history of mosquito.