AN INTRODUCTION TO
NATURE STUDY.

MACMILLAN AND CO., Limited

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AN INTRODUCTION

TO

NATURE-STUDY

BY

ERNEST STENHOUSE, B.Sc. (Lond.)

ASSOCIATE OF THE ROYAL COLLEGE OF SCIENCE, LONDON; JOINT-AUTHOR
WITH A. T. SIMMONS, B.SC. OF “SCIENCE OF COMMON LIFE”

MACMILLAN AND CO., LIMITED
ST. MARTIN’S STREET, LONDON
1910

First Edition 1903.
Reprinted 1904 (twice), 1905, 1906, (with additions) 1908, 1910.

GLASGOW: PRINTED AT THE UNIVERSITY PRESS
BY ROBERT MACLEHOSE AND CO. LTD.

PREFACE.

One of the most encouraging of recent educational movements is the increasing importance attached, both in this country and abroad, to what is called Nature-Study. It is evident that the instruction contemplated differs as widely, on the one hand, from the traditional object-lessons on polar bears and ironclads, as it differs from formal Biology on the other. This difference is abundantly shown, not only by the circulars and syllabuses issued by our own Board of Education, but by the publications of the leading educational authorities of Europe and America. The aim of Nature-Study, as thus laid down, is not primarily the acquisition of the facts of natural history: it is rather a training in methods of open-eyed, close, and accurate observation, especially of familiar animals and plants, which shall teach the student to see what he looks at, and to think about what he sees.

It is in a spirit of entire agreement with these views that this book has been written. No previous knowledge of Biology on the part of the reader is assumed, and technical terms have as far as possible been dispensed with. In drawing up the course, I have had in mind throughout the attitude of an intelligent youth of sixteen, and the work will be found to be well within the powers of such a student. Teachers will, however, find no difficulty in adapting the exercises to the needs of younger pupils.

Care has been taken to select, as types for study, animals and plants which are at the same time representative and easily obtainable,[1] and I have been further guided in the selection by the Board of Education Syllabuses of the King’s Scholarship Examination and Section I. of the Elementary Stage of General Biology, the subjects of which are included in the volume. The book has, however, a considerably wider scope than is indicated in these syllabuses, and will therefore, I hope, be found useful not only in schools and training-colleges, and to examination candidates, but also to members of field clubs and to students of natural history generally. It has been necessary to arrange the chapters with some attempt at logical sequence, but it is not supposed that this order will be adhered to in practice; by the aid of the monthly nature-calendar, together with numerous cross-references, it will be found easy to take up the work at any point.

The chapters are divided into sections, each of which consists of two parts: First, precise instructions for practical observations and experiments, designed to exercise the reasoning faculties of the students; and, second, a descriptive portion, in which the meaning and relation of the results obtained are discussed. At the end of each chapter is a number of additional exercises, either original or taken from past examination papers. Of the latter class, questions to which dates are affixed have been set by the Board of Education, while those marked “N.F.U.” are selected from National Froebel Union tests. In many cases, the exercises provide subjects for further observation and experiment, as well as for written description.

Much trouble has been taken in the selection of the illustrations, many of which have been expressly drawn or photographed for this book. Through the kindness of the publishers I have been able to include illustrations from Strasburger’s Text-Book of Botany, Parker and Haswell’s Text-Book of Zoology, The Cambridge Natural History, and other books; and Mr. Ernest Evans has courteously consented to the use of a number of figures from his Botany for Beginners. The following illustrations have been prepared from photographs supplied by Mr. J. C. Shenstone, F.L.S., Vice-President of the Essex Field Club: Figs. [27], [57], [65], [67] to [71], [74], [75], [80], [81], [83], [84], [85], [87], [89], [92], [94], [95], [102] to [110], [120], [136], [145], [149], [152] and [153]; while Figs. [180], [196], [200], [201], [203], [205], and [211] are reproduced, by permission, from Pike’s Woodland, Field, and Shore (Religious Tract Society).

Finally, I must acknowledge gratefully the continuous help which, at every stage in the preparation of the book, I have received from Professor R. A. Gregory and Mr. A. T. Simmons, B.Sc.—help as valuable as it was generous.

The issue of a new edition has provided the opportunity of adding a section on School Journeys, originally contributed by me as an article to The School World, and reprinted here by kind permission of the Editors of that journal. For the illustrative sketch-map ([Fig. 237]), and for Figs. [19], [21] and [138], I am indebted to my friend Mr. T. D. Tuton Hall.

E. STENHOUSE.

CONTENTS.

PART I. PLANT LIFE

CHAPTER I.
SEEDS AND THEIR EARLY STAGES OF GROWTH.

1. THE STRUCTURE OF SEEDS WITH TWO COTYLEDONS.

1. Preparation of the seeds.—Obtain several seeds of the broad bean, pea, mustard, yellow lupine, vegetable marrow, and sycamore; soak them in cold or slightly warm water until they are soft enough to be cut through easily with a sharp knife. The time necessary will vary with different seeds according to the size of the seeds, and with the temperature of the water. The beans should be left in the water for a few days. When the seeds are soft enough, examine one or two of each, and in the meantime put about six of each (except the mustard) in damp sawdust in a warm place. Put the mustard seeds on damp flannel in a saucer.

2. The outside of a broad bean.—Notice the flattened oval shape, with an indentation at one place ([Fig. 1]). What is the colour of the skin (seed-coat) of the bean seed? Is all the skin of this colour? A black scar extends along the edge from the indentation for about ¾ in. What is this scar? If beans in the pod can be obtained, see that the scar is the place of attachment of the seed stalk. Make drawings to scale, showing side and edge-views of the seed. Wipe the bean dry and then squeeze it gently. Notice that a drop of water comes out at a point at one end of the stalk scar. There is evidently a little hole here. This little hole is called the micropyle. Mark its position by a dot on the drawing.

3. The inside of a broad bean.—With a sharp knife cut the seed-coat open, beginning at the side of the seed furthest from the micropyle, and carefully remove the seed-coat. Notice that near the micropyle the seed-coat forms a funnel-shaped depression, and that the point of the funnel is at the micropyle. Does anything fit into the funnel? A little cone may be seen to fill the funnel; this conical body is called the radicle. Make a drawing of the seed after the removal of the seed-coat. Look at the edge opposite the radicle and notice that a crack divides the body of the seed into halves. Put the point of your knife blade into the crack, and gently force the halves apart. They come apart without tearing, showing that they are naturally separate, although they fit so closely together.

These two swollen bodies are called the cotyledons. Separate them and see, at the point where they join the radicle, a little curved rod, evidently a continuation of the radicle, lying between them. This rod is the plumule. Take off one cotyledon, and make a drawing of the inner face of the other cotyledon, with the adhering plumule and radicle ([Fig. 2]).

4. Starch present in the cotyledons of the bean.—Scrape the inner surface of a cotyledon and then pour on it a drop of iodine solution.[2] Is there any change? Pour also a drop of iodine solution on a piece of laundry-starch. Is a similar blue colour formed? What substance is probably present in the cotyledons of the bean?

5. The pea.—Examine a pea in a similar manner. Make drawings showing the stalk-scar, the micropyle, and the plumule and radicle with their manner of connection with the cotyledons. Does the end of the radicle point towards the micropyle? How many cotyledons has this seed? What shape and colour are they? Do they contain starch?

6. The seed of the yellow lupine.—Compare this with the bean and the pea, and find out how many cotyledons it has, and whether they contain starch. Can you find the plumule? It is very small, but occupies a position similar to that of the plumule of the bean. Does the end of the radicle point to the micropyle?

7. The vegetable marrow seed.—Notice the peculiar shape (somewhat like a pocket-flask) of the seed, and the thickened margin which runs round it. Carefully cut the seed-coat away so as not to injure the part inside. How many cotyledons are present? What is their colour? Do they contain starch? Can you see the plumule and radicle clearly? If not, do not decide that they are absent, but leave the question to be settled later, when you watch a vegetable marrow seed “come up.”

8. The mustard seed.—Notice how much smaller this seed is than the others. With a balance, find how many mustard seeds are equal in weight to one bean seed. Observe the stickiness of the seed-coat of the soaked seed, and then remove it carefully with needles, exposing two thin plates, each one folded on itself, and one tucked inside the other, like two sheets of note-paper. These are the cotyledons; it seems that the smallness of the seed may be mainly due to the small size of the cotyledons. What is their colour? Remember these characters and try, when you watch the young plants come up later, to find an explanation of them.

9. The sycamore fruit.—The seed of the sycamore is enclosed in a case which has a wing attached to it. The wing, the case, and the enclosed seed together constitute the fruit of the sycamore. The fruits occur in pairs ([Fig. 137]). Notice that a cord runs out to each fruit from the stalk on which the pair of fruits is borne. Make a drawing of a pair of fruits, then separate the fruits.

10. The sycamore seed.—Cut open a fruit. Can you see anything between the seed and the fruit-case? Would the hairy covering of the seed tend to keep it warm during the winter? Why? Why do you prefer to wear flannel in winter and linen in summer? Flannel is more fluffy than linen.

Remove the seed-coat carefully. Running down one side you will see a little curved rod. This is the radicle. Gently raise it with the point of your knife. Notice that the rest of the seed seems to consist of a green part, which is curled up. Uncoil the curls carefully. You find that they are two green leaves, fixed at the top of the radicle. These are the cotyledons. In the seed each cotyledon is first folded in two across the middle and then coiled up. Make a sketch showing the coils ([Fig. 4]). Can you see the plumule? It is just at the top of the radicle, where the cotyledons are fixed on.

Plants are living things.—One of our foremost naturalists[3] tells us that when he goes out into the woods, or into one of those fairy forests which we call fields, he finds himself welcomed by a glad company of friends, everyone with something interesting to tell. Such a feeling would be quite impossible to one who did not vividly recognise the fact that plants are alive; for it is precisely this recognition or its absence which makes the observation of the forms and habits of plants fascinating or the reverse. Let the Nature-Student, then, at the outset of his work, keep the idea of life inseparably bound up with his every thought about plants. It may at first require a little effort, but before long it will enable him to understand how the friendship of the more silent half of animate nature may form one of the great pleasures of life.

The study of seeds.—The manifestation of life is so striking, and the changes in form and size take place so rapidly, in the germination of seeds, that the study of plants cannot better be commenced than with this stage of their growth. The method has also the logical virtue of beginning at the beginning, or nearly so.

These early changes can be well observed by taking various common seeds, soaking them in water until they are soft, and then allowing them to germinate in damp sawdust, taking a few out at intervals and noting their progress. The growth of the seeds takes place more rapidly if they are kept in a warm room, but in any case some days will probably elapse before much change is noticeable in them.

During the interval of waiting, some of the seeds themselves should be carefully examined, and drawings of all the parts should be made. The drawing ought on no account to be omitted. It compels the student’s attention to details which would otherwise pass unnoticed; and a careful sketch is a much better record of an observation than any amount of description alone could be. The drawing need not be elaborate; an outline pencil-sketch to scale will usually be sufficient.

Fig. 1.—A Broad-Bean seed. A, side view;  B, edge view: st. sc., stalk-scar; m, micropyle. (× ⅔.)

The seed of the broad bean.—The seed of the broad bean ([Fig. 1]) is large, having a diameter of perhaps an inch and a half, and a thickness of half an inch. In shape it is oval, but at one region the edge is indented, and a black scar (st. sc.) runs from the indentation along the edge for a distance of about three-quarters of an inch. This scar is the place of attachment of the stalk which formerly carried the seed in the bean-fruit (pod). It may be called the stalk-scar. If a soaked bean is wiped dry and then gently squeezed, a small drop of water escapes from the end of the stalk-scar nearest the indentation. The hole out of which the water comes is very small and difficult to see, but its position is thus made clear. This hole (m) is called the micropyle,—a word meaning the “little gate.”

The bean seed is covered by a tough brown skin, the seed-coat ([Fig. 2], s.c.), a funnel-shaped depression in which leads to the micropyle (m). The depression is occupied by a part of the seed which is shaped like a conical peg and called the radicle (R); the point of the radicle is directed toward the micropyle. The great body of the seed is composed of two fleshy, cream-coloured lobes, easily wedged apart by inserting a knife-blade between them; these fleshy lobes are the cotyledons (Cot.). Between them, and continuous with the radicle, is a small yellow body, the plumule (pl.). The relations of the radicle, plumule and cotyledons are best seen by removing one cotyledon ([Fig. 2]).

Fig. 2.—Broad-Bean seed, seen from the inside, after the removal of half the seed-coat and one cotyledon. Cot., the inner face of remaining cotyledon; C′, area of attachment of other cotyledon; m, micropyle; pl, plumule; R, radicle; S.c., seed-coat; st. sc., stalk-scar. (× 1.)

A scraped cotyledon at once turns blue when a drop of dilute iodine solution is poured on it, thus showing the presence of starch. We shall see in [Chapter II.] what use the growing seedling makes of the starchy food which is stored in its cotyledons.

The seed of the pea.—Except in size and shape the seed of the pea is very similar to the bean seed. Its form is spherical, and the scar left by the stalk which formerly attached it to the wall of the pea-pod ([Fig. 3]) is plainly to be seen. Pointing towards the micropyle is the peg-like radicle; the plumule lies between the hemispherical cotyledons. As before, the cotyledons can be proved to contain starch, by the blue colour which is formed when a drop of iodine solution is poured on the scraped surface.

Fig. 3.—Pods and Seeds of Pea. (× ½.)

The seed of the yellow lupine.—The seed of the yellow lupine is about as large as a pea, but it is slightly flattened in shape. The seed-coat is prettily mottled; when it is removed, the greater part of the seed is found to consist of two cotyledons. They are somewhat swollen, but the stored food is not starch. The plumule and radicle occupy positions similar to those of the bean and pea.

The vegetable marrow seed.—This seed has a rather curious shape, and somewhat resembles a pocket-flask. It is flattened, and the border of the seed-coat is thickened and of silky appearance, the rest of the “skin” having some resemblance to kid. The two cotyledons, which compose the greater part of the seed, are white and only slightly fleshy. The plumule and radicle are at the pointed end of the seed, and are difficult to see.

The mustard seed.—In comparing the mustard seed with those already described, one is struck with the great difference in size. An average broad-bean seed weighs about 600 times as much as the mustard seed. While the two fleshy cotyledons make up the bulk of the seed of the bean, pea, lupine and vegetable marrow, the cotyledons of the mustard seed are thin and leaf-like. They are folded on themselves, one inside the other (as at g, [Fig. 61]), and enclose the radicle. The characters of the cotyledons account very largely for the small size of the mustard seed. It will be seen, when the growth of the young plants is watched, that the difference is associated with the special duties which the cotyledons perform in the various cases.

Fig. 4.—Sycamore Fruit, cut through in the plane of the wing. s.c., seed-coat (indicated by a thick, broken line); f.w., fruit-wall; h, layer of fine hairs; R, radicle; pl., plumule; cot. 1, cot. 2, cotyledons (diagrammatically shaded). (× 2.)

The sycamore seed.—What is generally called the seed of the sycamore is really a fruit. The fruits are in pairs (Figs. [ 33] and [ 137]), and each half consists of a flat wing and a rounded case in which the seed itself is enclosed. The round seed-cases of the two fruits are connected together. When they come apart, a scar marks the place where they were formerly in contact, and a little cord runs out to each fruit from the stalk on which the pair of fruits is borne.

Between the sycamore seed and the wall of its case is a layer of fine hair (h, [Fig. 4]), which forms a warm nest for the seed in winter. The seed is surrounded by a thin, brown seed-coat, and consists mainly of two cotyledons, but these are very different from any yet described. Each is a green leaf, measuring, when unfolded, about an inch in length. It is first folded across the middle of its length, and then rolled up into a close coil with its fellow. The coils are very plainly to be seen when the seed coat is removed, or when the whole seed is cut through, by a sharp knife, in the plane of the wing. Running down one side of the seed is a green rod, the radicle (R, [Fig. 4]). The two cotyledons (cot. 1 and cot. 2) spring from its upper end, and between them is the tiny plumule (pl.)

The sycamore seed bears more resemblance to the mustard seed than to the others, but it is on a much larger scale. In each of these two seeds the cotyledons are plainly leaves, while in the others their nature is disguised by the great accumulation of stored food in them.

2. THE EARLY STAGES OF GROWTH OF SEEDS WITH TWO COTYLEDONS.

If the seeds which were sown in damp sawdust and on flannel are kept warm they will soon be ready for study. You should remember that at present your object is not so much to rear the plants as to find out how they grow. As soon, therefore, as any sign of growth is to be seen when you take a seed out, you should begin to examine them at regular intervals, taking one or two out every day and leaving the rest to continue their development. Keep the sawdust damp, but not wet.

1. The pea and bean.—(a) General development.—Very soon the seed-coat splits at the micropyle-end of the stalk-scar, and the end of the radicle protrudes. Does the radicle grow upwards or downwards? Observe that even if the seed was so planted that the micropyle was at the top, the radicle turns over and grows straight down. Turn over a seedling and see if you can persuade the radicle to grow upwards. Open a seed when the radicle is about an inch long, and see what the plumule is doing. It is still enclosed in the seed-coat, and lies between the cotyledons, but is larger than at first. As the growth proceeds the cotyledons begin to separate near the top of the radicle, and you can get a glimpse of the plumule.

(b) The growth of the radicle.—Day by day the radicle becomes longer. Is it all growing longer, or does the increase in length take place more at one part than another? To answer this question, take five or six inches of cotton thread and moisten the middle part with Indian ink. Lay the seed on a flat ruler, so that the radicle lies over an inch divided into—say—tenths. Hold the thread tight, and press the inked part gently on the radicle, making about five marks at equal intervals from the point upwards. The ink will dry almost immediately. Then carefully replant the seed, taking care not to injure the radicle. After a few days take it out again, lay it once more on the ruler, and measure the distance between the marks.

The radicle is evidently the young root.

(c) The root-cap.—Hold up the radicle to the light, and examine its tip with a lens. Try to see that the tip is covered by a little cap, somewhat like a very small thimble. This is called the root-cap.

(d) The root-hairs.—Hold a seed, with the radicle about an inch long, against a dark surface. Is the surface of the radicle smooth, or can you see any fluffiness on it? Is all the radicle fluffy, or only a part? Which part? As you examine older and older seedlings notice how much of the radicle is fluffy, and where the fluffy part is. The fluffy appearance is caused by fine, closely-set hairs, called root-hairs.

(e) The plumule.—How soon after planting does the plumule become free? Does it grow upwards or downwards? The plumule is evidently the young stem.

As soon as the young stem is old enough mark it with Indian ink as you marked the young root, and replant it to find if there is any difference in the rates of growth of its different parts.

(f) The fate of the cotyledons.—From time to time examine the cotyledons and notice that as the seedling grows larger they become more and more shrunken. Something is evidently being taken from them, perhaps to feed the young plant. We shall inquire into this by further experiment ([Chapter II.]).

Do the cotyledons remain in their original position, or are they carried upwards with the growing stem or downwards with the growing root?

2. The yellow lupine.—In the same way observe how this seed grows. Do the cotyledons shift their position or change in colour? Do they become leaf-like? How do they differ from later-formed leaves? What becomes of them at last? What becomes of the seed-coat?

3. The mustard seed.—Notice that, soon after the radicle has come out of the seed-coat, a sort of hump forms at its upper end, and at length the cotyledons are pulled out of the seed-coat and turn up towards the light. What is their colour? Observe that the two cotyledons are soon raised on the end of a little stalk. Like the cotyledons of the yellow lupine they are plainly leaves. Notice their shape. Are they of equal size? Why not? When they are about three inches above the seed-coat gently separate them and notice the little bud between them. Draw the seedling. How large can you get a mustard seedling to grow on damp flannel? Plant a few mustard seeds on earth, and notice the difference between the shape of the cotyledons or seed-leaves and that of the leaves which appear later. What becomes of the cotyledons?

4. The vegetable marrow seed.—Make similar observations upon the vegetable marrow seeds, noticing particularly whether the cotyledons remain in their original positions and shrink up as the plant increases in size, or whether they are pulled out of the seed-coat by the elongating stem, and become green and leafy. How does the plant hold down its seed-coat whilst it pulls out its cotyledons?

5. The sycamore seed.—From what you have seen of the cotyledons of the sycamore seed, will you expect them to behave like those of the mustard seed, or like those of the pea and bean? Even in the seed they are green, and plainly leaves. How do they escape from the seed-coat? What is their shape? Do they come out before or after the radicle? Do they get any larger as the stem grows? How large can you get a sycamore seedling to grow in damp sawdust? As large as a seedling of pea or bean? Plant some sycamore seeds on earth and compare the shape of the cotyledons with that of the next-formed leaves. How soon do the “true” leaves appear after the cotyledons have escaped from the seed? Do any “true” leaves grow on the plants in sawdust? What becomes of the cotyledons at last?

The embryo.—The plumule, radicle, and cotyledons, which have now been seen in the seed, form the embryo of the plant. The adult plant will be wholly formed by the growth and development of these parts, and we must now follow carefully the changes which take place when the seed germinates, and try to find out what becomes of each part. It is better to put the seeds at first in damp sawdust rather than in earth, as the young roots can then be more readily cleaned and observed. With small seeds the early stages of growth are better seen if damp flannel is used.

Germination.—Under the influence of moisture and warmth the embryo in the seed begins to swell and unfold its parts. The radicle makes its appearance first ([Fig. 5]), breaking through the seed-coat at the micropyle; it is the young root. The radicle always grows downwards, that is, toward the centre of the earth. If the seed lies in such a position that the micropyle is directed upwards, the point of the radicle turns over and grows downwards as soon as it escapes from the seed-coat. As the young root becomes longer and thicker ([Fig. 6]) the seed-coat opens more and more, showing the cotyledons beneath, and these, too, are gradually forced apart.

Fig. 7.
—Mustard
Seedling, showing
root-hairs and
cotyledons. (× ½.)

The cotyledons.—During the germination of various seeds, a very marked difference in the behaviour of the cotyledons is to be seen. In the case of the broad bean and pea the cotyledons remain in their original positions, partially enclosed by the split seed-coat. Presently a hump (Figs.[ 6] and [ 11]) forms at the upper end of the radicle, as if the plant were making an effort to pull its plumule out of the seed. It soon succeeds ([Fig. 12]), and the plumule turns up to the light. It is the young stem. At its end is a little bud, formed by a number of small, overlapping, green leaves which surround the growing point. Henceforth the stem grows upwards, that is in a direction precisely opposite to that of the root’s growth. Both stem and root are attached to the cotyledons, which gradually shrivel up as the stem and root become larger and larger.

When, however, the seed of the mustard, or sycamore, germinates the cotyledons behave very differently (Figs.[ 7] and [ 8]). Soon after the root has become well established the cotyledons come quite out of the seed-coat and unfold themselves. Instead of remaining on or under the surface of the ground they are carried upwards at the end of a stalk toward the light, and for some time the little plant appears to consist of root, stalk, and cotyledons only. If, however, the cotyledons are gently pressed apart, a tiny bud is seen between them. This evidently corresponds to the bud at the end of the stem of the bean or pea.

In the case of the lupine ([Fig. 9]) or vegetable marrow ([Fig. 10]) the cotyledons appear to combine these two conditions. They are swollen and contain stored food; yet they come out of the seed-coat early, become green, and open out to the light. They are evidently leaves, though their shape differs from that of the later leaves.

Fig. 8.—Three stages in the growth of a Sycamore Seedling. cot., cotyledons; fol., first pair of foliage leaves. (Slightly reduced.)

The germinating vegetable-marrow seed possesses a curious contrivance for pulling its cotyledons out of the seed-coat. This is a peg (p, [Fig. 10]) which develops at the top of the radicle, and holds down the lower half of the seed-coat whilst the other half is forced upwards to allow the cotyledons to be withdrawn.

Fig. 9.—Three stages in the growth of the Yellow Lupine. On the right the cotyledons are still enclosed in the mottled seed-coat. In the middle plant the cotyledons are spreading out; the first foliage leaves have not yet unfolded. On the left, the first two foliage leaves are unfolding, and the cotyledons have spread out flat. (Slightly reduced.)

After a little thought a possible explanation of these differences in the cotyledons suggests itself. It may be that, in the case of the mustard and sycamore, leaves are required as early as possible, while the bean and pea have no immediate need for leaves because their cotyledons contain so much stored food. The cotyledons of these plants shrivel up as the seedling grows, and this seems to indicate that during its early stages the plant lives upon this food. In [Chapter II.] we shall make experiments to see if this explanation is the true one. If so, the lupine and vegetable-marrow seeds evidently rely partly upon their stored food and partly upon setting the cotyledons to work as leaves, whilst the plant is still very young.

The true leaves.—The cotyledons are really makeshift leaves, which are already formed in the seeds. Even when they expand and become green they do not live long, but as soon as the next few leaves are well established, shrivel up and wither. The true or “foliage” leaves first make their appearance as a bud which surrounds the growing point of the stem. As this part of the stem increases in length, the foliage leaves become separated from each other and spread out to the light and air.

The lengthening of the stem and root.—Unless an experiment to test the truth of the matter is really made, it might be supposed that the different parts of the stem and root of the seedling grow in length at the same rate. This can be tested by marking the stem and root with lines of Indian ink at equal distances. In one experiment with a pea seedling five lines were marked upon the young root at regular intervals of one-tenth of an inch, beginning at the tip ([Fig. 11]). The seedling was carefully replanted and examined again a few days later. Between the tip and the first mark there was then ([Fig. 12]) a distance of seven-tenths of an inch; that is, this part had grown to seven times its former length. The second interval was four times as long as before, the third was one and a half times as long, while the fourth and fifth intervals had not increased in length at all. Such experiments prove that the root grows in length either at or just behind the tip. When a young stem is treated in the same way the lengthening is found to take place more evenly.

Rootlets.—After a time the radicle begins to put out branches called rootlets. These come off the main root in rows. In some cases rootlets make their appearance whilst the radicle is still very short, as in the vegetable marrow of [Fig. 10], but in others the radicle may be a few inches long before it produces rootlets.

The root cap.—The tip of the root, and of each of its branches, is covered by a little cap, shaped somewhat like a thimble ([Fig. 13]). This protects the tender growing point from the friction of particles of soil, and is continually renewed by growth from within as its outer layers are worn away.

Root hairs.—When a young root is held against a dark background it appears fluffy. This appearance is caused by a large number of very fine hairs upon its surface. The hairs are not found all over the root and its branches, but only for a short distance a little way behind the tips (Figs. [ 7] and [ 11]). These root hairs are of very great importance to the plant, as will be seen in [Chapter II.]

3. THE STRUCTURE OF GRAINS OF
MAIZE AND WHEAT.

1. Preparation of the seeds.—Soak grains of maize (Indian corn) and wheat in water until they are soft. The grains of maize will need soaking for several days. Plant about a dozen of each in damp sawdust, and in the meantime examine others.

2. The maize grain.—A grain of maize is really a fruit, as a pea-pod is. A pea-pod contains several seeds; a maize fruit contains only one seed, which fills it. Notice the shape of the grain—flattened, rounded along one edge, and bluntly pointed at the opposite edge ([Fig. 14]). Notice a whitish patch on one of the flattened sides; a ridge (E) down the middle of this marks the position of the embryo. Cut through the grain lengthwise, so as to divide the embryo into two equal parts, and examine the cut surface ([Fig. 15]). Identify:

(a) The embryo, lying somewhat obliquely, and to one side. The radicle (rad.) is directed towards the pointed end of the grain, and the plumule (pl.) towards the rounded end.

(b) The endosperm (end): a mass of material outside the embryo, and forming at least half of the grain.

(c) The scutellum (scm): a plate lying between the endosperm and the embryo.

(d) The coats of the seed and fruit, surrounding the whole.

Draw. Add a drop of iodine solution to the cut surface of the half-grain. The endosperm turns blue. What does this indicate?

3. The wheat grain.—A grain of wheat is also a one-seeded fruit. Notice the groove along one side, and—on the opposite side near one end—the white patch marking the position of the embryo. At the other end is a tuft of very fine hairs. Cut the grain lengthwise, so as to divide the white patch into two equal parts, and make out the embryo, endosperm, and scutellum ([Fig. 16]). Draw. Test the endosperm with iodine solution. Does it contain starch?

Grains of maize and wheat.—A grain of maize or wheat is really a one-seeded fruit. In other words, the grain consists not only of the seed with its seed-coat, but also of the seed-case. In this respect it resembles the fruit of the sycamore ([p. 8]). When, however, a grain of maize or wheat is carefully examined, it is found to differ greatly from all the seeds hitherto mentioned. A maize grain is somewhat flattened, and rather pointed along one edge ([Fig. 14]). On one flat side, near the pointed end, may be seen a whitish patch, and, along the middle line of this, a ridge which marks the position of the embryo.

A wheat grain has a different shape. It is oval, with a deep groove running down one side. One end is clothed with a tuft of very fine hairs; near the other end, on the side opposite to the groove, is a white patch beneath which is the embryo.

These differences in shape are of small importance, for the two grains are really of very similar structure, as may be seen when they are cut through lengthwise with a sharp knife, in a direction which divides the embryo along its middle line. It is then plain that each grain consists of two principal parts: the embryo and the endosperm (Figs. [ 15] and [ 16]). The embryo lies to one side and in the lower half of the seed. At its upper end the young stem and at its lower end the young root—each still enclosed in protecting sheaths—are easily seen. The greater part of the seed is quite outside the embryo; it is a mass of food called the endosperm, which has been stored up for the use of the young plant during its earliest stages of germination. This food mass at once turns blue when a drop of iodine solution is placed upon it, showing that it contains a considerable amount of starch. It is the endosperm which is made into flour when corn is ground.

Lying between the embryo and endosperm is a flat plate called the scutellum. Covering embryo, scutellum, and endosperm is the seed-coat proper, and outside that come the various layers of the fruit case.

Fig. 16.—Diagram of a longitudinal section
through the middle line of a wheat grain.

In the seeds previously examined, the embryo—consisting of plumule, radicle, and cotyledons—was seen to fill the seed-coat completely. In some cases the cotyledons were found to be more or less swollen with stored food-material. In the maize and wheat, however, the embryo forms only a comparatively small proportion of the seed, the bulk of which consists of stored food called endosperm. This is a difference of some importance. Still more important, however, is the fact that the two cotyledons, which were so conspicuous a feature of the other seeds, cannot apparently be seen at all in these seeds. Are there then no cotyledons in the seeds of the maize and wheat? If there are, how and when do they appear, and what is their number?

4. THE EARLY STAGES OF GROWTH OF
MAIZE AND WHEAT.

1. The roots.—Watch for the appearance of the roots. Is there, as in the seedlings previously studied, one principal root, or are there soon several, all apparently of equal or nearly equal importance? Do the roots grow straight down as before, or do they spread horizontally?

2. The stem and the cotyledon.—Notice that a rod, somewhat thicker than a root, grows out near the origin of the roots, and curves upwards towards the light. When this is about an inch long on a maize seedling, slit it open carefully, and observe that it consists of a pale outer sheath and a green core. The sheath is the single cotyledon; the green core is the young stem enclosed in a young foliage leaf. Cut open the grain and notice how the endosperm has shrivelled. As the seedlings become larger watch the young stem growing out at the end of its sheathing cotyledon. What is the length of the cotyledon when the stem first appears (a) in the maize, (b) in the wheat?

3. The foliage leaves.—As soon as a foliage leaf unfolds make a drawing of its shape. Contrast it with the young foliage leaves of the other seedlings. Hold up the leaves to the light and compare the arrangement of their veins.

How maize and wheat seeds grow.—When the maize and wheat fruits have been kept in damp sawdust for a few days, the seeds—one in each fruit—begin to germinate. As a rule the wheat plants have grown to a height of some inches before the roots and stems of the maize plant have emerged from the seeds.

The roots.—As might be expected, the first signs of life make their appearance at the white scar which indicates the position of the embryo or young plant. Instead of one main root growing out, several little roots make their appearance almost at the same time. They do not grow as directly downwards as the radicle of a pea or bean ([Fig. 6]), but tend to spread in a horizontal direction ([Fig. 17]). It is clear that in this way the roots are more independent of each other than if they grew directly downward side by side.

The cotyledon and the stem.—Growing out from the seed close to the roots is another rod ([Fig. 17], C), rather thicker than the roots, which at once curves upwards to the light. It is pale green in colour. This is the cotyledon or first leaf. When it is carefully slit open, it is found to be a hollow sheath, enclosing a bright green core. In the seedlings which are left undisturbed, the core at last breaks through the tip of the cotyledon. It consists of the young stem and its surrounding foliage leaves. As the growth of the plant continues, these sheathing leaves unfold themselves into the long narrow blades characteristic of grass leaves ([Fig. 98]). The bottom of each leaf is tubular and forms a sheath round the stem.

Fig. 17.—Young wheat seedling.
C, cotyledon; r, r, r, r, roots. (× 3.)

The endosperm.—The endosperm, which at first made up more than half the seed, gradually shrivels up as the little plant continues its growth. The food material which it contains is absorbed by the scutellum and is passed on to afford the plant the necessary nourishment for those early stages when it is too young to feed itself. By the time the first few foliage leaves are well developed, all that remains of the grain is an empty husk.

Comparisons and contrasts.—The examination of these seeds and seedlings will enable the student to see that differences, which at the first glance appear great, are often of only minor importance; while apparently small variations may prove, on closer inspection, to be caused by deeply-seated differences of structure and habits of life. He should always set himself the questions, “In what ways do such and such objects resemble each other; and in what ways do they differ from each other? Which of the differences and resemblances are of most importance?” He should also notice that a mere difference of size is often of very small consequence.

Above all, the student should get into the habit of asking the reasons for the differences and resemblances which he notices in his nature-study. To learn what these reasons are he must observe closely, think carefully, and then make experiments to test the accuracy of his conclusions. “Be sure you are right; then look again”[4] should be his motto.

It is at once plain that the seedlings fall into two classes, according to the number of cotyledons or seed-leaves which they possess. The wheat and maize have only one such seed-leaf, while the mustard, bean, pea, sycamore, and vegetable marrow have two each. We shall see later on that one-seed-leaved plants differ from those with two seed-leaves not only in the number of their cotyledons, but also in the characters of their leaves and flowers and in their method of growth. These differences are so constant and so important that botanists have agreed to call all plants of the first class (such as maize and wheat) Monocotyledons, and plants of the second class Dicotyledons.

One of these differences is that the main roots of dicotyledons are formed directly by the growth of their radicles; while in monocotyledons there is, after a short time, no such main root to be found, but several roots of almost equal size spring from the base of the stem and spread outwards in all directions ([Fig. 17]).

Both maize and wheat seeds contain—outside the embryo—a large store of food called endosperm (Figs. [ 15] and [ 16]), which is not seen in any of the dicotyledonous seeds described in this chapter. This is not a very important difference, for, if we examined a very large number of dicotyledon seeds, we should find that most of them possessed endosperm. On the other hand, many monocotyledonous seeds are destitute of endosperm. Only after observing a very large number of facts is it safe to make general statements.

Confining our attention to dicotyledons, we are impressed by the great variation in size of the cotyledons. Those of the bean and pea are swollen with food material and form a large proportion of the bulk of the seed. As a consequence, the seedling has enough food to enable it to grow into quite a sturdy little plant before it needs any foliage leaves. The cotyledons of the mustard and sycamore, however, are thin, and they unfold almost immediately into green leaves, and set to work to help to maintain the plant until the first foliage leaves can be formed. The cotyledons of the lupine ([Fig. 9]) and vegetable marrow ([Fig. 10]) serve a double purpose. They not only contain a store of food ready to hand, but they also set to work early to make new food, until the new leaves are sufficiently advanced to take up their duties. It should be remembered that cotyledons are makeshift leaves.

EXERCISES ON CHAPTER I.

1. Make a collection of the seeds of various trees; try to find, in each seed, the cotyledons, radicle, and plumule. Which of the seeds contain stored starch?

2. Soak pine and larch seeds in water for several days and then sow them, with a covering of half an inch of soil. Make notes of the number, shape, size, and behaviour of the cotyledons. How large are the seedlings at the end of the first season?

3. Make similar observations on the growth of sycamore, ash, and beech. Cover the seeds with an inch of soil.

4. Plant seeds of oak and chestnut two inches deep, and make drawings and notes of the stages of growth.

5. Investigate the structure and method of germination of a barley seed, and find out whether barley is a dicotyledon or a monocotyledon.

6. Make experiments to discover the effects, upon the germination of various seeds, of differences of temperature, moisture, and light, and write full accounts of the results obtained.

7. Draw from memory a young seedling of maize, and notice its chief peculiarities. (1898)

8. Draw the seedling of the sycamore in two or more stages, and add short notes. (1898)

9. Draw the root of any seedling that you have studied, giving its name. Mark the exact position of the root-hairs. (1898)

10. Open the nut provided. Draw what is to be found in it in one or two positions. Name the parts and give short explanations. (1901)

11. Explain, with drawings, how certain seedlings withdraw their seed-leaves from the seed-coat. (1901)

12. Describe and explain as far as you can the principal changes to be observed during the germination of a bean or pea. (1901)

13. Describe the germination of a bean, and compare it with that of a grain of wheat. (1898)

14. Describe the structure of a grain of wheat, and contrast it with that of an acorn. (1896)

15. Plant seeds in wet, sticky soil (so that the air cannot easily get to them), and compare their growth with that of similar seeds in a light, open soil.

16. Two acorns are allowed to germinate, one in the neck of a bottle full of water, and the other in an ordinary flower pot. What differences will be noted in the two plants as they grow? (Certificate, 1904)

CHAPTER II.
HOW A GREEN PLANT FEEDS.

5. THE FOOD WHICH A GREEN PLANT
OBTAINS FROM THE SOIL.

1. A plant cannot grow permanently in damp sawdust or clean sand.—Notice that the seedlings which were grown in damp sawdust presently wither and die, while those which were grown in soil flourish, and, with proper care, come to maturity. Obtain some clean sand, and, to be sure that there is nothing in it which water can dissolve, wash the sand in several changes of clean water. Germinate some seeds in the sand, keeping it damp. The resulting plants in this case also wither and die. Evidently soil contains some plant-food which the plant cannot obtain from sawdust or clean sand. What is this food?

Fig. 18.—Porcelain crucible heated by Bunsen burner.

2. The amount of water and mineral matter in plants.—Take a healthy plant, say a bean plant, and weigh it. Then dry it thoroughly in the oven and weigh it again. It will be found very much lighter; the difference in weight represents the water which has been driven off. Burn the dried plant. When the flame goes out notice the black charcoal which is obtained. Continue the heating and observe that at last nothing is left but a little grey ash. This experiment can be performed over an ordinary fire by using an old shovel or a tile, but if you can use a porcelain crucible (without lid) and a Bunsen burner ([Fig. 18]) you will get better results. If a chemical balance is available, weigh the ash and compare it with the weight of ash obtained from an ordinary bean seed, such as that which gave rise to the plant you have used.

The ash from the plant is much greater than that got from the seed. This extra ash must have been taken from the soil during the plant’s growth.

3. A nutritive solution.—Make, or ask a dispensing chemist to make, the following solution:

(A few drops of a dilute solution of sulphate, or chloride, of iron should be added.)

Water, with this solution, a plant growing in wet sand, and when it is well grown, dry and burn it. As much ash is obtained as from a plant of the same size grown in soil. Notice the difference between such a plant and one which has had water only supplied to it.

4. Water culture.—Fix two similar young plants in corks as shown in [Fig. 19], and put the corks into two bottles, the first of which contains pure water and the second the nutritive solution, and let the roots of the seedlings dip into the liquids. Cover the outsides of the bottles with rolls of paper to keep out the light. Notice that the plant living in the nutritive solution thrives, while the other presently withers. Dry and burn the former, and observe that it yields more ash than does a seed such as that from which it sprang.

5. Plants obtain their mineral food from the soil by their roots.—As the roots are the only parts of the plant which are in contact with the nutritive solution, or which (under ordinary conditions) are in the soil, the mineral matter must be taken in by the roots.

6. The root-hairs.—Take up a seedling which has been growing in damp sand, and observe the small particles of sand adhering to the root-hairs ([p. 17]). The hairs of a plant’s root and rootlets apply themselves very closely to particles of soil ([Fig. 20]), and the mineral food (dissolved in water) passes into the hairs and so gets into the root and thence to the other parts of the plant.

7. Roots as storehouses of food.—Examine, before the plants flower, the roots of a turnip, a carrot, and a radish, and notice how greatly they are swollen. You know that these roots are valued as foods; of what use do you think the stored food is to the plants themselves?

The food of a young seedling.—When such a seed as that of a bean is germinated in damp sawdust or wet clean sand, and kept in a warm and light place, it puts out a radicle, which grows downwards and becomes the main root, and a stem which grows upwards and bears green leaves. After a time the main root branches, giving off side roots, which spread in all directions through the sawdust or sand. The main root and the rootlets bear very fine fluffy hairs for a short length, which is situated just behind their points ([Fig. 11]), and these root hairs come into very close contact with particles of damp sawdust or sand ([Fig. 20]), the moisture passing into them and thus reaching the main root, from which it is distributed to the various parts of the plant. The stem likewise flourishes, growing in length and thickness, and putting out new leaves.

All this time the young bean plant is living on the food material stored up in its cotyledons ([p. 6]); and if the sand or sawdust is kept moist, with even pure water, this seed food is at first quite sufficient. When at last the seed food is all used up, however, and all that remains of the cotyledons is a shrivelled skin, the plant begins to droop and wither from lack of food.

Plants obtain food from the soil.—Contrast this with the condition of a seedling which has been grown in soil. It still flourishes, even when the seed food is used up, for it is drawing up food from the soil—food which could not be obtained from the damp sawdust or clean sand.

That the plant really has taken up some solid matter from the soil can be proved by a few simple experiments. A plant which has been growing in soil for some time after its seed food is used up is dried and burnt, and the ashes are weighed. The weight of ash or mineral matter thus obtained is found to be considerably greater than that of the ash obtained from an ungerminated seed, or from a seedling grown in damp sawdust or sand which has only been supplied with pure water.

The mineral food of plants.—The composition of the ash obtained from various plants has been carefully determined by chemists, and in this manner they have been able to find out what substances must be present in soil in order that the plant may obtain all the mineral food it requires. A mixture of potassium nitrate (nitre), sodium chloride (common salt), calcium sulphate (plaster of Paris), magnesium sulphate (Epsom salts), calcium phosphate, and chloride (or sulphate) of iron—dissolved in water in the proportions specified on [p. 27]—has been found to supply the necessary elements of the mineral food in a form which the plant can readily use. That such a mixture is capable of supporting the plant, while water alone is incapable of doing so, may be seen by growing a plant—in the manner shown in [Fig. 19]—in this solution. If, in addition, the plant is supplied with light and fresh air, it will grow in a perfectly healthy and normal manner. If any of the constituents (except the common salt) are omitted, the plant will suffer. On the other hand, a plant which is growing in pure water will presently die, from the lack of the necessary mineral food.

Fig. 19.—Plant growing in a nutritive solution
of salts. The bottle should be covered with a
roll of paper to keep out the light.

The work of the roots.—These experiments show that water—of which a large proportion of a plant consists—and the mineral constituents of its food (dissolved in the soil-water) are taken up from the soil by the roots. In ordinary soil the rootlets spread out on all sides, dividing and subdividing, seeking for this very weak solution of mineral salts. Even when soil appears practically dry, a very thin film of moisture covers each little particle of earth, and the root hairs become closely applied to these little particles ([Fig. 20]), so that the water passes through their walls and gradually makes its way to the main root, the stem, and the leaves.

Fig. 20.—Tip of a root hair with adhering particles of soil. (× 240.)

Roots sometimes perform other duties in addition to those of fixing the plant in the soil and providing it with water and mineral food. It is usual, for example, for biennial plants—which produce flowers and seeds in their second year, and then die—to take in much more food during their first season than they require at the time, and to store up the surplus in readiness for the great effort of the second year. These reserve materials are often stored in the roots, which then become swollen and fleshy, like those of the turnip, radish, and carrot.

6. THE FOOD WHICH A GREEN PLANT
OBTAINS FROM THE AIR.

1. Plants contain much carbon.—Char a stick, and notice the black charcoal which is formed. Charcoal is an impure form of carbon.

2. Carbon dioxide gas is formed when wood burns.—Fasten a shaving or a splinter of wood on a piece of wire, light it, and lower it into a clean glass jar. When the wood has burned for a few seconds take it out, and pour a little clear lime-water into the jar. The lime-water turns milky. Similarly, pour a little lime-water into a jar in which nothing has been burning, and notice that it remains clear. There is evidently a difference in the nature of the air of the two jars. The difference is caused by the burning of the wood, during which some of the carbon unites with the oxygen of the air in the jar, forming an invisible gas, called carbon dioxide. Carbon dioxide can always be detected by the milkiness it causes in clear lime-water.

3. Carbon dioxide present in ordinary air.—Pour some clear lime-water into a blue saucer and let it stand exposed to the air for half an hour, then examine it. A white scum has formed on the surface of the lime-water. Stir with a glass rod; the solution becomes milky. The scum and the milkiness are produced by the union of the lime with carbon dioxide from the air. Carbon (in the form of carbon dioxide) is therefore present in the air.

4. No carbon in the food solution.—Examine again the list of elements ([p. 27]), which compose the mineral salts which have been found to replace satisfactorily the food which a plant obtains from the soil. There is no carbon in it. A plant evidently does not depend on the soil for the carbonaceous part of its food. From what other source can a plant obtain its carbon? Carbon has just been proved to be present in the air. Does the plant obtain its carbon from the air?

5. Green leaves contain starch after exposure to sunlight.—Take a green leaf from a plant which has been exposed to the sunlight, boil the leaf in water for a minute or two to kill it. Then put it in methylated spirit until the leaf-green is dissolved out. When the leaf is bleached rinse it in water, then put it into a dilute solution of iodine ([p. 2]) and notice that it becomes blue or purplish brown. The formation of this colour proves the presence of starch in the leaf.

6. Starch contains carbon.—Char a piece of laundry starch and observe the charcoal formed.

7. Green leaves do not contain starch after being left in the dark for 24 hours.—Keep a leafy plant in the dark for 24 hours and then test a leaf as in the previous experiment. No starch can be detected. Put the plant in the sunlight for an hour or two and test another leaf. It contains starch. Plainly, starch is only formed in leaves if they are exposed to light, and any starch previously present disappears when the plant is kept in the dark.

8. A green plant kept in air from which the carbon dioxide has been removed will not form starch in its leaves.—Obtain a large glass bottle, such as those used by confectioners, and fit it with a cork or india-rubber stopper through which passes a glass tube bent[5] as in [Fig. 21]. Care should be taken to make all the joints tight, and it may be necessary to soak the cork in melted paraffin to ensure this. Pack the bend of the tube loosely with pieces of soda lime, and in the bottle place a small jar containing lumps or a strong solution of caustic soda. When the apparatus is ready, place in the bottle a small plant or a leafy twig in water (fuchsia answers very well), which has been kept in the dark for 24 hours. The caustic soda in the jar very soon absorbs all the carbon dioxide which is present, and the soda lime in the bend of the tube prevents any carbon dioxide from getting into the bottle from the outside air. Place the jar in bright sunlight for a few hours and then test a leaf for starch. None can be detected. It is plain that one of the carbonaceous food stuffs—starch—is not formed in the leaves of plants unless the plant is grown (in the light) in air containing carbon dioxide.

9. Seedlings are at first independent of light.—Germinate pea or bean seeds in wet sand or sawdust in the dark. Notice that for some time the seedlings grow almost as well as when in the light. Soon, however, the stem becomes long, weak and straggling, and the leaves are pale in colour, even if the plant is supplied with mineral food.

10. The formation of sugar in a germinating pea-seed.—Take up a pea-seedling when the stem is one or two inches long, and chew the partly shrunken cotyledons. Notice the slightly sweet taste. Contrast this with the taste of an ungerminated seed.

A plant contains much carbon.—When a piece of wood or other part of a plant is strongly heated it first blackens or chars, showing the presence of a large proportion of charcoal or impure carbon. On continued heating, this carbon “burns away.” In the process of burning it unites with some of the oxygen of the air, and forms a colourless, invisible gas known as carbon dioxide. Though this substance cannot be seen, its presence can be easily detected by means of clear lime-water, which, when exposed to the gas, absorbs it and becomes milky owing to the formation of a white precipitate of chalk. If, for example, a splinter of wood is burnt in a glass jar, and a little clear lime-water is immediately afterwards poured into the jar and shaken up, a milkiness at once proclaims the presence of carbon dioxide.

The air contains carbon.—If lime-water is poured into a blue saucer and left exposed to the air for half an hour, a white scum of chalk is seen to have formed on its surface, showing that carbon dioxide is present in the atmosphere. It is important that the student should realise this presence of carbon—as invisible carbon dioxide gas—in the air. Although the proportion is very small, amounting to only 3 parts of carbon dioxide in 10,000 parts of fresh country air, it is of incalculable importance to plants, and indirectly to ourselves and all other animals.

A green plant obtains its carbon from the air.—Since the parts of a plant contain much carbon, and the food which a plant obtains from the soil need not contain any carbon; while the air, on the other hand, does contain carbon, it seems likely that a plant obtains its carbonaceous food from the air. This surmise is confirmed by experiments. One of the most easily recognisable of plant products containing carbon is starch, for it yields a very characteristic blue, or purplish-brown, colour when treated with iodine solution. By means of this test starch can easily be proved to be present in the green leaves of a plant which has been exposed to the air and sunlight. The leaf is first killed by being boiled in water for a minute or two, and then its green colouring matter is dissolved out by immersion in alcohol (methylated spirit). The bleached leaf is rinsed in water and then put in iodine solution, and the blue or purplish-brown colour which is formed shows the presence of starch. There is a marked difference when a leaf, which has been kept in the dark for twenty-four hours, is similarly tested. In this case no starch can be detected.

One compound of carbon—i.e. starch—may thus be recognised easily; and if we found that a leaf made no starch when supplied only with air from which the carbon dioxide had been removed, this fact would be strong evidence in favour of the conclusion that a green plant obtains its carbon from the carbon dioxide of the air. To test this, a large bottle is fitted, by means of a tightly fitting cork or stopper, with a tube containing lumps of soda lime, a substance which eagerly absorbs carbon dioxide from air. A small jar of caustic soda is placed inside the bottle ([Fig. 21]). A green plant or a leafy twig, which has been kept for twenty-four hours in the dark to free it from starch, is then put in the bottle, and the whole exposed to sunlight for a few hours. At the end of this time it is found, on testing the leaves, that no starch has been formed. By this, and other experiments, botanists have proved that green plants obtain all their carbon from the carbon dioxide of the air, and that sunlight is indispensable for the process. We shall examine this question more fully when we study leaves ([Chapter III.]).

Fig. 21.—Experiment to prove that leaves do not make starch
unless the air with which they are supplied contains carbon
dioxide. S, tube containing lumps of soda lime;
S′, jar containing a solution of caustic soda.

The carbonaceous food of a young seedling.—Just as a bean or pea seedling is for a time independent of an outside supply of mineral food—its roots needing only to be supplied with water—so there is enough carbonaceous food also stored up in the seed to satisfy for some time the needs of the growing plant—the stored starch being gradually changed into sugar and absorbed. For this reason a young seedling will live healthily in the dark. When, however, the seed food is exhausted, or nearly so, the plant draws upon the store of carbon, which is present as carbon dioxide in the air, for a renewal of the starch and allied substances which are necessary to it. As it cannot make use of this atmospheric carbon in the dark, it must henceforth be supplied with sunlight or it will not thrive. Plants kept in the dark after their seed-food is exhausted are pale in colour and unhealthy. Their stems grow long and straggling ([Fig. 22]), but are usually too weak to stand upright.

EXERCISES ON CHAPTER II.

1. Make experiments to discover the effects, upon seedlings growing in a nutritive solution ([p. 27]), of each of the following modifications in the composition of the solution: (a) omit the sodium chloride; (b) omit the potassium nitrate; (c) omit the compound of iron; (d) omit the magnesium sulphate; (e) substitute sodium nitrate for potassium nitrate.

2. Explain how it is that a green plant cannot carry on its nutrition in darkness.(1892)

3. What part of its food does a green plant obtain from the air? In what form, and under what conditions, is it taken in? (1889)

4. Describe the method of water cultures, and give the general results of a set of experiments. (1898)

5. Give experimental proof that green plants require to be fed with combined nitrogen. (1897)

6. What are the necessary conditions for the formation of starch in a plant? Mention experiments which support your statements. (1896)

7. Explain the influence of light on a growing plant. Illustrate your answer by reference to the changes in a ripening and germinating bean. (King’s Schol. 1903)

8. How can it be proved experimentally that a green plant draws some, but not the whole, of its nourishment from the air? (1904)

Fig. 22.—Two mustard seedlings of equal age.
E, grown in the dark;
N, grown in ordinary daylight.

CHAPTER III.
THE FORMS AND DUTIES OF LEAVES.

7. THE FORMS OF LEAVES.

1. The shapes.—Make a collection of the leaves of a large number of different plants, for example, elm, beech, lime, oak, birch, ash, blackberry, pine, yew, horse chestnut, rose, holly, woodsorrel, grass. Lay each in turn flat in your notebook and trace the shape of the leaf blade by passing the point of your pencil round the edge. Measure the length and greatest width; write down these dimensions. Is the greatest width at, above, or below, the middle of the leaf blade?

Most of the leaves are flattened green plates. Those of the pine and yew are long and needle-shaped. Do you know any other leaves like these?

2. The veins.—What enables the leaf to keep stretched out? Turn it over and notice the “veins” on the lower side. Do they act like the ribs of an umbrella? Fill in the positions of the main veins in your drawings. Are the veins parallel to each other in any of your leaves? Write a list of as many leaves as you can find which have parallel veins.

3. Skeleton leaves.—Put some leaves in a saucer with a little soft water, and allow them to rot. Clean away the soft stuff from time to time by gently brushing the leaves with an old tooth-brush. Notice the “skeleton” which remains. Skeleton leaves may be made much more quickly by soaking the leaves for some time in a weak solution of bleaching powder. Wash them well before drying.

4. The colour.—Is the green deeper on the upper or the lower surface of a leaf? Which surface usually receives more light?

5. The apex.—In how many of your leaves is the apex (a) pointed, (b) blunt, (c) rounded?

6. The margin.—Examine the edge or margin of each leaf. In how many is it (a) quite plain, (b) hairy, (c) wavy, (d) saw-edged, (e) doubly saw-edged, (f) spiny? Do you find spines on holly leaves which are so high on the tree as to be out of the reach of cattle? What is the use of the spines?

7. Blackberry leaves.—Gather several leaves from a blackberry bush. Notice that in addition to the “saw cuts,” the margins of some are cut into slightly, while others are divided quite to the midrib, the leaf being thus cut into two or more leaflets. Select specimens which form a gradual series between the “simple” leaf and the “compound” leaf (consisting of three or five leaflets) and draw them.

8. The horse chestnut leaf.—Draw the compound leaf of the horse chestnut, and draw an even curved line joining the points of the leaflets. You can imagine that the compound leaf may have been formed by a leaf of this shape being cut into until it was divided into seven complete leaflets.

9. The rose leaf.—Draw an imaginary simple leaf such as may have been the original form from which the compound leaf of the rose was derived. Notice the difference in the arrangement of the main veins of the leaves of the horse chestnut and the rose. Does this account for the leaflets coming off at the sides of the midrib in the rose leaf, and springing from one point, like fingers from the palm of the hand, in the case of the horse chestnut?

10. Sycamore and ivy leaves.—If the large indentations in these leaves were continued to the midrib, would the compound leaves thus formed be of the type of the rose leaf or of that of the horse chestnut leaf?

11. The leaf-stalk.—What attaches the leaf-blade to the stem or branch of the plant? Can you see signs of the main veins joining the top of the leaf-stalk? Do you know any plant with the blades of the leaves fixed directly on the stem, i.e. without any leaf-stalks?

12. Stipules.—Examine the rose leaf again and notice the two leaf-like outgrowths at the bottom of the stalk. These are called stipules. Make a list of as many leaves as you can find which have stipules. How many leaves can you find with a sheath at the bottom of the stalk?

13. The leaf of the sweet pea.—Notice the large stipules of this compound leaf ([Fig. 28]). What are the tendrils? Do you think they may be mainly the larger veins of the upper leaflets? Is the leaf of the type of the rose or of the horse chestnut leaf?

14. Other compound leaves.—Compare and contrast ash, lupine, woodsorrel, strawberry, and other compound leaves with those of the rose and horse chestnut.

A leaf.—The leaves of different plants vary much in size and shape, but in general a leaf is a thin, broad, and more or less oval blade of green colour, attached by a leaf stalk to the stem or branch. In some cases, however, the leaf stalk is absent and the blade is attached directly to the stem or branch.

The veins.—The leaf is kept taut by a number of branching ribs, somewhat as the silk of an opened umbrella is stretched tightly by the ribs. The ribs or “veins” of the leaf run beneath the skin, but are generally nearer the lower surface than the upper, and are easily seen when the leaf is turned over. If a leaf is allowed to rot in a little soft water, the skin and the soft green stuff of the interior decay and leave these veins as a white “skeleton” ([Fig. 23]). The process may be assisted by gently brushing the leaf from time to time. A skeleton leaf may be obtained still more quickly by putting the leaf in a weak solution of bleaching powder until the skin and interior are soft enough to be brushed away. Care should be taken to use a weak solution, or the veins also will be rotted. The skeleton should be well washed in water before drying.

The arrangement of the veins in a leaf varies widely, but it falls broadly into two classes, according as the main veins run parallel or nearly parallel to each other ([Fig. 24]), or form a less regular network ([Fig. 23]). The venation of a leaf is curiously associated with the number of cotyledons possessed by the seedling; for nearly all dicotyledons ([p. 23]) have net-veined leaves, while the leaves of monocotyledons are almost invariably parallel-veined. Careful drawings of several typical leaves should be made, and the principal veins indicated on them.

The shapes of leaves.—Although the blade of a leaf is most commonly flattened, and roughly oval in outline, there are several exceptions. The leaves of pine, spruce, larch, and yew are needle-shaped; those of grasses (Figs. 102-110) are very long in proportion to their width; while the leaves of many moorland plants are rolled up into hollow cylinders. There is some reason—could we find it—for every such variation, and the significance of some of these shapes will be referred to later ([p. 47]). When the dimensions of leaves are carefully measured, the proportion of the length to the width will be found to vary much in the leaves of different plants, but will be found to be pretty constant for the same sort of plant. This holds good, too, for the position of the greatest width (e.g. at, above, or below, the middle of the blade), the form of the apex of the blade (blunt, pointed, spiny, or rounded), the nature of the margin (smooth and “entire,” hairy, saw-edged, doubly saw-edged, lobed, etc.), and the extent and positions of the larger indentations. Thus, while any particular elm leaf ([Fig. 124]) is probably slightly different from every other elm leaf in the world, it resembles every other elm leaf more than it does any leaf from any other plant than an elm. No leaf of this shape ever grows on an oak tree or a sycamore. Thus, in spite of minor variations there is a wonderful conformity to type, and the student will find that by carefully examining the shape, venation, margin, apex, etc., of all his leaves, and above all by drawing them, he will soon be able to recognise them at sight. It is by doing this and noticing in each case the methods of folding and arrangement of young leaves in the bud that it may be possible in the future to explain some variations which are at present not understood. It has already been seen that the peculiar forms of the leaves known as cotyledons are associated to some extent with the shape and size of the seeds containing them, and with the amount, if any, of the food stored in them.

Simple and compound leaves.—Blackberry leaves ([Fig. 76]) well repay close examination. Some of the leaves on the bush will be found to be simple—having one blade only on the leaf stalk. Here and there, however, a leaf may be discovered which is so deeply cut into along one side, that it is almost completely divided into two leaflets; and other leaves will easily be found which consist of three or five leaflets, much resembling the leaves of the rose ([Fig. 25]), a near relative of the blackberry. Here, then, we have a plant which produces simple or compound leaves according to its needs. It seems as if the blackberry were still trying, as an experiment, a device which the rose tree has found so advantageous as to have adopted for good. Some other plants, the ash, for example, have compound leaves broadly similar to the rose leaf—the leaflets springing in pairs from the sides of the midrib.

The compound leaf of the horse chestnut ([Fig. 26]) is of a different type, for the seven leaflets all arise from one point; and the leaves of the lupine ([Fig. 9]) are arranged on the same plan. When the venation is examined, the reason for this becomes plain. In these cases the main veins all diverge from the top of the leaf stalk; whereas in the rose and ash the midrib gives rise to side ribs in pairs. The leaflets are naturally arranged so that one of the larger veins shall support each. The next question arising is, “What causes the differences in the methods of branching of the midrib?” At present this is a mystery. Compound leaves consisting of three leaflets are found in woodsorrel, strawberry ([Fig. 50]), clover, etc.

Intermediate leaves.—The ivy ([Fig. 27]) and sycamore ([Fig. 33]) have leaves which seem intermediate between truly simple and truly compound leaves. From the arrangement of the veins it is seen that they approach the horse chestnut type more than that of the rose. On the other hand, if the deep indentations of the oak leaf ([Fig. 113]) were carried to the midrib, the simple leaf would be divided into leaflets arranged, somewhat like those of the rose or ash leaf, along the sides of the midrib.

Stipules.—At the bases of many leaf stalks, close to the stem, are leaf-like outgrowths called stipules. They are well seen in the rose ([Fig. 25]) and pea ([Fig. 28]). Some leaves have a sheath at the bottom of the stalk, partially enclosing the stem.

Tendrils.—The pea also affords an interesting case of leaflets being modified to do special work. Here the upper leaflets seem to have remained undeveloped except for their main veins, and these have acquired a remarkable power of twining round suitable objects and so supporting the stem. Many other plants have tendrils, but these are not always modified leaflets.

8. HOW LEAVES ARE ARRANGED ON THE STEM.

1. Opposite leaves.—Examine a deadnettle plant ([Fig. 92]). Do the leaves come off the stem haphazard? How many come off at each level? Are both leaves on the same side of the stem, or opposite each other? Are the two leaves at the next level above just over these, or do the directions cross? Do the leaves get as much light or more than they would if each pair were just over the pair next below? How many other plants do you know which have leaves arranged in this manner? Examine leafy twigs of sycamore, horse chestnut ([Fig. 40]), and ash. Are the stalks of the lower leaves of these twigs of the same length as those of the upper ones? Is it any advantage for the lower leaves to have longer leaf stalks?

2. Box leaves.—What is the arrangement in the box? Examine particularly the young leaves near the end of the twig. Are the lower ones twisted? Can you suggest a reason for this twisting? Can you find any twigs in which no twisting has taken place? Are these untwisted twigs so placed that they are equally exposed to the light on all sides?

3. Alternate leaves.—Examine the arrangement of the leaves on a wallflower stem. They come off alternately, each springing from a rib on the stem. How many ribs are there? Look at the bottom of the stem, where the leaves have fallen off, and notice that each has left a scar. Mark one of the scars with your pencil and then count how many scars you pass before coming to another on the same rib. How many times do you wind round the stem in doing this? You pass five leaves and wind spirally round the stem twice. This is always the case in the wallflower.

Examine leafy twigs of oak and pear trees. Here, too, the leaves are alternate, and every sixth leaf is above the first, and a line joining all the leaf-bases or scars between the first and sixth leaves would wind spirally round the stem twice.

What is the arrangement in the elm and lime?

4. Leaves which form a rosette.—Examine plants of primrose ([Fig. 81]) and daisy. The leaves in these cases spring from close to the ground and form a rosette. Notice that the bottom of the leaf blade is much narrower than the upper part. Is any saving of material obtained by this arrangement?

5. The position of branches and buds.—Look in the upper angle between a leaf and the stem in all your specimens. This angle is called the axil of the leaf. Can you see a bud in the axil of the leaf? Can you find that a bud or a side branch ever arises in any other position? (The former positions of fallen leaves are marked by scars.)

To the beginner in nature-study leaves seem in the majority of cases to be arranged on the stem of a plant in a haphazard and confusing manner, and it is only after very careful observation that a definite order and regularity is seen to be always maintained.

Nodes and internodes.—The level at which a leaf springs from the stem is called a node (Lat. nodus, a knot), and the length between two consecutive nodes is called an internode (Lat. inter, between).

Opposite leaves.—It is best to begin the study of leaf-arrangement by examining some such plant as the deadnettle ([Fig. 92]). The leaves come off in pairs: two at the same level, set opposite to each other. The next pair above or below springs from the stem in a direction at right angles to the first—a device which allows the leaves to get a more equal share of light than if each pair were placed directly over the next below.

A similar arrangement is adopted by various other plants, including the horse chestnut ([Fig. 40]), sycamore ([Fig. 34]), box, privet, etc., but in some instances it is disguised. Box twigs afford an interesting example of this. Those twigs which are equally, or almost equally, illuminated on all sides, have their leaves arranged in pairs at right angles to each other like those of the deadnettle. Some twigs, however, receive the light from one direction only, and in these cases the leaves turn themselves until they face the light; so that at a casual glance the pairs of leaves seem to lie all in the same plane. One only needs to examine the end of the twig, where the leaves are just unfolding, to see that the arrangement is really in pairs alternately at right angles. In the case of the privet the efforts of the leaves to face the light often cause the stem itself to be twisted between the leaf-levels.

The alternate, or spiral, arrangement.—Perhaps the commonest leaf-arrangement is one in which only one leaf is given off at any particular node, the next leaf being a little further round the stem, and so on. As a result, an ink line or a piece of thread joining the leaf-bases would wind spirally round the stem. In the case of the wallflower, oak, pear, and many others, such a line would wind spirally twice round the stem before coming to a leaf vertically above the first, and in so doing it would pass five leaves. This may be shortly described as the ²/₅ arrangement. A less common one is ⅜, where in winding spirally round the stem 3 times, 8 leaves would be passed.

Efforts of leaves to obtain light.—It would be difficult to imagine any order of insertion which would secure a more equal distribution of light to each leaf than the spiral arrangement; but here also cases of leaves twisting in order to face the light are not uncommon. Lime leaves very often turn for this reason, so as to lie in almost the same plane—adopting a device similar to that of the box and privet described above. Further, the lime leaves arrange themselves at such angles that there is very little overlapping. Elm twigs also often exhibit similar instances of a mutual accommodation of leaves to each other’s light supply.

The lower leaves on a horse chestnut twig have longer stalks than those nearer the end ([Fig. 40]). This enables the leaves to stand well out to the light and escape the overshadowing of those above.

The positions of branches.—A branch of the stem of a flowering plant always arises as a bud in the upper angle between a leaf and the stem. This position is called by botanists the axil of the leaf, from the Latin word axilla, the arm-pit. Clearly, then, the arrangement of the branches is primarily dependent upon that of the leaves, and we shall, for example, never find “opposite” branches on a tree which bears its leaves on the “alternate” system. It is easy to notice, however, that not all the buds develop into branches. In other words there are many buds which remain dormant, and the final arrangement of the branches is often somewhat irregular on this account. But wherever an ordinary bud or a branch occurs, we may be perfectly sure that there was once a leaf immediately below, even if the leaf-scar can no longer be seen.

Economy of leaf surface.—All these things seem to indicate that a good supply of light is of the greatest importance to leaves, and this conclusion is supported by the fact that leaves are usually either narrow or actually cut away in places where the light cannot reach them. The leaves of the daisy and of the primrose ([Fig. 81]), for example, all spring from nearly the same point, and form a rosette. Evidently there would be a certain amount of overlapping at the leaf-bases, unless the blades there were very narrow, as they are. Again, the greatly-indented leaves of the ivy are often arranged so that a point of one leaf fits over an indentation of another—a beautiful example of plant economy.

9. THE WORK OF LEAVES.

1. In sunlight leaves make starch.—Expts. 5, 7, and 8 (Sec. 6) have already proved (a) that leaves of a plant growing in ordinary air and exposed to the sunlight make starch; (b) that in the dark this starch somehow disappears; (c) that in air destitute of carbon dioxide leaves are unable to make starch even in sunlight.

2. The parts of a leaf which are not exposed to light do not make starch.—Keep a plant, say of tropæolum—or, if not convenient, a single leaf ([Fig. 31])—in the dark for 24 hours to free the leaves from starch. Split a small cork and pin the halves on opposite sides of a leaf, and then expose the plant to bright sunlight for an hour or two. (If a single leaf is used let the end of the stalk dip into water.) Take off the cork, kill the leaf with boiling water, dissolve out the green colouring matter with methylated spirit, rinse, and test with iodine solution. The part from which the light was excluded remains bleached, and therefore contains no starch; while the rest of the leaf becomes blue or purplish brown owing to the presence of starch.

3. Parts of a leaf which are not green do not form starch in sunlight.—Take a variegated leaf from a plant (e.g. the variegated geranium or maple) which has been in bright sunlight for some hours. Apply the usual test for starch. The parts which were originally green contain starch; the originally white parts remain bleached.

4. Leaves supplied with carbon dioxide, and exposed to sunlight, give off oxygen gas.—(a) Take a bunch of fresh watercress or any green water-weed and put it in a beaker or glass jar. Cover the plant with an inverted funnel which is shorter than the beaker. Now fill the beaker with ordinary tap water or river water (not distilled water), so that the end of the neck of the funnel is covered. Completely fill a narrow test tube with water, close it with the thumb, and invert it over the neck of the funnel. If this has been done carefully the test tube will still be full of water. Expose the arrangement ([Fig. 29]) to bright sunlight, and notice the bubbles of gas which are given off from the plant and collect at the top of the tube. When a few inches of gas have collected, raise the test tube, close it with the thumb whilst still under water, and hold it mouth upwards. In the meantime, light a splinter of wood with the other hand. When it is well burning, blow out the light, remove the thumb from the test tube, and plunge the glowing splinter into the gas. It bursts into flame again, showing that the gas is oxygen.