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THE STUDY OF PLANT LIFE

First Impression October 1906.
Second Impression February 1907.
Second Edition 1910.

BY THE SAME AUTHOR

ANCIENT PLANTS

BEING A SIMPLE ACCOUNT OF THE PAST VEGETATION OF THE EARTH AND OF THE RECENT IMPORTANT DISCOVERIES MADE IN THIS REALM OF NATURE STUDY

Illustrated. Demy 8vo, 4s. 6d. net

“Miss Stopes’s book is an enterprising and able attempt to popularize a difficult subject. The really keen student will undoubtedly be stimulated to pursue the study of fossil plants further, and even those who are not students will get some new ideas and derive a certain amount of interest from a book which is sometimes brilliant but never dull.”—Nature.

“Dr. Marie Stopes has made a name for herself in this special line. Anyone who takes an intelligent interest in the subject cannot fail to be charmed with the pleasant manner in which Dr. Stopes conveys her information.”—Athenæum.

PLATE I.

COMMON BRACKEN FERN (see p. [133])

THE STUDY OF
PLANT LIFE

BY
M. C. STOPES
D.Sc.(Lond.), Ph.D.(Munich), F.L.S.
Lecturer in Palæobotany at the University of Manchester

Second Edition

LONDON: BLACKIE & SON, LIMITED
NEW YORK: D. VAN NOSTRAND COMPANY
1910

PRINTED AT
THE VILLAFIELD PRESS
GLASGOW

PREFACE

As a result of the present efforts to raise the standard of education in this country, many different “Methods of Teaching” are receiving our grave consideration. So insistent are their advocates, that we stand in some danger of forgetting that learning, rather than teaching, is the essential factor in education. It is not the knowledge given us ready-made by the teacher, but that which we learn, acquiring it by our own efforts, which enters into our being and becomes a lasting possession.

Therefore this little book does not pretend so much to teach as to act as a guide along the road for those who desire to learn something about the plants around them; hence it points out how much they can easily see for themselves of the wonderful life and work of the silent plants.

It is planned for children, whose quick sympathies are more readily drawn towards the life of things than to the dry facts of morphology or classification. Its “Leitmotif” is therefore the story of life, and those of its activities which find expression in the plant world. Perhaps it may serve to awaken interest in some older people who have not yet been initiated into these mysteries.

As is inevitable, most of the actual facts in this book are already the common property of botanists, though some of the suggested work, such as the mapping, is only now being adopted by the Universities.

The most interesting subjects are often left out of the more elementary books, or even if given are frequently set forth in such a lifeless and pedantic fashion, that little real interest or understanding has been awakened in the young student. The present work attempts to avoid the time-worn methods of arranging the subject. Children generally know more about the behaviour of animals than that of plants (being themselves animals and frequently having kittens or other pets); hence, the parallels between the life-functions of plants and those of animals are pointed out whenever possible. Once the idea of their “livingness” has been fully realised, it is time to go on to the study of the details of the plant’s body, and then to the communities of plants which grow together. In this way the child can work out from its own observations a complete and logical idea of the living plant, instead of having merely acquired a detailed but fruitless knowledge of barren facts.

To burden a child’s memory with long names is not only useless but harmful, therefore an effort has been made to use only short and simple words. A few scientific terms are introduced where they are really of value as describing things which are not generally noticed, and so do not come into the usual English vocabulary. In such cases it is far better for the child to learn the correct scientific name than to be provided with a clumsy translation consisting of several English words which can never give the precise meaning.

The use of a microscope is not to be recommended for those beginning the study of plant life, and the chapters have been planned so that no greater magnification than that of a good hand lens will be needed. This, however, makes it difficult to explain the life histories of the fern and other primitive plants; hence in the chapters bearing on them stress has not been laid on many of the fundamental points which are only to be seen with the microscope, but on those facts which can be observed without it.

The chapters on the families of plants attempt to bring out the reasons for the separations of the few great groups only; detailed classification of the flowering plants has so long been considered the chief part of botany, that it is to be found in nearly every schoolbook on the subject.

If this book should be used as the text-book for young children, the teacher will probably find it necessary to enlarge on the instructions for the work suggested in the last three chapters, which were added chiefly for the guidance of those who may assist the youthful students in carrying out the practical work therein outlined.

I sincerely hope that those who wish to learn, and are prepared to study the plants themselves, may get some help from this little guide-book.

M. C. Stopes.

The University, Manchester,
July 1906.

PREFACE TO SECOND EDITION

The public and the critics have been so kind to the first edition of this book that I am encouraged to offer them a second. There are no considerable changes in it, but I have profited by some suggestions regarding points of detail which several friends have been good enough to offer, and hope that the book has now fewer blemishes, and will be more useful. In Chapter XXXIV. two interesting photographs of drowning trees have been added, which illustrate a problem in Ecology less generally studied than its converse.

It has been very pleasant to hear from many teachers, some in distant parts of the earth, that the book has been useful to them, and I hope they will continue to allow me the privilege of their criticism or appreciation.

M. C. Stopes

The University, Manchester,
October 1910.

ACKNOWLEDGMENT

For the right to reproduce the photographs I am much indebted to the following gentlemen, to whom I express my warm thanks: viz. to the Rev. J. S. Lea, of Kirkby Lonsdale, for Plate VII. and fig. 149; to Prof. F. W. Oliver, of London, for Plate VI.; to Dr. O. V. Darbishire, of Manchester, for Plate IV. and fig. 130; to Prof. K. Fujii, of Tokio, for Plate III.; to Dr. F. F. Blackman, of Cambridge, for fig. 144; to Mr. Crump, of Halifax, for fig. 140; to Mr. R. Welch, of Belfast, for Plates I. and V., and fig. 138; to Dr. H. Bassett for figs. 154 and 155.

To Dr. W. E. Hoyle of Manchester, and to Miss Mary McNicol, B.Sc., I am also much indebted for their kindness in reading the proof-sheets.

I have drawn all the text illustrations specially for this book.

M. C. S.

CONTENTS

PART I.
THE LIFE OF THE PLANT

CHAPTER I.
INTRODUCTORY

Many people do not realise that plants are alive. This mistake is due to the fact that plants are not so noisy and quick in their ways as animals, and therefore do not attract so much attention to themselves, their lives, and their occupations.

When we look at a sunflower, surrounded by its leaves and standing still and upright in the sunlight, we do not realise at first that it is doing work; we do not connect the idea of work with such a thing of beauty, but look on it as we should on a picture or a statue. Yet all the time that plant is not only living its own life, but is doing work of a kind which animals cannot do. Its green leaves in the light are manufacturing food for the whole plant out of such simple materials that an animal could not use them at all as food. Even its beautiful flower is creating and building up the seeds which will form the sunflowers of the future. All animals directly or indirectly make use of the work done by plants in manufacturing food, for they either live on plants themselves, or eat other animals which do so.

Plants are living, and therefore require food of some kind as well as air and water in the same way, and for the same purposes as do animals. As a rule, we cannot see them breathing and eating, but that is because we do not look in the right way. In our study of plants we must first learn how to see and question them properly, and when we have done this they will show themselves to us and tell us stories of their lives which are quite as interesting as any animal stories.

Now the sunflower we have just thought of is probably growing in a garden well looked after by a gardener, who sees that it gets all the light and water and just the kind of soil it needs. It is therefore protected and cared for to a certain extent, but who looks after the wild plants which manage to grow everywhere? These have not only their own lives to live, but by their own efforts must overcome difficulties which are not even felt by the cultivated ones.

They succeed in a wonderful way, and some plants manage to grow under very difficult conditions, even in places where they get no water for months under a burning sun, or in forests where the overshadowing trees cut off the light and rain, or under the water where they get no direct air. They have to do all the usual work of plants, and at the same time struggle against the hardships of their surroundings. They are like men fighting for their lives with one hand and doing a piece of work with the other.

The result of this is that they sometimes make themselves strange-looking objects, and in some plants which have had a very hard struggle it is difficult to know which part of the plant is which. Look, for example, at a Cactus (see fig. 48), which grows in the desert; it appears to have neither stem nor leaves like an ordinary plant, and to consist merely of a roundish green mass covered with needle-like prickles. Yet when you come to study the Cactus you will find out that the thick, fleshy mass is really its stem, and the prickles its leaves which have taken on these strange shapes. By means of its unusual form the Cactus can live where our common plants would die of the dry heat in a day or two. The power plants have of changing their bodies so as to fit themselves to live under all kinds of conditions is one of the strongest proofs that they are alive.

All the parts of plants have some special life-work, just as we have legs and arms for different purposes, and every part is formed in some way to suit the needs of the plant and help it to get on well in its home.

The main thing to realise at the beginning of the study of plants is that they are living things, and therefore to try to discover the importance of the shape and arrangement of all their parts and their relation to the life of each plant as a whole.

We will begin by looking carefully for all the signs of life in them, and noting how often these are the same as those of the animals, even though the whole plant-body is so different from that of an animal.

CHAPTER II.
SIGNS OF LIFE

Fig. 1. Jar (A) with well-fitting cork, in which young bean plants are growing. The tube leading from the jar dips into dish of water (s) which has risen to levels marked in the course of three days. (b) Small tube of caustic potash.

If you were asked to give the signs of life in an animal, it is likely that you would think at once of its power of breathing, eating, growing, and moving. Now when we ask the same question about plants the answer does not appear to be quite so easy to find, because at first sight plants do not seem to do any of these things except the growing. However, the same answer would be quite correct for plants, as well as animals, for they are really able to breathe, eat, grow, and move; all you have to do is to watch them in the right way to see that this is the case.

We are not in the habit of treating dry seeds as though they were alive; beans are stored away in sacks all the winter and may be left for months in dry cellars, and the precious seeds which will give us our beautiful flowers in the summer are put away in boxes through the winter. Yet you know that if you place seeds in the earth and keep them warm and moist, little plants will come up and will grow. What gives them the power of growth which is not possessed by the stones and earth around them? Warmth and moisture alone could not put this power into the seeds when we planted them. This power, which only belongs to living things, was there all the time, but was lying asleep, shut in and protected so that it was not easily disturbed till suitable conditions made it time for it to wake.

You know when you are asleep that you do not eat or run about, but simply lie still and breathe. This is what the seed was doing before the baby plant began to break through its protecting coat and show itself to the world as a living thing.

Let us watch some of these young plants just waking up to activity, and see if we can find in them the four signs we take as being the tests of animal life.

First let us see if we can show that they breathe.

You know that when you breathe you take air into your lungs, use some of it, and give the rest out. You can show that plants also use up some part of the air. If you would actually prove this to yourself or anyone else, take some peas or beans, soak them in water, and leave them in damp sawdust for a day or two till the tiny plant has just begun to show. Then put them on wet blotting-paper in a jar which has a very well-fitting cork with no leakage, and through which a fine bent glass tube is fitted. Place a small tube of caustic potash in the jar. Then place the end of the bent tube in a dish of water, which acts better if you have dissolved some caustic potash in it (see fig. 1). Once it has begun to rise in the tube, mark the level of the water with a small label. If then you mark it daily the labels will show how much water has risen each day, and the amount of water rising in the tube shows us the amount of air which has been absorbed by the growing beans.

This tells us, therefore, that air is absorbed by plants in the course of their growth. But there is another thing we must notice about breathing which is equally important.

You will find that you yourself, as well as all animals, not only use up a part of the air, but also give out a waste product which we call carbonic acid gas. You can see one of the characters of this gas from your lungs if you take a jar of lime water and breathe into it for some time. Compare this with a similar jar of lime-water through which ordinary air has been pumped at about the same rate for the same time, and you will see that the one you have breathed into has gone very much more cloudy-white than the other (see fig. 2). The cloudiness in jar A is caused by the waste gas (carbonic acid gas) which you breathe out, and which combines with the lime in the lime-water to make solid grains of chalk. Fine white chalk grains always form in lime-water when this gas is present, so that a jar of clear lime-water is a very good test for the presence of the gas.

Fig. 2. Jar A contains lime-water through which human breath has passed. Jar B, lime-water through which ordinary air has been pumped for the same time. Note how much greater is the milky deposit in A than in B.

The giving out of carbonic acid gas is one of the most characteristic things about animal breathing, and we can show that plants in breathing give out this gas too.

To prove this, take another jar with a well-fitting cork, and put some beans and peas, which are just beginning to grow, into it, with a little damp blotting-paper to keep them sufficiently moist. Leave the jar closed for a day or two and then open it and quickly and gently pour in some lime-water. Put the lid on again at once and shake it up. You will find that the lime-water turns quite milky, showing that the same waste gas is given out by the plants as was given out in your own breath.

These experiments show us that plants breathe in a part of the air, and also breathe out some of the same waste gas which is given off by animals in breathing. So that we have found that plants do breathe.

Now to go to the other signs of life. I think you will hardly need to do any special experiment to show that seedlings grow into big plants, you must have seen it so often for yourself in the woods and fields and gardens.

We have still to show that plants eat and move, but before we can do this properly, we must learn a little more about the parts of the bodies of the plants themselves, for they have quite a different set of organs to those we are accustomed to in animals, and their way of eating is so different from that of animals that we cannot understand it immediately.

CHAPTER III.
SEEDS AND SEEDLINGS

If we wish to follow the whole life of a plant, we cannot do better than begin by watching the baby plant “hatching” out from its seed at the beginning of its active life.

There are many seeds which would be good to begin work on, any kind would be interesting, but it is best to use some nice big ones which allow us to see the parts easily. Good ones to choose would be broad beans or peas. Notice first the size and shape of the dry seed of the bean, make a drawing of it, and then place it in water. After a few hours you will see that the outside skin wrinkles up; this is because the skin absorbs water and increases in size, and so becomes too big for the rest of the seed (see fig. 3, A, B). After the water has soaked right into the substance of the seed you will find that the outer skin fits again and is once more smooth, and that the whole seed is larger than it was before it was soaked (see fig. 3, C).

Fig. 3. A single Bean seed, A dry; B half soaked, when the skin wrinkles; C fully soaked and swollen.

Take one of these soaked beans and examine its structure. Notice the black mark where it was attached to the parent pod, and the little triangular ridge pointing towards it (see fig. 4, A). Now carefully peel off the skin, noticing that there are two skins, an outer thick one and an inner thin one, which protect the parts within. When you have removed the skin, you will find that the inner portions of the seed split very readily into two thick fleshy parts, and that lying between them is a tiny young plant. Notice how this young plant is connected on either side with the fleshy parts, so that to separate them you must tear one side or the other as in fig. 4 B, where at (a) we see the scar left where the tiny plant (p) was torn from the side. The two big fleshy parts are really portions of the young plant, and are in fact its two first leaves, but they are very different from ordinary leaves, and are packed with food substances, and are called the cotyledons, or “nurse-leaves.” Notice also the tiny root of the baby plant or embryo, as it is called; it bends a little to the outer side, and fits into a kind of pocket in the skin of the coat. You can see the shape of the root even from the outside of the dry bean (see fig. 4, A (r)). You will find in the pea, cucumber, and many other seeds, that there is also the tiny embryo with its two nurse leaves, the whole being protected by strong coats. The differences between the bean, pea, and cucumber seeds are only in the details of shape and colour, not in the actual parts of the seed.

Fig. 4. A, outside of Bean; (h) black scar showing where the bean was attached to the pod; (r) ridge made by young root; B, bean split open; (n) nurse leaves; (p) baby plant; (a) scar where the baby plant was separated from the nurse leaf on that side.

In the case of maize and corn, however, you will find that the seed does not split into two equal parts like the bean, but that the young plant lies at one side of the seed, and a solid white mass fills the rest of the space (see fig. 5). There are also differences in the seedlings which you will notice when they begin to grow.

Fig. 5. A, outside of Maize fruit, showing the embryo (e) on one side; B, sprouting plant, showing the root (r) and shoot (s); C, the same further grown.

Now that you have examined some seeds, you should start a number growing, so as to have plenty to watch. They will grow more quickly if you soak them in water for a night before you plant them in damp sawdust, and keep them moist and fairly warm all the time. You should have a number of seeds of each kind planted together to provide enough for you to dig up one of them every day and examine it fully, inside as well as out. Make a drawing of each one so that you will have a complete series of drawings showing how the young plants grow. This will kill them, so that you must leave at least one seedling which is never touched, and which you can watch all through its life.

Fig. 6. Growth of Bean seedling: A, the root only showing; B, the root lengthening and shoot appearing.

As the young plant grows, notice how it breaks away from the protection of its nurse leaves; first the root comes out and bends downwards into the sawdust (see fig. 6 A), then the little shoot which bends up into the air.

Whichever way you plant the seeds you will find this is always the case, for even if you start with the root pointing up, it will bend round and grow downwards while the shoot bends up (see p. [41]).

As the plant gets bigger, side roots grow out from the main one, and the little leaves of the shoot begin to open out—the whole plant is growing (see fig. 7).

Fig. 7. Later stage in the growth of Bean seedling; side roots developed, and the shoot enlarged.

Now we may perhaps begin to find out something about the question of feeding in plants. What are the nurse-leaves doing all the time the plant is growing? You will find in the bean that the seed coats may split open a little, but that on the whole the cotyledons remain all the time enclosed in them, and attached to the young shoot (see fig. 7). Examine the nurse leaves of seedlings of different ages, and you will see that they are much less thick and fleshy in the older seedlings. As the plant gets bigger the nurse-leaves get thinner and less until they become merely dry shrivelled remnants.

Now, what use could the cotyledons be if they only shrivel away?

Take a freshly soaked seed and cut a thin slice of the nurse-leaves and drop it into a little solution of iodine;[1] the tissue will go a violet blue colour. Then drop iodine on a piece of bread, a piece of potato, and some boiled rice, and you will find that they also go blue, or almost black. The food in the nurse-leaves is in some ways the same as that in bread, potato, and rice, and in many other things we eat.

The part of the food which goes blue with iodine is starch, and this blue colouring of starch with iodine is an easy and safe test for it. You will see the same colour if you take some ordinary laundry starch and stir it up with hot water and a little iodine. Look now at the corn seed; the white solid mass in the seed contains starch, as you can prove with iodine, and although it is not in the cotyledon, yet it is quite near the young plant, which can get at it easily.

We have found, therefore, that young plants have a store of food in their nurse-leaves, or near them in the seed, and that this food is the same as very much of our own food, that is, it is starch. There are other food substances present, too, but they are more difficult to find. The seed, therefore, contains not only the young living plant itself, but also a storehouse of food for its use, and as the plant grows we see this store getting less and less in the shrivelling cotyledons. This shows that the young plant uses up this food in the course of its growth.

But you must not forget that, although we find the young plants provided with food in this way, we have not yet settled the question of the food supply for all plants. As we see, the cotyledons shrivel up and are emptied of their store long before the plant is full grown. Remember that baby calves have milk for food, while old cows have grass. And when the store of food supplied in the seed is finished the older plants must find new supplies for themselves.

In growing seedlings you must always keep them well supplied with water, the soil or sawdust in which they grow must be kept moist. If you take one out of the sawdust and try to grow it only in the air, you will find that it soon dies. Even for the seedling the storehouse of food is not enough; it requires to have water too.

You can keep seedlings growing quite well, however, if you place them in glass jars so that their roots are in water, or even in closed glass jars standing over water, so that the air is thoroughly moist. You will then be able to see very well numbers of fine white hairs on the roots (see fig. 8). These hairs are very important and absorb the water which keeps the whole plant moist.

Fig. 8. Maize seedlings growing enclosed in damp air, supported on a wire stand over dish of water so that their roots do not touch it, but grow in the air. Notice the “root hairs” growing out from the roots.

You have now seen that seedlings require water for their life just as animals do; and also that young plants are provided by their parents with a store of food which is largely starch, and which they use up during their early growth.

CHAPTER IV.
FOOD MATERIALS OF THE OLDER PLANT
(1) IN THE SOIL

As we have just seen, young seedlings are supplied with stores of food, starch, and other things, which are packed in their cotyledons and are used up by them as they grow. But we also saw that as the plant gets older these stores get emptier, and finally the nurse leaves shrivel up entirely when their contents are exhausted. All the same, however, the plant continues to grow. Surely it cannot do this on nothing, any more than an animal could? When the young calves cease to be fed with milk, their food changes, and they begin to eat grass; this gives them individually more work, for grass is not a “prepared food” like milk. Very much the same thing happens with seedlings. Their prepared food supply gets used up, and they must find food for themselves. Where do they find it?

When you remember the fine hairs on the young parts of roots which absorb water from the soil or sawdust, it is quite natural to think at once of the soil as a possible place for them to find their food; and, indeed, this is partly the case. The water in the soil is not perfectly pure, for there are many different “salts” dissolved in it. By “salts” one does not mean only table salt, but also any kind of mineral in solution, such as salts of iron or portions of chalk or limestone, or even some of the minerals which make up granite. These may all be dissolved in rain-water just as sugar is dissolved in your tea, and so spread equally through it. As the water enters the roots of plants through the hairs, these dissolved salts come in with it, and so get distributed over the whole plant. The root hairs cannot “eat” particles of soil, but they twine in among the fine grains and absorb the little films of water which cling to them.

Fig. 9. Root hairs growing among soil particles.

(Much magnified.)

You can find out some of the importance of these mineral salts in the life of the plant, if you do the following experiment.

Take several seedlings which have already grown enough to have nearly exhausted the supply of food in their cotyledons. These you must grow in jars of pure distilled water, to which you have added certain salts which have been found to be the important ones in the soil water and plant food. By giving the plant nothing but these salts and distilled water you know just what it gets. Distilled water is made by catching and condensing steam, and it has no salts dissolved in it; while ordinary tap water has run off some mountain side or risen in some spring from the rocks, and it has many salts in it already, so that it is useless for this experiment.

Take three big glass jars, each with one litre of distilled water, and label them A, B, and C. Into A put nothing further, into B put the following salts, which have been weighed out carefully either by you or by a chemist:—

then add to C all these salts, and also one or two drops of a dilute solution of iron chloride.

Into the jars fit corks which are split, with a hole in the centre, and pack a plant into each with the part of the stem between the corks wrapped round with cotton wool (see fig. 10), and so fix the plant that its roots are in the solution and its stem and leaves in the air[2] (see fig. 11). Wrap black cotton or paper round the jars so as to keep the roots dark as they would be in the soil.

Fig 10. Plant packed in split cork. (h) Hole in cork; (c) cotton-wool packing the stem.

Do not use too small vessels; in fact, if you had bigger jars and took double quantities of everything it would be better.

You may make the experiment more complete by preparing a whole series of solutions with one of the salts left out each time. In this way you would be able to see the effect of the different elements on the growth of the plants, and you would find nitrates are very important. Put a plant, similar to the one you are experimenting with, into a pot of soil or the garden, and keep it well watered. This is called the “control plant.”

Very soon you will find that the plant in jar A (the one with only distilled water) is not growing so fast as the others, and after a time will die off completely. The one in jar C with all the salts, on the other hand, should grow quite as well as the control plant in the garden, which you should take as the standard.

The plant in jar B, when it has everything but iron, should act in a curious manner. At first it should grow all right and outlive the one in distilled water, but after a time its leaves should get paler, till the new ones formed are quite yellow instead of green, and soon after this the plant will die. If, however, you add two drops of the iron solution before it dies, it may recover, become green again, and go on living. It turned a whitish yellow colour because there was no iron in its supply of salts and water. Just as when people get pale and white the doctor orders them iron, so it is necessary for plants to have iron when they begin to lose their green colour. Later on you will find how very important the green colour is, for without it they cannot grow (see Chap. VI.).[3]

Fig. 11. Three jars in which seedlings of the same age are growing; A, in distilled water; B, in the food solution without iron; and C, in the complete food solution.

From these experiments you see that it is not the soil which is necessary to the plants, but that certain salts in solution in the water held by the soil particles are very important. When all the salts are present in the water, as was the case in jar C, the plant can grow just as well as one in the soil; but when it has not these salts it must die. The salts in solution, therefore, must be a very important part of the food. Are they the only food the plant gets?

CHAPTER V.
FOOD MATERIALS OF THE OLDER PLANT
(2) IN THE AIR

The experiments you have just done show that plants absolutely require the mineral salts dissolved in the water of the soil or of their food solutions. Yet although these salts are so necessary, they do not use a large quantity of them, as you may prove by taking the solution C, which is left after the plant has grown in it, and slowly drying off all the water (taking care not to destroy a part of the salt crystals) by gentle heat, and then weighing this dry salt, and comparing its weight with that of the salts you put into C. You will find that the growing plant has only removed a small quantity of the salts. Yet the plant should have grown to some considerable size. Of course, the water itself goes into the plant tissues, but you can drive this off by gentle heat. Before drying it, however, cut off a part of the plant which is equal in weight to the weight of the young plant you put into the food solution at first (see p. [16]), so that you have only to deal with the amount of its growth while using the food solution. Then if you weigh the fully dried plant, you get the weight of the solid structure added to its body while it was growing in the food solution, and you will find that this is much heavier than the amount of the salts it used during its growth.

What is this extra substance?

Now let us examine the dried plant more carefully. Heat it on an open dish, and you will find that it goes black and chars, very like the charred wood on a fire or specially prepared charcoal. The black charcoal is well known to consist chiefly of carbon, and so does this black plant-ash. You know that charcoal can burn, and so will this charred plant if you heat it more strongly. Although you can burn the carbon (that is, you can make it combine with oxygen gas and go off in an invisible form), yet you cannot absolutely destroy it. Like all elements it is not to be made or destroyed by us, nor can the plant make carbon for itself.

If you examine the list of substances you put into the food solution once more, you will find that carbon is not among them, nor is it contained in any of them.

Carbon, then, is the extra substance which makes the weight of the plant greater than that of the salts used from its food solution.

Where does the plant find this carbon?

You may know that there are three chief gases in the air: oxygen and nitrogen, which are the important parts for our breathing, and a little carbonic acid gas, which you may remember is breathed out by animals and plants (see p. [6]), and is made of carbon joined with oxygen. As there was no carbon in the food solution, and the plant was surrounded by air containing carbon and oxygen in the invisible form of gas, the idea is suggested that perhaps it is from the air that the plant gets its carbon. Now let us see if this is true by trying the effect of removing the carbonic acid gas from the air in which the plant is growing.

To do this we must set up an apparatus which will allow only air freed from carbonic acid gas to surround the plant. Such an apparatus is shown in the figure 12. The plant is grown in the closed bell jar D, which stands over the dish C filled with lime-water, which prevents carbonic acid gas entering through the cracks between the foot of the jar and the table. All the air which enters the jar D must come first through jar A, which is filled with a solution of caustic potash that has the power of absorbing the carbonic acid gas, and then through jar B with lime-water. You can draw plenty of air through jar D for the use of the plant by sucking at the indiarubber tube G, which must be carefully shut with a clamp when you stop the current. The bell-jar D will now be filled with air which is quite free from carbonic acid gas, and the small quantity which is breathed out by the plant itself will be absorbed by the lime-water in dish C. Place the whole in a light or sunny position, and change the air every day or two in the way you filled it, that is by drawing at G so that the fresh air comes in through A and B, and is free from carbonic acid gas.

Fig. 12. Apparatus used to keep a plant without any carbonic acid gas. A, jar of caustic potash, B, jar of lime water, which absorb the carbonic acid gas, through which all the air entering jar D must pass; C, basin of lime water to absorb any of the gas given out by the plant growing in D; G, indiarubber tube which can be closed or attached to a siphon to draw air through D.

If you keep the plant growing under these conditions for some time you will find, in comparison with another quite similar plant growing in the open near it, that its growth is very slow. The leaves it forms are smaller, and finally its growth almost ceases. Further, if you test the leaves of the plant growing out in the air for starch (see pp. [24] and [25]), you will find that they contain plenty, but that the leaves on the plant in the bell-jar are empty of starch. Now all healthily growing green leaves contain starch, so that this is a good proof that something is seriously wrong with the plant, which has been deprived of the supply of carbon in the air. This shows us that plants use the carbonic acid gas in the air for their growth.

Carbonic acid gas is composed of a union of carbon and oxygen gas. If, then, the carbon is used by the plant, what happens to the oxygen?

Fig. 13. Jar of Elodea in water, giving off bubbles of oxygen gas in the sunlight.

You must have noticed bubbles rising from the “pond scum” and water-plants when they are in the sunlight, the little bubbles sometimes coming up in a quick, regular succession from the leaves and stems. Let us collect this gas and test it to find out what it is. This is more easily done if the plants are living in glass jars, where you can see them and get at them readily. A very good plant to use is the common Canadian water-plant (Elodea), which you can buy in aquarium shops if you cannot get it from the ponds for yourself. Place a handful of this plant in a tall, glass jar filled with fresh water, and cover it with a glass funnel, so as to collect the bubbles as they rise. See that the funnel is well under the water and support over it a test tube full of water, as in fig. 13. Place the jar in as bright sunlight as possible, when you should see the bubbles beginning to come off quite quickly. As the bubbles rise in the tube A, the water is forced out till the whole vessel is filled with gas. Then place your thumb over the mouth of the tube of gas, and remove it quickly from the water. Test it by plunging into it a splinter of wood which has been burning, but just blown out, so that it is still glowing. If you plunge it quickly enough into the tube, it should catch fire and burn brilliantly. Now this is the test for oxygen gas, so that we have proved that the tube was full of oxygen. This oxygen is the part of the carbonic acid gas which is given off by the plant as it uses the carbon and frees the oxygen it does not need.

You will find that the gas bubbles are given off much more rapidly when the plant is placed in bright sunshine than when it is shaded, and that when the plant is in darkness the bubbles stop altogether. This seems to show us that the sunshine must assist the plant to split up the carbonic acid gas, and we will find out more about this later on (see p. [25]).

We have now found that carbon forms a large part of the plant body, that plants cannot grow in air in which there is no carbonic acid gas, and that in getting the carbon from the carbonic acid gas, they split it up and give off the oxygen. So that we see that plants use the carbon in the air as well as the salts dissolved in the water of the soil as raw materials, with which they finally build up their food. We must now try to find what food substance it is that they build up from these raw materials.

CHAPTER VI.
THE FOOD MANUFACTURED BY THE PLANT

You will remember that much of the food provided in the nurse leaves consisted of starch, and that the baby plants use this food as they grow.

In the full grown plant we also find much starch; in fact, nearly all the parts of plants which we eat as food contain large quantities of starch, as you can test with iodine in potatoes, turnips, radishes, oatmeal, flour, and a host of our other vegetable foods. This is also the case in many parts of plants which we do not generally use as food, for example, in the lily and tulip bulbs, underground stems of Solomon’s Seal, and the stems and leaves of most plants. So that we find that the food grains of starch are developed in grown plants, and are not only provided for the young ones.

What is starch made of? Try heating a piece of laundry starch on an iron plate or the bars of the grate, and you will see that it blackens, and finally, if you put a light to it, may burn. If you simply heat it without quite burning it, you will find that it chars and goes black like a piece of charcoal. The solid element of starch is carbon. Now you may remember that in the plant growing under the bell-jar from which we shut out all the carbonic acid gas, we found that the leaves did not show any starch (see p. [20]). The plant had not been able to build up starch without the carbon obtained from the air.

The leaves of a plant are spread out in the sunshine and air, and it is in the leaves that we get the starch first formed. The leaves, in fact, are the food factories of the plant. You should study the appearance of starch in the leaves. As their green colour hides the iodine colouration, it is better first to remove it from any leaves you are studying in the following manner. So soon after picking them as possible, throw them on to some very hot or boiling water for a moment. This kills them quickly and makes them soft; then put them in a jar or tube of alcohol,[4] and leave them in it overnight. By next day the green colour should be gone, having been absorbed out of the tissues by the alcohol, and the leaves left yellowish or white. Then put them to soak in water till the stiffness caused by the alcohol has gone, when you should add the iodine. If you examine ordinary leaves in this way you will find that they go violet or brownish blue, showing that they contain starch.

Now do leaves always contain starch? You will remember that the oxygen bubbles were given off much more quickly from the plants in the sunlight than from those in the dark (see p. [21]). This shows that the leaves in the sunlight split up the carbonic acid gas more quickly than the others, which would give them more carbon to work on, and therefore it seems that they should be able to build up more starch in the light than in dull weather or darkness. You can see if this is true by doing a simple experiment.

Fig. 14. Leaf partly covered with cork sheets, A, and place in sunny position (compare Fig. 15).

If you take a leaf growing in the sunlight, and cover a portion of it, leaving the rest exposed, you will be able to see the effect of light and darkness on the starch-building powers of this particular leaf. To do this use two flat pieces of cork or thick cardboard, covered with silver paper or tin foil about 1 in. to 1½ in. big, and of the same size and shape. Place a part of a healthy leaf between them and bind them tightly together, as in fig. 14. If the weight of the cork makes the leaf bend down out of the full sunlight, then support it so that it lies in a position where it is well lighted. Leave it untouched for three days, and then in the middle of a bright day cut the leaf from the tree, remove the cork when you get into the house, and immediately treat it as described above for the iodine test. You will find that the part of the leaf which was exposed shows a good violet colour, proving that starch is present there, while the part which was covered is only yellowish, showing that starch has not been developed in this portion (see fig. 15). This proves that the covered part of the leaf could not build starch, so that exposure to the light and air seems to be necessary, as we expected. This further suggests that it is only in the daytime that the plant can build starch. You can see that this is actually the case by testing leaves from the same tree at different times of the day and comparing the starch in them. For example, test a leaf from a certain plant in the early afternoon, when it has been exposed all day to good sunlight, and compare it with one which is gathered just before sunrise, if you can get up so soon (this is, of course, easier in the spring or autumn, when the sun does not rise so early as in midsummer). You will find that the leaves picked in the early afternoon are packed with starch, while those picked before the day begins show very little or none.

Fig. 15. Same leaf as in Fig. 14 treated with iodine. It shows that the covered part had formed no starch.

What then becomes of the starch during the night?

You will remember that we found much starch in potatoes, which you know grow right underground, and therefore, according to the experiments we have just done, should not contain starch. But it is found that the starch is made in the leaves through the day, and is slowly carried down the stems in solution, and then stored (not made) in the underground parts, such as bulbs, potatoes, thick roots, and many others. It is like the shopkeeper, who collects some money each day and sends it every evening to the bank to be stored for him.

The leaves of the plant are then fresh next day to begin the work of building up more starch.

Fig. 16. Striped leaves; the white stripes show no starch when stained with iodine.

One of the great contrasts between the leaves in the air, and the parts of the plant underground, is that the leaves are bright green in colour, and the underground parts are yellowish or brown. It has been found that the green colour in leaves is very important in the building up of the starch. You can see this in the case of leaves which have parts quite colourless, as in those which are variegated or striped. Take the leaves of such a plant, which have been exposed to a good light, and test them in the usual way for starch. You will find that the pale stripes of the leaf show no colour with iodine, because they are empty of starch, owing to the fact that the green colour was not there to build it up. The value of the green colour is that it absorbs the energy of the sunlight, and uses it to get the carbon from the carbonic acid gas, and then to build the carbon into starch.

Now you will remember in doing the experiments on the food solutions (see p. [17]), that one of the plants lost its green colour, turned yellowish, and finally died. That was the plant which had no iron in its food solution. We have found, therefore, that without iron a plant cannot build up its green colour, and without its green colour it cannot use the store of carbon in the air to build up its food. This is only one example of the importance of mineral salts to the plant. Salts containing nitrogen are equally vital, while a number of mineral compounds are necessary for healthy growth. So that we see that the minerals absorbed in solution by the roots, as well as the carbonic acid gas absorbed from the air by the leaves, and the energy of light absorbed by the green colour are all equally necessary to the life of the plant, as all help in the building up of its food.

We have now seen that plants require food just as much as animals do; but that they use different and simpler elements from which they build it up for themselves, unlike the animals, which require their starchy foods to be ready built up for them. The foods which plants make they use in growing, and the other activities of their lives, just as animals do.

CHAPTER VII.
THE CIRCULATION OF WATER

As we have already found out, water is one of the things which are necessary for the well-being of plants. Seedlings can begin to sprout only when they are well supplied with it, and in the growing plant it is the water in the cells which keeps it firm and fresh. Directly the plant is deprived of some of its water it becomes limp and flabby, and “withers.” We noticed in Chapter IV. that the rootlets absorb the water (with its salts contained in solution) from the soil, and from them it travels all over the plant. The salts dissolved in water, however, are in very weak solution, and to provide the plant with sufficient of them for its growth it is necessary that a continuous stream of water should enter the plant. How is this stream kept up?

Fig. 17. Experiment to show that leaves give off water. Notice the drops collecting in the tube, which is closed with cotton-wool.

The leaves play a very important part in the water circulation, their thin expanded surfaces giving a large area from which the evaporation of water can take place. The water which comes off from them is not generally visible to us, because it comes off as vapour. However, you can easily make experiments which will show you that it actually does come off from the leaves.

Take a large test tube or a small glass flask, and place it over a good-sized fresh green leaf, which you leave attached to a healthy plant or a branch in water. Round the leaf-stalk wrap cotton wool till it fits like a cork in the neck of the flask, so that it shuts the leaf into the vessel, leaving no communication with the outer air, and at the same time does not injure it in any way (see fig. 17). Very soon, even after an hour or two, you will find a misty appearance inside the glass, and this will settle gradually in the form of drops of water which collect together and run down the sides of the flask. You do not see all this water coming off from the leaf under ordinary conditions because it goes into the air as invisible vapour, but when it is given off continually into a closed space the air soon gets saturated with all it can hold, and the rest must form liquid drops which we can see. If you keep a record of the time of your experiment, and also measure the amount of water collected in the flask, and then measure the size of the leaf, it only needs a little simple arithmetic to give you a rough idea of the quantities of water which must be given off every day by a single leaf. From that you can imagine the amount passing away from a whole plant or a great tree; and I think you will be surprised to find how much it is.

Another simple experiment shows us that the leaves play an important part in giving off water. Take three flasks with long, thin necks, and of as nearly equal sizes as possible. In one place a branch to which a number of fresh, green leaves are attached, in another a branch of the same size with only small buds (cut off the leaves if necessary), and leave the third as a check to show how much water has simply evaporated away. Fill all the flasks up to the same level with water, and mark this in all three when you start. Leave them for a day or two and then mark the level of the water, some of which will now have evaporated (see fig. 18). This will show clearly that more water has gone from the one with the branch than from the empty flask, and that a great deal more water has gone from the one in which was the branch with big leaves attached.

Fig. 18. Experiment to show that leaves give off water. The flasks were all filled to the same level I., and left for the same time. The one with the leaves in it lose far more than the others.

You can see roughly the rate at which the water goes off from the leaves by completely filling with water an apparatus like that in fig. 19. As the leafy branch (which is firmly fastened in the cork with no air leakage) uses up the water, it must be drawn along the narrow tube, which is graduated so as to show the quantity lost.

Fig. 19. Experiment to measure the amount of water given off by leaves in a given time. At first the tube is full of water, which is drawn back to points 1, 2, etc., as the leaves use it.

From these experiments we find that even although we do not actually see it coming off, yet the leaves of the plant give off a great deal of water in the form of vapour. By this process large quantities of water are drawn through the plant, and the salts in weak solution in it are kept and used by the plant as they are needed for building up its structure.

Now you may think that the loss is simply the result of evaporation from the leaves, because the surface of the leaves is great, and they would therefore naturally lose a considerable amount of water by evaporation. But this view is only partly correct, because the giving off of water by leaves or “transpiration,” as it is called, is regulated by a number of little pores in the skin of the leaf, which can open and close. You can see the importance of these pores as water regulators in plants which have them only on one side of the leaf, because practically all the water escapes from the side on which they are situated.

Fig. 20. Leaf A greased on the lower side, leaf B on the upper side, and C not at all. B withers as fast as C.

To see this, take three leaves of the indiarubber tree, which is grown so often in rooms. Choose three which are as nearly as possible just alike in size and shape. Of one of them carefully cover the whole of the lower side, and the cut-end of the stalk, with vaseline or coco butter; do the same to the upper side and the cut-stalk of the second leaf, and leave the third untouched. Fasten all three separately on to a string so that they all hang with both sides exposed to the air, and leave them for some days. The leaf which was not greased will shrivel up; as it gives up its water and can get no more, it “withers” and dies completely. The leaf which was greased on upper side also withers at about the same rate as the ungreased one, but the one which was greased on the lower side remains fresh and green (see fig. 20). This is because all the pores are on the lower sides of these leaves, and in the one greased on the lower side the vaseline had completely closed them, and so prevented the water from passing away through them. The upper surface is well protected against ordinary evaporation by a thick skin which does not allow the water to pass through it. The leaf greased on the upper side had all its pores left open, and so in this way was withered as quickly as one not greased at all. Not all leaves have their pores only on one side, but in nearly all plants the pores can open and shut. These facts show that transpiration is more than mere evaporation; it is a “life process,” that is, a physical process which is regulated by the structure of the living plant.

Transpiration is very important for plants, for it helps to keep the continual stream of water going through them, which brings with it the necessary food salts. Some plants cannot afford to let much water pass away, for they find it very hard indeed to get enough to keep them fresh; such plants as live in deserts or on bare, sandy places, for example, protect themselves from much transpiration by various devices and special arrangements, which we will study in Chapter XVIII.

We have already observed the fact that water enters the plant at its roots, and have just seen that it passes off as water-vapour from its leaves. Let us now consider for a moment the manner of its entrance. How can water enter the roots of plants?

Let us first look at a somewhat similar case in non-living things which will, perhaps, help us to understand the process in living plants.

Take a small “thistle-funnel” and tie tightly over the wide opening a piece of bladder; then pour some very strong solution of sugar into the funnel and place it in a glass of pure water. Mark the level of the sugar with a label (see fig. 21, S). Leave this for a short time, and you will find that the water has entered the funnel tube and run up it for quite a long way.

Fig. 21. “Thistle funnel” covered with bladder B, filled with sugar solution up to level S, and placed in a jar of water. After a time the water is seen to have risen to W.

You should take another similar tube and do everything in the same way, except that you leave out the sugar solution. Then you will find that the water remains inside the funnel at just the same level as in the outer jar. This is the usual behaviour of water, and in the first case, where the water rose inside the funnel, the rise was due to the influence of the sugar, which has the power of drawing in water. Now we can compare the skin of the root hairs (see fig. 9) to the bladder membrane covering the funnel, and it has been found that inside the cells are substances which have the same power of attracting water as we found was possessed by the sugar. So that the entrance of water into the roots depends chiefly on the attraction of the substances within its cells.

That a large amount of water enters the root in this way you can see if you cut off a quickly growing plant (a vine is very good if you can get it) just near its base, and attach to the cut-end a long glass tube in place of the shoot you have cut away. You must fasten this tube by a very well-fitting indiarubber tube, which you bind tightly so that it will allow no leakage, and support the glass. Pour a few drops of water down the tube to keep the cut-end of the plant from drying up at the beginning of the experiment. Then mark the level reached by the water, and do this every day as it rises in the tube. You should find that for some time it steadily rises day by day (see fig. 22).

Fig. 22. Plant P, which has been cut off near the root, is attached by the indiarubber tube I to a tall glass pipe, which is supported by stand S. On the glass are marked the levels reached by the water rising from the root.

We see in this way that the roots take in a large and continual supply of water, and this must get pressed up the stem even without the influence of the transpiring leaves. This is called the “root pressure,” and is a very important factor in supplying the plant with water. In a plant which is growing under usual conditions, both the transpiration of the leaves and the root pressure are at work, and are both necessary to keep a good stream of water passing through the plant. This stream of water provides it with its mineral food materials, and also keeps it stiff and fresh, and is, as we have seen, absolutely necessary for the growth of the plant.

CHAPTER VIII.
LIGHT AND ITS INFLUENCES

When we were experimenting on the building of starchy food in leaves (Chapter VI.) we saw how very important and even essential light is for the activity of the plant, and it is therefore natural to expect that light should influence its growth very considerably.

You may see the effect of light which comes only from one side on plants grown in the windows of rooms. If they are left in one position they grow in a one-sided manner with only the bare stalks toward the darker side of the room and all their leaves turned towards the window through which the light comes. If you want them to look pretty towards the room side also, they must be turned round frequently, so that the leaves are drawn in many directions instead of one only. The usual effect of light is to make the leaves grow towards it. You may see this still more clearly by placing a pot of seedlings in a blackened box with a small hole on one side. Very soon they will bend over towards the light entering by it (see fig. 23).

Fig. 23. Grass seedlings growing in an earthenware dish enclosed in a strong box blacked inside so that the light only enters at a. (Note how the seedlings bend towards it.)

Leaves can absorb most light when their upper surfaces are at right angles to it, and you will find some leaf-stems will bend right round in order to allow their leaves to get into this position. For example, if you take a pot of nasturtiums growing in the usual way, and support the pot on a stand, and cover it over with a bell jar which has been blackened, or with a black box, so that all the light reaches the plant from below, you will find that in a day or two the leaves will have turned completely round on their stalks and are now facing the light, so that they are upside down in their relation to the position of the whole plant (see fig. 24).

Fig. 24. Nasturtium covered over, so that the light only enters from below. The leaf surfaces bend over to face it.

Fig. 25. Spray of Maple showing stalks of leaves of the same pair of very different lengths, so as to place the leaves well as regards light.

In a small plant, or one with only a few big leaves, this desire for the light is easily arranged for, as there is room for each of them. But if all the leaves of a great tree were turned in the same direction, you will see that many of the under ones must be shaded by the others. This is not so bad as one might expect, however, owing to the wonderful way in which the leaves arrange themselves so as to use every bit of space they can, and yet to overlap and screen each other as little as possible. Particularly in plants which grow flat on the ground or against walls, and which therefore get all their light from one side, this is very well shown. In plants with the leaves in opposite pairs you will often find one leaf of the pair big, and the other one small, or that the leaf-stalks are of different lengths, and if you examine this pair in relation to the rest of the branch, you will see how it is developed in this way so as to use every bit of space it can and get as much light as possible without overlapping its neighbours (see fig. 25). Although it is true in one way that each leaf works as a separate individual, yet each separate leaf is only a small part of the plant, and they all work together for the good of the whole. Branches which have their leaves arranged in this way so that they seem to fit into a pattern, form what is called “Leaf Mosaic.” You may see this kind of arrangement among the leaves of very many plants (see figs. 25 and 26).

Fig. 26. Leaves of Ivy growing out from the stem so as not to overlap each other.

If, as we have already seen, light is so very important for the plant, what is the result of growing it in the dark? As you know, it will not be able to build itself food, and so would finally starve and die. If, however, we choose a plant which has already much food stored up and can therefore grow for a time without making a new supply, then we can study the effect of darkness on its growth.

Take some beans which are just beginning to sprout, plant them in a pot, and place the pot in some quite dark place such as a cellar or a dark room, or cover them with a well-made blackened box which shuts out all the light. Also take a potato which is just beginning to sprout at its “eyes,” and keep it in the dark. Both these plants have food in reserve; the beans have much in their nurse-leaves, and the potato is packed with starch, as you saw before. At the same time grow a potful of beans and a potato plant in the light, so that you can compare the growth of the plants under the two different conditions of light and darkness.

You will find that those grown in the dark are very straggling and of a sickly yellowish colour, and are a great contrast to the shorter sturdier young green plants grown in the open air. The stems of those grown in the dark are long and limp, and not able to support themselves upright, while the distance between the leaves is very great, and the leaves themselves are small and useless (see fig. 27).

Fig. 27. Seedlings of Bean of the same age, A grown in the light, B in the dark.

Why should these plants have such a great length of stem? It shows us, that when the plant is already supplied with food, darkness does not prevent mere growth in length. In fact it grows faster in length in the dark, which is an effort on the part of the plant to grow away from the darkness into the light. It economises in material and does not form stiff, thick stems and big leaves which would be useless until it reaches the light.

If you now make a small chink in the black box with which you cover the plants, you will find that they grow towards it and through it into the light. Once the tip of the stem is outside in the light, it will form the usual leaves at the proper intervals from one another.

The power of rapid growth in length of a plant growing in darkness, which economises the material generally used for strengthening the plant, and its power of growing towards the light, combine to be of practical use to a bulb or seed which is planted too deep in the earth. You will find that the part underground has much the same character as a plant grown in artificial darkness, until it reaches the surface. These weak underground stems bring the growing part into the light, and the plant does not waste material in forming large leaves and strong stems underground where they would be useless.

Although light is so important, it does not follow that the stronger the light, the better it is for the plant; just as it does not follow that because we like to be warm, we like to be as hot as possible. It has been found that plants bend away from the light when it is too strong for them, as you may see in some plants near one of those very brilliant electric lamps. The sun even is sometimes too brilliant (English plants, however, do not suffer from that very much), and many plants living in the tropics and regions of strong sunlight, protect themselves from its direct rays by a number of different devices.

CHAPTER IX.
GROWTH IN SEEDLINGS

When once the young plants start growing under suitable conditions they steadily get bigger. At first sight they appear to grow equally all over, stretching out in each direction as indiarubber does when it is pulled. Let us try to find out whether this is actually the case.

Fig. 28. A Bean seedling: A, with divisions marked on root and stem; B, after further growth, showing where most of the stretching has taken place.

Take a well-grown straight seedling and measure off along its stem and along its root, beginning from the tip, distances 1 or 2 mm. apart, marking them with a fine brush and waterproof ink. Take care not to injure the plant, and also not to make the mark blurred or too big. Draw the plant showing the marks on it as accurately as you can, and make the drawing exactly life-size. Grow it in damp, but very loose sawdust, so as not to rub off the marks, and after one or two days take it out and compare it with your original drawing.

You will find that the whole plant is bigger than when you first drew it. Look carefully at the marks on root and stem, and you will find that they are not all the same distance apart, as they should be if the plant had grown equally all over. The marks which are widest apart are those just behind the tip of the root and below the top of the stem, thus showing that there has been much more growth in these two regions than in the rest of the stem or root (see fig. 28). If you repeat this often with many plants you will find that these are the actively growing parts of the stems and roots; the individual leaves, of course, are also growing. Thus we see that growth is not a simple stretching of the whole, but that there are two definite regions where it is specially active. That of the stem and first root carry on the growth in opposite directions, as we noticed before (see p. [11]), the normal stem growing up into the air and the root down into the soil.

Fig. 29. A, Bean seedling planted upside down. The root has bent right over and is growing vertically down. B, later stage of the same. The shoot has bent up.

You can see how very determined the directions of growth are by planting upside down a bean which is just beginning to sprout, so that its root points up into the air. As it grows you will see the root bending over till it points vertically downwards, while the stem bends up and grows straight into the air (see fig. 29). The same thing happens if you plant a seedling on its side, and even if you take quite a big seedling, which has grown in the usual way, and then place it upside down in moist air, you will see the root and shoot bending in order to get into their right positions. This very determined growth on the part of roots and stems seems to show us that they must have some means of “perceiving” and regulating their position. It is not an accident that they always grow in these very definite directions. Let us find out what we can about this question.

Take a seedling and mark its root as you marked the roots for the experiment on the region of growth (see fig. 28), lay this seedling on its side on soft, damp sawdust, so that the root can easily bend into it. Next day you should find that the end of the root has bent, and that the bend is in just about the same region as that which showed the most active growth.

Is this actively growing and bending region therefore the part of the root which “realizes” that the whole is in a wrong position, and which therefore bends to put it right?

To answer this question quite fully would require a great deal of work, but there are three simple experiments which you can do, and which will tell you the most important facts about it.

(1)[5] Take a seedling with a fairly long root which has been growing straight down, then very quickly and with a sharp knife or razor, cut off the last 2 mm. of the tip of the root. Lay the seedling on its side on damp sawdust and examine it next day. It will not have bent, even though it has grown in length (see fig. 30, A).

Fig. 30. Experiments on the bending of the root tips in Beans. (See description in text.)

(2) Take another like it and leave it lying on its side for an hour, and then cut off the tip in the same way as in number one, placing it on its side once more. Next day you will find that it has bent in the same way as one which had not been cut (see fig. 30, B).

(3) Take a third, as like the other two as possible, and lay it on its side all night; do not cut it till next day, when it has definitely begun to bend (see fig. 30, C), then quickly cut off the tip, and place it in the upright position (C¹). You will find that it continues to grow in the bent form, the root tip going on to one side. It does not seem to know that it is growing along instead of down. If you keep it in this position for a few days it will then get a new tip and begin to grow downwards in the usual way (see fig. 30, D).

Think over the results of these three experiments, and you will see that it is only when the tip of the root is not cut off that the plant seems to “realize” that it is not in the right position. When the tip is removed it does not bend down even when the whole plant is lying horizontally, and in the other case (fig. 30, C¹, D) it will keep on bending even after it has been put in its right position.

We noticed that it is not the very tip itself which bends, so that we see that the very tip is the part which “feels” what is happening, while the part just behind it grows and bends according to the need of the plant.

This is a somewhat similar case to what happens when you realize with your brain that you are in danger on the road, and your feet hurry you across.

When we come to consider why the root should grow downwards in this persistent way, we find that there is an outside influence at work on the plant. You know when a stone is left without any support that it always falls to the ground, and we say that it is attracted toward the centre of the earth by the force of gravitation. It has been proved that the strong tendency of roots to grow down into the soil is largely the result of the same attraction, while the stem is not attracted by it but driven away, and therefore grows away from the centre of the earth. To prove this, however, requires more complicated apparatus than you are likely to be able to use at present.

From the experiments which we have done already we see that plants, as well as animals, are affected by their circumstances, and can in some measure realize them, and move to alter themselves in accordance with them. Later on we shall find that plants have a similar power in relation to light, supply of water, and other things. Have we not already observed in plants nearly all the signs of life we set out to look for? (see p. [4]).

There is one very important point about the growth of plants which is strikingly different from the growth of animals. A young kitten has four legs, a head, and a tail, and as it grows to be a cat these only alter a little in shape and get larger and stronger; the number of its legs remains the same. A baby plant, on the other hand, has its little root and shoot with a few tiny leaves, but as it gets older these increase very much in number, till it may have many branches and thousands of leaves. In fact, the number of its parts is much more indefinite than those of an animal; its body is built on quite a different plan. Yet both plants and animals show the same important thing in their growth, that is the increase of their living body, which they build up out of their non-living food.

CHAPTER X.
MOVEMENT

While we have been examining plants to find out some of the facts about their other life properties, we have at the same time seen many cases of movement in their different parts.

For example, we found (Chapter IX.) how the tips of roots move round to get back their vertical position if they are placed horizontally, and how the shoots of young plants bend over towards the light when they are grown in a dark box where it can enter only from one side (Chapter VIII.). Then, too, as the root tip grows into the soil or between the crevices of rocks it bends round the stones or other things in its way, and it is also attracted towards water, thus showing a continual, slow movement in its growth. The shoot shows a parallel kind of movement in following the light and placing itself as advantageously as possible with regard to it.

Fig. 31. Tendrils of the Pea; A young tendrils which have not yet been touched; B beginning to curl fifteen minutes after being rubbed with a twig.

You may see a still faster movement if you carefully examine a twining tendril. Notice how the young tendrils of a sweet-pea are at first almost straight, growing out into the air (see fig. 31). Now choose such a one for the experiment, and another like it which you do not touch, but keep to compare with the one on which you have experimented.

Gently rub one side of the tendril with a small rough twig, and then leave it alone. You will see that in about five or ten minutes it has begun to curve, and in a quarter of an hour may have bent round completely. Such movement is more rapid than that in the ordinary growth, and this power of bending so quickly is one of the special characters of tendrils, and one that is very important in helping them to do their work for the plant and to seize on any support within reach as quickly as possible.

Fig. 32. Leaves of Wood-sorrel; A in the day position, B “asleep” at night.

Then there are other movements, one of which you must have often observed in the “sleep” of plants. Many flowers and leaves close up and bend down at night, taking up their usual position again next day. This is not the same thing as the opening of buds, for it may occur again and again in the fully grown parts of plants. For example, you may mark certain leaves of wood-sorrel or common clover, and watch them close up at night and re-open in the morning many times. These movements are not very fast, and you cannot see the plant moving as you can see a kitten waving its tail, but the difference is only one of degree.