Botany: The Science of Plant Life

by Norman Taylor

1. Light and Its Importance To the Plant

PRACTICALLY all that has been said in the first chapter relates to what plants are, their organs, or what we may call the architecture or plan of their framework. But what they do with this elaborate structure is as important as what we do with a house that may contain every modern improvement but is never a home until these things have been put to use. One of the chief concerns of any architect is to see to it that the house has as much sunshine by day and as attractive illumination by night as possible. Nature, that greatest of all architects, also sees to it that plants get the utmost necessary sunlight, but for a much more important reason than the mere attractiveness of sunshine, be that ever so beautiful. For light, the life-giver of all green things, is so absolutely essential to plants that experiments to grow them in the dark have always failed, and many gardeners now use electric light in greenhouses in order to prolong the short daylight of winter. It is the lack of light that makes celery blanch.

Plants grown in the house inevitably turn toward the windows, even plants growing against a wall turn their leaves away from it—nowhere can one find living green things that do not find the light as surely and persistently as men try to get their food or their mates. Many examples of this could be given and must have been noticed by everyone.

Sometimes seeds germinate under a barn floor for instance, and the puny pale little plantling reaches out slender stems, all of which turn, as a compass turns to the north, to perhaps a crack of light in one corner of the building. We have already seen how the search for light will carry the slender rattan palms of India hundreds of feet to the topmost leaves of the forest. Individual plants, and, as we shall see later, whole forests make desperate efforts to get to the light. We know already, that the struggle for light is just as bitter as the struggle for food by roots. And finally if, as we have many times proved by experiments, plants die when grown in a dark room, what is it that light does for plants and how is a process carried on that everything leads us to think is of the greatest possible importance? Quite obviously it is not the mere beauty of sunshine dancing upon the landscape, as entrancing a picture as that may be any summer afternoon, with the play of sunshine and shadow on the tracery of foliage. That green color of the foliage, the almost universal green of so much of the earth’s vegetation, restful to tired eyes, providing us with the most pleasant shade, has wrapped up within it the secret of just what sunshine does for plants. For under the magic of light acting upon this greenery one of the most important industries in the world, the manufacture of food, is constantly going on.

LEAVES AS FACTORIES FOR THE MAKING OF FOOD

It must be clear enough from the start that to call a leaf a factory for the making of food forces us to decide at once whether this is a mere way of speaking, or whether, incredible as it may seem, anything as thin as a leaf can really produce food. As we eat lettuce, and millions of cattle graze every day, leaves as food producers win handily on that score. But to understand how food is produced in such a tiny factory demands that we walk about in it for a bit, study the inside of it and especially its many small chambers within which is not only the machinery, but some of the finished product stored up for later use.

Unlike modern factories there are many entrances, from any one of which we can begin our tour of inspection. On the under side of nearly all leaves and on the upper side of some there are scores or even hundreds of small pores called stoma, so small that only with a microscope can they be seen. These entrances through the factory wall, are carefully guarded by a pair of watchmen whose business it is to see neither too much dry air gets in nor too much of the product of the factory gets out. They see to it, also, that waste products are thrown out at the proper time. These watchmen, or guard cells, as they are called, are constantly on the job, work almost automatically, but their chief function is connected with the proper ventilation of the place, and will be discussed later under “How Plants Breathe.”

Once past the entrance it is obvious that we are in one of the strangest of all factories, for none of the rooms are truly square or oblong and their irregularity as to outline would drive your average foreman into profanity. Yet they are certainly divided into distinct classes, at least as to size and as to what the rooms contain. Some are apparently filled with nothing but air and have direct connection through the stoma with the outdoors. These are called intercellular spaces. Others, and these are most important, are filled mostly with the green coloring matter that gives the leaf its color. This substance is known as chlorophyll, its individual units as chloroplasts, or literally, chlorophyll bodies. Quite independently of these chlorophyll cells or rooms, or the intercellular spaces which correspond to halls, there are some large and many small tubes. These are the veins of the leaf and their finer branches and by their direct connection through the stem to the roots, serve as the ducts through which some of the raw materials are brought into the factory.

This green coloring matter or chlorophyll is perhaps the most important substance in nature. Without it all except a very few plants would die, and even in those beautifully colored leaves like coleus or caladium chlorophyll is always found, but in these colored leaves it is merely obscured by other coloring substances. It is in the chlorophyll that the ability resides to take the inorganic substances through the roots or from the air, and by the aid of sunlight transform them into organic substances like starch and sugar. Nothing else in all nature can do it; without this faculty, which the commonest green leaf possesses, the earth would prove uninhabitable within a single year. Just what chlorophyll is chemically is not yet thoroughly known, but the thing of chief interest is that it is hardly ever found in parts of the leaf not exposed directly to the sunlight, and that during the autumnal coloring and before the fall of the leaf chlorophyll is carried to other parts of the plant, and quite possibly stored for use the following season.

While the composition of chlorophyll is not surely known, iron is certainly one of its constituents, as plants deprived of iron lose their green color. It also is known to contain oxygen, carbon, hydrogen, and nitrogen, but merely to catalog what we know about its make-up does not tell us that it is a living green substance and that sunshine sets it in motion. Just exactly how light acts on chlorophyll no one really knows; we merely know that it does so act and that the result is one of the marvelous secret processes of nature, perhaps like the secret of life itself forever hidden from man. In our tiny factory, then, we have raw products coming from the roots and through the stoma from the air; machinery of the most efficient type, for chlorophyll works night and day, and constantly renews itself while producing the finished products; energy from the sun; and finally the complete manufactured products which are foods in the shape of starch and sugar. During the growing season there is no banking of the fires, no stoppage of this most important of all industries, no strikes or lockouts. Each part of the whole works smoothly and with the nicest precision—in fact so perfectly does this process keep on going, so complete is the orderliness of the place, and so regular are the completed products turned out, that no modern factory manager or workman but can learn something from a rather close study of this smallest but most efficient factory in the world.

Some of the raw products are delivered to the leaf from the roots where they have been absorbed by another process that will be considered a little later. These consist of water and the inorganic substances dissolved in it, popularly called sap. Carbon and oxygen come mostly from the air, sometimes separately, more often in the form of a combination called carbon dioxide which is one of the chief constituents of the gas thrown off by man as he breathes out. Now these inorganic substances, contained in the sap or derived from the air, are literally mixed by the chlorophyll and form, always with the aid of sunlight, substances known as carbohydrates, the commonest example of which is sugar. Some form of sugar is one of the earliest results of this process, but sugar is quite easily dissolved in the sap which has contributed to its manufacture, and the excess sugar is thus removed. Otherwise it would clog the machinery and prevent the production of fresh supplies. This first step in the manufacturing process has not inaptly been called photosynthesis, the meaning of which photos, light, and synthesis, combining by means of, suggests in a word the necessity of light and the combination of the inorganic substances mentioned above. Of course this process of photosynthesis is not as simple as the brief account of it suggests, for it is actually a complicated chemical process only part of which is yet understood. It is fairly certain that it goes step by step; it is quite certain that the beginning is inorganic and the end organic compounds like sugar. Something is known also of the wear and tear on the chlorophyll, its waste products, and how it keeps itself not only fit but provides for its own constant renewal. One of the excess or by-products in this initial manufacture of sugar is oxygen. This is either used in other ways by the plant, or more generally it is thrown off through the stoma into the outer air. Oxygen, as one of the necessary constituents of the air that man breathes in, is thus thrown off, while, as we have seen, carbon dioxide, a poisonous gas which we breathe out, is a necessity for this manufacturing process in all green plants. Hardly any trick of nature so completely fulfills the wants of animal and plant life as this mutual exchange of by-products—in the case of animals it is the waste of respiration, in plants it is the wastage of sugar making and some other changes that go on in the plant just after this stage.

The amount of sugar made, carbon dioxide taken in, and oxygen given off by this process suggests that while leaves may be very tiny factories they are among the most efficient in the world. Assuming an area of leaf surface equal to about a square yard the amount of sugar made would be about one-third of an ounce in a day or nearly three pounds in a single growing season. Carbon dioxide withdrawn from the air would average from the same area of leaf surface about two gallons a day or over three hundred gallons for the season. As an equal amount of oxygen is given off by the leaf, it becomes clear that as all of this interchange must go through the stoma the functioning of these and their guardians must be nearly one hundred per cent perfect. As we shall see a little later, they perform still other duties with even greater perfection. When we stop to reflect what an absurdly minute fraction one square yard of leaf surface is to the total leaf surface in the world, we come to some realization of the gigantic proportions of this process of manufacturing sugar and exchange of gases mutually useful to animals and plants. While in the United States most of the leaves fall in the autumn, the great bulk of the vegetation of the world holds the greater part of its leaves all the year, notably in the vast evergreen forests in the north, and of course practically all tropical vegetation. Chlorophyll in such places works continually and what the total of sugar production may be no man can even guess.

Sugar, although the first step in the process, is not the final one, and the leaf has still other tasks to complete. Some of the sugar is used up in the process of renewing the chlorophyll, some of it is moved to other parts of the plant where in sugar cane it forms the world’s chief sugar supply; but the remainder is transformed into starch, a substance that is not dissolved by the water of the sap, and is therefore capable of permanent storage either in the leaf itself or in other parts of the plant, notably in the tubers of the potato, the solid part of which is nearly all starch. The conversion of sugar to starch, which is really a means of contriving to properly store the product of the factory, is done by certain ferments known as enzymes. Just what enzymes are or even how they work is not well known, but apparently they have the faculty of converting certain substances like sugar, and in the process they neither use up nor materially change their own composition. It is certain that the conversion of sugar to starch is an elaborate chemical process, but it is accomplished by these enzymes, the very existence of which has only recently been discovered. Enzymes not only do this, but they convert starch which is insoluble into a kind that may be dissolved and thus carried to different parts of the plant. Upon this power depends the storage of starch in roots, tubers, seeds, or wherever else it is found in the plant, and it is of course upon this power man depends for the food supply of the world. Wheat or corn, potatoes, rice, all the foods that are rich in starch produce none in that part of the plant harvested by man. All of it has come by the process which is only sketched in its briefest outlines in the foregoing paragraphs. All of it must come from that green coloring matter of nearly all plants which, while mostly confined to leaves, is not always so. And wherever chlorophyll is found this process goes on even in the simplest plants. Because it is so overwhelmingly a characteristic of leaves and, as we have seen, leaves are the one organ of the plant upon which man pins his only hope of future food supply, the leaves of all plants may be truly likened to a factory the work of which is never ending, the product of which the leaf will never use, but the result of which has far-reaching consequences to us all.

EFFECT OF LIGHT AND DARKNESS ON INDIVIDUAL PLANTS AND VEGETATION AS A WHOLE

Now that we understand the importance of light to all except a very few plants, and its very close relationship to the green coloring matter of all leaves, many things about the arrangement and position of leaves, and indeed of the whole plant, may be understood, which, without this knowledge, seems the result of mere caprice or chance. It would seem as though the habit of plants growing toward the light, and against the pull of gravity, a character almost universal, no matter from what mountain declivity or rocky cliff it may spring, might be the result of the “pull” exerted by light on the green coloring matter in the leaves. While light does aid in plants having a generally erect habit it is not the cause of it, as we have many times proved by experiments. As a seed sprouts and the roots go down into the earth, the shoot, before it has broken through the surface and while still in the dark, always grows upward. This property of growing in two opposite directions at the same time, the roots always with gravity and the shoot nearly always against it, is known as geotropism. In the case of vines or other trailing plants there is the same tendency exhibited, even though the plant is not erect. We must think of geotropism as a growth habit of all plants, not caused by light, for it has been shown to act in the dark, but of the greatest advantage to all plants in their initial start toward the light. If this were not the case, it may be imagined into what chaos the vegetable world would be thrown. We are so accustomed to roots going down and shoots going up that we are not apt to think of it as the result of two antagonistic growth habits, the true cause of which is not understood, the result of which is common knowledge. Geotropism is one of those mysteries with which the book of nature is crowded, and merely to describe it and realize its force is by no means to arrive at its true inwardness.

But, quite independently of this peculiar growth habit, the stems and often whole plants do show response to light and many times the response, in its effects, cannot be distinguished from geotropism. Perhaps the most homely illustration of this is the common house geranium which, no matter how often it is turned, always grows toward the window, and if not turned at all becomes hopelessly lopsided, with the leaves all bending sharply toward the light. Trees growing on a cliffside, while always growing upward, nearly always may be seen bending away from the cliff where light is scarce and toward the unobstructed light. The position of hundreds of twigs and branches on any tree have been dictated by their exposure to light, and the habit of practically all trees in the forest of being clear of branches for many feet from the ground is another illustration of the profound effect of light. In the latter case the taller the trees the farther from the ground are the first branches, and in the big trees of California the first branches are frequently over a hundred feet from the ground. In their young stages all these trees were furnished with branches, the leaves of which in their day performed their appointed tasks. But in the strife and hurry of the crowns of the forest to overreach their neighbors these lower branches, from the bottom upward, gradually die off. So inexorable is the plant’s demand for light, that these lower branches, in spite of being nearest the source of their food from the roots, are doomed to be killed. Nature plays no favorites and these lower branches, once the pride and support of the young tree, are ruthlessly dropped off when they can no longer play the game. This wholesale slaughter of lower branches in a forest, more complete than any pruning by man could ever be, gives us, if the story of the factory leaf has not already done so, some conception of the part played by light in the plant world.

The shade of certain trees is so much denser than others that they have been planted for this purpose, notably the horse-chestnut and Norway maple. Foresters have long recognized this difference in trees and it would be strange if nature had not taken advantage of it also. If certain trees can still maintain themselves in the forest without producing a dense crowd of leaves, such as the silver maple for instance, they would have a decided advantage over a tree like the sugar maple which casts a much denser shade. A walk through any forest will show scores of examples of trees that live and produce seeds by virtue of the fact, not that they demand all available light, as their more vigorous neighbors do, but that by a compromise, by an almost diabolical cunning, their light demands, and of course their leaf exposure, have been cut down to a point where the tree can grow in a place impossible for trees that lack this ability. It is, of course, not a trick which any individual tree can perform at will. Rather is it a characteristic found in all individuals of certain kinds, where the comparative disadvantage of making less food and having less leaf exposure is more than overcome by the enormous advantage of being able to fight their way into a forest that would otherwise be impossible for them. We shall see, in the chapter on Plant Distribution, how this peculiar response to light has had effects of considerable significance upon forests, particularly after forest fires, lumbering, or other disturbance of the natural conditions. Trees in the forest, and the shrubs and herbs under them are not the quiet stately things about which the poets are so fond of singing. They are places, on the contrary, of intense warfare, and perhaps some of the greatest casualties occur in the battle for light.

Leaves, as being the most directly involved in the matter of utmost exposure to light, show the greatest amount of response to it, by their shape sometimes, by their position nearly always, and very often by the character of their leafstalks. In many herbs the first young leaves are relatively short-stalked, while as the plant grows upward the lower leaves are progressively longer stalked, which is a direct response to the fact that the upper leaves take their full share of light, leaving little or nothing for the lower ones. To avoid complete shading their leafstalks are often many times the length of their more fortunately placed neighbors above them. In those plants like the garden primrose or common weedy plantain, which bear all their leaves in a close cluster or rosette at the level of the ground, we see an almost fiendish cleverness in their earlier and later habits of growth. When the leaves first start, as they nearly always do among grasslike vegetation in which these plants usually have to fight for a chance of life, the leaves grow straight up, so that they may get above the level of the surrounding grass. Once there, and the precious light an assured fact, they gradually flatten out their leaves to form a rosette, of course cutting off the light from the grass about them and killing it just as certainly as though it were pulled up by the roots. Hundreds of different kinds of plants do this, apparently with the utmost cruelty to their inoffensive neighbors, with whom they start upon nearly equal terms in the race for life. If they began at once to spread out their rosette while it was still in its small spring state, the upward pointing grasses would smother it, and as if in anticipation of this the leaves grow up with the grass, only to flatten out when the proper time comes for them to show their true colors.

Light not only affects leaves in their habits of growth but it actually causes movements in some leaves which are as regular as clockwork. The best known cases are those in the pea family and wood sorrels, all of which bear compound leaves. During the day these leaflets are spread out in the ordinary way and catch the light, but at sundown, as though this were a quite useless exertion for the night, they fold up and the leaf “goes to sleep.” On cloudy days they partly fold up, as if in recognition of the fact that for their business of getting light it is an off day; but also if the sun comes out they hurriedly expand their leaflets. It is not yet certain whether these apparently intelligent movements of leaves in relation to light are of any real advantage to the plant as a whole or not. They are surely one of the most interesting things to watch and may be seen in locust trees and wood sorrel any night.

Just as we can have too much of a good thing, it is possible for plants to have too much direct sunlight. In open spaces, where the struggle for life centers not about the fight for light but over other matters, we find leaves actually protecting themselves against too much exposure, and by a variety of ingenious ways. The texture of the upper or lower side, the kind of hair growing on their surface, and the number and size of their pores, are the most usual ways of leaves arming themselves against an oversupply of the one thing that their neighbors in the cool forest fight to the death to obtain. There seems to be a fatality against which plants, like ourselves, are nearly helpless. Their attempts to overcome it, again like our own struggles against an apparently overmastering fate, develop those characteristics that insure survival to the fittest, death to the puny or unaccommodating.

We could hardly leave the subject of light and plants’ relation to it without mentioning, perhaps, the most remarkable case of adaptation to peculiar light conditions. All those aquatic plants that grow beneath the surface of the water need and get much less light than ordinary land plants. But from the island of Madagascar comes the lace leaf or water yam, which grows in quiet pools that are mostly in the depths of the tropical forest. Add to the dense shade cast upon the gloomy surface of such ponds the amount of light naturally lost in its passage through the water, and we get some notion of the singularly secluded home of this aquatic plant. What, now, is nature’s response to these peculiar conditions? How do the leaves of this well-shaded inhabitant of quiet pools behave? Their leaves are about a foot long and three or four inches wide, quite unnecessarily large for a submersed aquatic, but they consist wholly of veins. There is no “meat” to the leaf, none of that soft, green tissue so familiar in ordinary leaves. The conditions under which it is doomed to live almost seem as if it recognized the futility of having a broad expanse of the usually constituted leaf blade to expose to a light which is not there. It is significant that this skeletonized condition is permanent, the leaf functions much as ordinary aquatic leaves do, but its network of quite naked veins almost seems a mute protest against its fate. The delicate, lacelike “foliage” of this aquatic adds a touch of beauty to one of the most curious plants in the world.