Botany: The Science of Plant Life

by Norman Taylor

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.