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 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 FOODIt 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 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 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 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 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 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 EFFECT OF LIGHT AND DARKNESS ON INDIVIDUAL PLANTS AND VEGETATION AS A WHOLENow 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 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 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 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. 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 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 2. How Plants Get Their Food and Water From the EarthIf we could stretch an apparently impervious membrane, like the inner white skin just inside an eggshell, or a piece of parchment, and so form a wall through the middle of a glass box, and then pour into one of the compartments pure water and in the other a mixture of water and molasses, a very curious result would follow within a comparatively short period. We should find that presently there would be a gentle filtering of the water through the membrane In our discussion of roots in Chapter I, we found that they end in very fine subdivisions, which are themselves split up into practically invisible root hairs. These root hairs are the only way that roots can absorb the food and water in the soil, and they are able to do this because they are provided with a membrane which permits osmosis to act between the solution inside the root hair and the water in the soil. The solution in the root hair is mostly a sugary liquid, some of that surplus sugar made in the leaves, and it is denser than the soil water, so there is apparently nothing to prevent an equalization of the liquids on different sides of the membrane. If this actually happened, as it would in the case of the simple experiment noted above, then roots would exchange a fairly rich sugary liquid for a much more watery one, and we should find that plants did not get their food from the soil, but really have it drained away from them by osmosis. But nature has a cunning device for stopping such robbery, which is prevented by the membranes of root hairs being only permeable to the extent of letting water in, not permeable enough to allow sugar to As root hairs are very much alive and work constantly, they must be provided with air, without which no living thing can exist. And here, again, it seems as though nature, with almost uncanny foresight, had deliberately planned for this requirement of roots. And, in this case, not by interfering with a physical process by an adjustment of plant structure, but by the arrangement of soil particles and the way in which water is found in all soils. Soil particles, even in the most compact clay, do not fill all the space occupied by the soil as a whole. There are tiny air spaces all through the soil, which insures a constant supply of fresh air. That is one reason why gardens are cultivated, to see to it that plenty of fresh air is allowed to permeate the soil. Around the finest soil particles there is always an almost incredibly thin film of water, which is renewed as soon as it is lost by its absorption by the root hairs or by evaporation. This renewal of the water film is itself a mechanical process, called capillarity, best illustrated by putting a few drops of water on a plate and placing on them a lump of sugar. The water will spread all through the lump of sugar in a few seconds and the capillarity that forces it up through the lump is the same as sees to it that the tiny film of water surrounding the finest soil particles Little do we dream, as we walk over the commonest weed, that buried at its roots are these delicate arrangements for securing food and water. Osmosis allowed to act so that the “exchange” of liquids is all to the advantage of the plant, capillarity providing a constant water supply, and the very piling together of the soil so contrived that the life-giving air filters all through it—does it not seem as if all this were, if not a deliberate plan, certainly a more perfect one than mere man could have devised? If you will turn back for a moment to the beginning of the description of how plants get their food, you will find that in osmosis the weaker liquid tends to permeate the denser one more rapidly than the denser one does the weaker. As we have just seen, the sugary liquid in the root hairs is denser than the soil water outside, and, furthermore, none of it is allowed to escape. This comparatively greedy process of taking everything and giving nothing results in a constant flow of soil water into the root hairs. When the flow of liquids in osmosis is not at once equalized, a gentle pressure is brought to bear to make them so. This is what is called osmotic pressure, and it is this pressure that forces the absorbed liquid through the roots and part way up the trunk of even the tallest trees. While we have just said it is a gentle pressure, that is true only in the case where the osmosis has free play, and the pressure is stopped with the perfect mixing of the two liquids. But what if they can never mix? What may not the accumulated osmotic pressure amount to in such a one-sided process as goes on in root Another result of this pressure is that it keeps leaves and the fleshy stems of plants in their ordinary position. The actual solid part of nearly all leaves is scarcely 5 per cent of their bulk and all the rest is water. The constant pressure of this water from the roots is sufficient to keep leaves comparatively stiff and rigid, how stiff is quickly realized if the pressure stops and the leaf wilts or withers. Sometimes this osmotic pressure, particularly during rainy weather, becomes so great as to cause injury to the plant, the splitting of tomatoes and occasionally of plums, being due to it. This osmotic pressure, together with the extra pull given by the leaves, is sufficient to account for the rise of water to the tops of the tallest trees. The tallest trees in the world are certain kinds of blue gum in Australia which frequently reach a height exceeding 300 feet. What the combined osmotic pressure and leaf pull must be to carry such a heavy thing as water to such a great height is easier to imagine than to calculate. The root hairs, then, by the process already described, absorb the water from the soil, but plants can no more live on water alone than we can. As we have seen, the membrane in the root hairs cannot allow the passage of even the tiniest particle of solid matter. In fact the root hair itself is so small that it can only be seen through the microscope, and of course the membrane is smaller still. Plant foods, then, can never be solids, but must always be such materials as can be dissolved in water. The chief of these are chemical substances, such as lime, potash, But neither starch nor sugar, important as they are to the plant, and absolutely necessary as they are to us, are the only things made by plants. Leaves may well be called factories, but plants are themselves the most wonderful chemical laboratories, beside which any built by man are as play-things. For plants, by processes too complicated to be explained here, work over their accumulation of starch and sugar, recombine some of their constituents, and store up in various parts of the plant the results, which are often such food ingredients as protein. This is the really essential food substance in wheat, as it is in eggs and meat. No chemist has ever succeeded in making a single scrap of it, yet it is such an everyday occurrence in practically all plants that it, with starch and sugar, forms the great food supply of the world. Not protein alone, but all the amazing plant products like the oils from the olive and the resin from pine, rubber, the drugs of plant origin, even tobacco—all these and hundreds of others are made by plants from those few simple foods absorbed through the roots, literally pumped up to the leaves and there, under the magic of sunlight, combined and recombined, worked over and changed utterly in their make-up. Nothing could be more perfect than the marshaling of forces and contrivances to secure the result; let Nor does the work of plants stop here. If it did, they would be not unlike a commission merchant who had gathered from the four corners of the earth a supply of eggs only to find he could not or more likely would not sell them all at once, and yet had failed to provide himself with proper storage. Plants, too, have times in their life when adequate storage is necessary for them. So true is this that unless there is food enough stored in seeds to give a start to seedlings before their own roots begin to work, they would die almost at once. In seeds and in many nearly dormant parts of plants these foods are stored away for future use. The tubers of potatoes and all our root crops, like beet and parsnip, are common examples of this. Even the manufacture of wood in the trunks of trees is a storage appliance on the part of the plant, for wood is just as much one of the food products of a plant as wheat or rice. 3. Borrowing From the Living and Robbing From the DeadWith such a beautifully perfected mechanism for getting food it might seem as though all plants would be satisfied to lead that life of independence for which they are so splendidly equipped. Some of them, however, are like men in one respect: there seems to be no end to the chase after getting something for nothing. Those that stand on their own roots, get their food honestly, and take nothing for which they do not make prodigal returns, make up the great bulk of the vegetation of the earth. Their independence has dubbed them with the title autophytes, literally solitary or self-providing plants, and this thrifty mode of life is called autophytic. But a few kinds of plants, actually many millions of individuals, have more devious ways of getting their food and provide strong contrast to their sturdier associates. These baser modes of life appear to have been rather insidiously developed, as though there had been some hesitation at even the smallest departure from the normal. Of course we must not forget that plants, while living things, are never reasoning ones, and that good and evil and all other qualities that are ascribed to plants are perfectly foreign to them. Throughout this book, and in many others, the habits of plants are spoken of as base, for instance, or good. What is actually the fact is that nature works in truly marvelous ways, and to our reasoning faculties these adjustments seem clothed with attributes they do not really possess. But the description of them in the terms of our everyday speech, the translation of their behavior into the current conceptions of mankind, does so fix them in our minds that they cease to be “just plants,” and we no longer put their habits in the category of those interesting things that nearly everyone forgets. One of the first signs of departure from the usual methods of getting food is the association of certain minute organisms at the roots upon which plants, otherwise autophytic, depend for aid in securing nourishment. This characteristic is fairly common, notably in all the plants of the pea family, such as peas, beans, locust trees, vetch, clover, and hundreds of others. If the roots of any of these be examined, it will be seen that attached to the smaller A much more gruesome habit of certain plants is their reliance for food only upon the dead. In the Indian pipe, some kinds of shinleaf, and in many other plants their roots and root hairs are changed or often nearly lacking, and we find them growing only on the dead bodies of other plants. One peculiarly repulsive characteristic of such plants is that they secrete at their roots a substance that hastens the decay of the dead, and, as if this were not rapid enough, there are associated with them certain kinds of minute fungus organisms that also speed up decomposition. Plants with this charming mode of life are known as saprophytes, literally sapros, rotten, and phytes, plants. “Rotten plants” they may be in their mode of life, but the pearly white stems and flowers of the Indian pipe have a certain ghostly charm, an almost statuesque beauty among the normal greenery of the gloomy dark woods in which they always grow. It is not without significance that Indian pipe bears no leaves, has none or almost none of the life-giving green coloring matter which we have seen to be the almost priceless possession of plants which lead a different, and perhaps a better life. The great bulk of saprophytes bear no leaves, and some only partially wedded to the habit appear to be midway between bearing normal green leaves and bearing none, or much reduced ones that are quite unlike the busy factories we know normal green leaves to be. Plants with this method of getting It might almost seem as if demoralization, so far as food habits are concerned, had reached its lowest point in these plants that literally rob the dead, but there are still lower depths to which certain plants have been reduced. This consists of robbing the living, and such plants are called parasites, a word perfectly familiar in other connections. Parasitic plants have no roots, but attach themselves to the roots of other plants, somewhat generously called hosts, from which they derive their food. The best known case is the common Christmas mistletoe, and the dodder (Figure 68), but there are hundreds of others. Nothing in all the realm of plant life so perfectly fits the action to the word as plants of this type, flourishing when the host flourishes, dying when it dies. Producing flowers and seeds, and often, by an irony of fate, perfectly green leaves, they are nevertheless the most debased of all plants in their mode of life. These successive steps in the degradation of food habits, are not always the clean-cut things they might be inferred to be from the foregoing. There are many intermediate stages; it may even prove to be the case that some plants are wholly autophytic 4. What Plants Do With Water and How They BreatheSome one has said that one day without water would make men liars, in two days they become thieves, and after the third or fourth day they would kill to get water. In the Army Records at Washington is a report of one of our expeditions, which in chasing Indians got lost in a desert, and in which the soldiers fought among themselves for even the most repulsive liquids. It hardly needs these gruesome examples, however, to confirm what everyone who has ever been mildly thirsty knows, that water is an essential for all animals, and that As we have already seen in “How Plants Get Their Food and Water from the Soil,” the water is the carrier of the food elements from the soil, but water as such does much more for the plant than act as a carrier. Osmotic pressure, a never-ending pump, keeps sending up a steady stream of water to the limits of its power. In everything except trees it seems fairly certain that this pressure is sufficient to drive water into the remotest leaves. It finally reaches these tiny rooms in the leaf about which we read in the account of Leaves as Factories. And just here a very curious thing happens. Each room is, as we have seen, a very busy place, crowded with all the necessary equipment to make sugar, and yet there is still room for water which is just as necessary as the other fittings; in fact so necessary is it that the whole interior of the room is bathed in water. This irrigation system works so well that the walls of the room literally bulge with the pressure of the water in them. If they did not—a condition known as turgor—the plant would at once wilt, and if no new supply came it would wither and die. But water cannot stay in this condition of pressure and stagnation for even a brief period. That would be as if a leaf were like a toy balloon which, after inflation, had the entrance pinched and so remained inflated. And while we have all along They do not always work unaided, for in many places the transpiration, even with their best efforts, would exceed the rise of water in the plant and death must follow if such a condition exists for long. This may be the case in certain bog plants, where, even with their roots in the water, they actually are in danger of drying out because the composition of bog water makes it partially unfit for most plants. And, again, in very open dry or windy places, such as deserts or the mountain tops above timber line, the actual supply of water may be insufficient. Many plants growing in such places have their leaves, particularly the under surfaces of them, clothed with various kinds of hairs. These may be quite velvety or cottony, but in any event, either by their texture or their color, they tend to reduce transpiration. An extreme case is a desert plant from Arizona where the whole leaf surface is covered with an ashy gray velvety coating, which, of course, absorbs less heat than a normal green leaf, and in addition there are much fewer pores through which transpiration could be carried on. In ever so many leaves nature has provided them with a thick coating of hairs in early spring, which they lose later in the summer. Shrubs and herbs, especially those that start earlier than the trees under which they grow, very often may be found with a dense woolly or silky covering in early spring. As the shade becomes denser and the need of the protection less, the wool or silk is shed, sometimes completely. Some of the most conspicuous cases of this In many leaves there is conflict between those forces that result in the leaf getting the utmost possible exposure to light and those that prevent too rapid transpiration. On the one hand there is the absolute necessity for light, on the other the ever-present danger that the response of leaves to this necessity will result in a transpiration rate too rapid to be held in check by the guard cells. The compromise between these two forces, each pulling in opposite directions, gives to some leaves a series of movements that are among the most interesting things in nature. One of the most marked examples is the common wild lettuce, a weedy plant of our roadsides introduced from Europe. In bright sunlight the leaves are turned so that the edge of the blade faces upward, and the surface is thus protected from the direct rays of the sun, but during cloudy weather or in the shade the leaves turn into the ordinary position of most foliage leaves. It is difficult to avoid the inference that photosynthesis, which, as we have seen is an absolute necessity to the leaf, is in the wild lettuce retarded by transpiration, to avoid the too rapid rate of which the leaf is turned on edge. In this plant the leaf base, as though to be While most plants are well provided with methods of losing water, so well provided in fact that in very hot or very long dry periods it is a common sight to see many plants literally panting for more water, there are some apparently more cautious individuals, who reverse this process. All throughout tropical America hundreds of relatives of the pineapple have their leaves so formed and arranged that they catch and hold considerable quantities of water. In one kind, called Hohenbergia, the long leaves are joined together toward their base into a water-tight funnel, which will hold a quart or two of water over a period of drought. In Africa the extraordinary traveler’s-tree, a giant herb growing twenty to thirty feet tall, has the overlapping leaf bases so arranged that they hold many gallons of water. And we have already seen how the giant cactus of our own Southwest will hold 125 gallons. The most remarkable case is the Ibervillea from the deserts This conservation of water on such a great scale offers striking contrast to the truly prodigal habits of certain plants that actually drip water, so charged are they with this precious liquid, and so little stress do their conditions of life put them under in this respect. Where water is plentiful and turgor maintained almost to the bursting point, evaporation in a moist or chilly atmosphere does not suck out water vapor fast enough. Sometimes, around the edges of the leaves of the common Whether it be desirous to retain water or to lose it by gradual evaporation, or expel an excess of it, each species of plant has developed the apparatus to best preserve its individual life. While only the barest outline of these adjustments to the water requirements of plants has been given here, the details form an almost dramatic picture of struggle of the different kinds of plants for survival. The extremes are the desert plants on the one hand and those of the rain forests in the tropics on the other. The chapter on Plant Distribution will show how important With carbon dioxide going in, oxygen, water vapor and, as we have seen, even liquid water coming out of the stoma of leaves, it might be surmised that these busy little pores and their guard cells had done work enough for the plant. And yet there is still one more act to play and the stoma have much to do with it. For this process of photosynthesis and the closely related one of supplying food and water to the leaf cannot go on without respiration, which is quite another thing. In plants respiration or breathing has no more to do with digestion than it does in man. Digestion in man is not unlike photosynthesis in plants, except that plants make food in the process while men destroy it. But plants must breathe just as we do, and, as we need oxygen to renew our vital processes, so do they. While respiration is a necessary part of plant activity it is not such an important part as photosynthesis, for which it is often mistaken. The thing to fix in our minds is that photosynthesis makes food, uses the sun’s energy and releases oxygen in the process, while respiration uses oxygen and might almost be likened to the oil of a machine—necessary but producing nothing. 5. Restless and Irritable PlantsIn walking through the quiet cathedrallike stillness of a deep forest or over the fields and moors, perhaps our chief thought is how restful the scene is, and what a contrast the quiet, patient plants make to the darting insects or flitting birds that our walk disturbs. We found at the beginning of this book that ability to get about is one of the main differences Perhaps the most difficult thing in the world is to keep an active growing child perfectly still for more than a few moments at a time. There seems to be some impelling force that makes young growing things in a constant state of restlessness, and it is perhaps not so extraordinary, after all, that practically all young plants are restless in the sense that they are never quite still. And, like many grown-up people who do not know what repose in their waking moments really means, there are a goodly number of plants that are restless until the day they die. Charles Darwin, perhaps the greatest man that the last century produced, wrote a book in two volumes on these restless plants, and proved by a series of experiments illustrated by charts which the plants themselves drew for him, that there were perhaps no plants that do not move at least some part of themselves during the early stages of their career. While he never could explain the cause of these movements he left in that book an imperishable record The tips or growing shoots of many plants will point in one direction in the early morning, a different way at noon and still a different one by nightfall. Hundreds of totally unrelated plants seem to have this habit of moving their tips through a definite cycle during each day and this restlessness does not appear to be of the slightest use to them. It cannot be response to the moving of the sun through the sky, for often the movement may be away from the direct sunshine, and sometimes the motion goes on in the dark, as experiments have proved. It is hard to see the movement of the whole upper part of a plant, although it is well known that they do move in many cases. But in the tendrils the movement is often easy to observe and even to induce. Some of these slender aids to climbing plants, if they happen to be swinging freely in the air, do actually make slow circular movements, that even if they were designed for the purpose could not more perfectly accomplish their obvious intent, which is to catch the nearest favorable support. These circular movements are to the left in the hop, honeysuckle and many other plants, to the right in the climbing beans, morning-glory and some others. When the tendril reaches a support it almost immediately turns about it, in the same direction as its free movements through the air have been. It is thus this apparently aimless swinging of tendrils through space that determines whether the vine is going to twine to the right or left. The speed with which a tendril will take its first turns about a This general restlessness, which by the imaginative has been thought of as a mild protest by plants at their otherwise fixed position, is not so spectacular as that of certain other plants, notably the poplars. A flattened instead of a round leafstalk makes the leaves of these trees flutter in the lightest air and in a gale the tree is a mass of animated foliage. No use has ever been found for this curious habit and it is not certain that it is of the least advantage to the tree. If anything, the constant movement may have the decided disadvantage of increasing transpiration. In our common wood sorrel the leaflets on cloudy days or during the night regularly “go to sleep.” That is, they are folded at such times, rather than spread out in the ordinary way. These sleep movements may have something to do with transpiration, but whether or not this is true they are very regular and in certain plants the habit is remarkably and rather mysteriously uniform. Why, for instance, do the leaflets of these wood sorrels, the beans, lupine, locust tree and licorice plant, always fold downward while the clovers, vetch, peas, and bird’s-foot trefoil are always folded upward? Such movements and their direction are among the unsolved problems of botany, and merely to know of them or observe them leads us nowhere as to their true inwardness. But quite apart from these merely restless plants, and there are thousands of different kinds which are known to move slightly, at least during their young stages, are a few more decidedly active ones that are seemingly irritable. At least they show peculiar movements if touched, and at night. One of the best known is the sensitive plant from tropical America. Its twice compound leaf is composed of From India comes the most remarkable of all plants so far as movements are concerned. For in the telegraph plant the movements are so regular and long continued that irritability might almost be said to be continuous. The plant is a low shrub or herb with compound leaves, and the terminal leaflet, which is much larger than its neighbors on either side of their common stalk, performs a motion that describes with its tip an irregular oval or ellipse. But the movement is not steady; it goes by a series of slight but perfectly distinct jerks. It takes about two minutes for the leaf to complete its cycle, and it is this jerky movement that has given the plant its name. During the night its leaflets stop this apparently quite useless performance, the cause of which is quite unknown. It is often grown in greenhouse collections where its strange movements may be seen on any sunny day. Many other cases of the restlessness or irritability of plants could be given, and nothing has been said here of the curious movements of some insectivorous plants as they have already been mentioned. It cannot have escaped the thoughtful reader that all of this chapter on plant behavior has dealt with those functions of plants in which roots, stems, or leaves play the chief part. These purely vegetative actions of plants, what might almost be called their bread and butter activities, would never lead to perpetuating their kind. For while all of these functions are necessary, except certain apparently wayward movements which still remain unexplained, they are in a sense only the preparation for an infinitely more important act, the reproduction of their kind. What the poetic have called the love of the flowers, or in more prosaic but perhaps more truthful words the fertilization, pregnancy, and birth of the new race, will be considered in a separate chapter. No other act of the plant world is so interesting as the mechanism of reproduction, the almost endless devices for securing it, and the ingenuity of nature in seeing to it that there are no flukes. |