Chapter II: Plant Behavior (2)

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 ready for whatever change transpiration or photosynthesis may demand, is so attached to the stem that such changes are made with the least possible delay or wrenching. In one of the many kinds of blue gum trees of Australia all the leaves turn one way in the light, and another in shade or on cloudy days. Ever so many plants have partial movements of their leaves, a good many of which are in response to these opposing demands, one pulling the leaf into the greatest possible light, the other holding it away from that condition. There are other movements of leaves, of parts of the flower, or even of the whole plant that are not so certainly the result of the conflict between light requirements and the necessity of conserving water supply. They will be considered presently.

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 of Arizona. In riding over this country one may find objects that look not unlike a burned pudding, about two feet in diameter and nearly as high. From the center comes a delicate stalk with the finest feathery foliage and tiny flowers. Of roots there appear to be almost none, and these curious objects, which are very hard and woody, might almost be taken for stones. But they are actually plants not distantly related to squash and pumpkin, and one of them collected years ago and brought into a museum behaved in quite the most thrifty fashion of any plant yet discovered. It was carefully cleaned and put in a museum case and locked up as a curiosity for the wondering public to gaze at. But suddenly, almost miraculously, it sent out its delicate growth which grew its appointed time and then withered. Imagine the astonishment of the curators of this museum to find it doing the same thing the next year, and the next. Finally after putting forth its shoot for five years it actually died and is now a peaceful museum specimen. No other such case of water storage is known, but thousands of plants have this remarkable ability to a less degree, all in response to conditions that would mean destruction to plants not so providently equipped.

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 garden nasturtium, drops of water may be found, literally forced out as drops, rather than transpired as water vapor. This happens to a considerable number of plants, during the night when transpiration is laggard, and such drops are usually mistaken for dew. The latter is actually the condensation of moisture in the air upon the leaves of plants which cool down more rapidly than the air, and seldom due to the forcing out of drops of water from leaves, although in rare cases it may be. In tropical forests, where the humidity is very heavy and water supply from the roots copious, certain leaves leak water so fast and are so constructed that this excess is prevented from accumulating on the leaf. The pipal tree of India has long drip tips to its leaves that conduct the excess water from the blade to the end of the slender tip where it drips off. The advantage of these dripping points is obvious, for in regions so humid that water is forced out of the leaf, the coating of the leaf with this extra moisture would by that much retard transpiration. Dripping points, which in less exaggerated forms than in the pipal tree are common in many parts of the world, are thus of decided advantage.

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 these water requirements of plants have been in determining what grows on the earth to-day.

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 PLANTS

In 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 between animals and plants. Like so many first thoughts, this is, however, only a half truth, for while most plants, seemingly by a kind of fatality, are anchored forever to the place of their birth, many of them do move certain parts of themselves and that quite regularly. While some of these movements have already been hinted at as a possible response to transpiration or too intense light, there are others where the advantage to the plant, if any, has yet to be demonstrated. These other movements, perhaps because their cause has never been discovered, seem the more mysterious as they certainly are more weird and interesting than almost any other of the curious things that plants do.

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 of the amount and direction of these mysterious movements, which are almost to be likened to the growing pains of young children.

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 support is so comparatively rapid that, once the support is caught there is scarcely a chance of the vine being torn away by the wind or other agency as would surely happen if tendril movements were the leisurely things that some folks think they are. In the case of one Passion-flower vine, which are gorgeous climbers mostly from the tropics, the tendril made a complete turn in two minutes after it first touched a possible support. And there is a quite noticeable movement in thirty seconds if the tip of the tendril be ever so lightly touched. Teasing tendrils to see how much or how fast they will coil has resulted in some extraordinary cases of the “comeback” of some of them. Darwin irritated a tendril for a few moments and induced a partial coiling which straightened out when the object causing it was withdrawn. To see how long the plant would stand this sort of thing and still not be literally tired of coiling he succeeded in making the plant partially coil, and by withdrawing the incentive uncoil again, over twenty times in fifty-four hours. An impulse to coil of such persistence as this naturally results in vines forming the impenetrable thickets they do in many forests. It emphasizes how restless are the growing points of these climbers, and serves as a striking illustration of those gradual movements of many other plants that seem to have some relation to growth, but in a way not yet understood. For while it is an obvious advantage for the vine to swing its tendrils through the air this advantage has not yet been proved the cause of the swinging. In fact if all possible supports are removed the tendril will often coil anyway, a perfectly futile proceeding, that looks almost like disgust.

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 many tiny leaflets which upon the slightest touch close up and apparently wither on their stalk at once. In five seconds after the leaf is touched it will appear like a wilted wreck. If the jar is sharp enough the whole plant will droop, and the response to a sudden jar is almost electrically quick in its action. And yet all this sudden wilting, actually caused by a quick loss of turgor, is slowly repaired and the plant carries on quite normally again until another shock renews its irritable response. This plant does the same thing gradually during the night, except that the leaflets recover their normal position only with the rise of the sun.

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. The very considerable movements of certain flower and fruit organs will also be considered elsewhere.

* * * * *

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.