Botany: The Science of Plant Life

by Norman Taylor

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.

CHAPTER III
HOW PLANTS PRODUCE THEIR YOUNG

THERE is perhaps no device of nature that more perfectly accomplishes its purpose than the one with which all living things are endowed—the instinct for the renewal of life. In man the dawn of the mating instinct has ever been the theme of poets, and some of its manifestations are the despair of ascetics. Through it some of the noblest of man’s emotions have arisen, and because of its perversion our daily newspapers chronicle the basest and most sordid tragedies.

But whether noble or ignoble, this instinct for mating is, in its simplest terms, only a provision of nature that all life contains within itself the means of renewing life. Without this, life, so far as we know it, would end with the present generation. Perhaps our understanding of this decree of an all-wise nature to increase and multiply will be heightened by looking at it not only from its familiar manifestations in man, but more broadly. Seen from this broader viewpoint, it is the inherent legacy of all living things from the dawn of life on the earth down to the present. Even the simplest one-celled organisms have the faculty of increasing. In all plants, both the flowerless ones and those producing flowers, the process is carried to a perfection almost unbelievable in its intricacy and in provisions against its failure. From the matings of flowers much may be gleaned; even man himself can learn from them the capacity for sacrifice, the sinking of individual aims and pleasures in the greater scheme of conforming to that necessity for renewal of the race upon which all progress must be based.

The equipment which different flowers have developed for this purpose, their almost uncanny devices to make certain that only the distant and foreign male can ever impregnate the female, the enormous wastage of both unfertilized females and males that will never become fathers, and the overwhelming effectiveness of it all, in spite of this prodigality—these manifestations of the production of young in the plant world will take up the rest of this chapter. All the first part will tell of this process in flowering plants, while the second shows how flowerless plants accomplish the same end in more secret ways. Finally, in a brief third part, we shall see how, without mating of the sexes, nature has still one other way to see to it that there is a constant supply of young.

We have already made clear that all plants are divided upon the basis of whether they bear flowers and their mating goes on before the world, or whether they bear none and the process is accomplished in more secret ways. Because flowers are so much better known, and it is simpler to see how the act is consummated in them than in the cryptogamous plants, we shall first consider the phanerogams or flowering plants, and in the second section of this chapter the cryptogams or flowerless plants.