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.