Page:Popular Science Monthly Volume 11.djvu/609

Rh Of the actions which organisms perform in order to accomplish a purpose, some, like those to which I have called your attention, are to be placed above, others below instinct. Many of the actions of plants, such as turning the leaves toward the light, the tendency of the roots to grow downward, and. the closure of the flowers at night, remotely resemble instinctive actions. In some the resemblance is more perfect—he groping of tendrils for support, for instance. Prof. Gray gives the following account of this action: "When a twining stem overtops its support, the lengthening shoot is seen thrown over to one side, and usually outstretched. One might suppose that it had fallen over by its weight, but it is not generally so. If turned over, say to the north, when first observed, it will probably be found reclining to the south an hour or two later, and, an hour later again, turned northward: that is, the end of the stem sweeps round in a circle continually like the hand of a clock. It keeps on growing as it revolves; but the revolving has nothing to do with the growth, and indeed is often so rapid that several complete turns may be made before any increase in length could be observed. The time of revolving varies in different species. It also depends upon the weather—being slow or imperceptible when it is cold, and more rapid when it is warmer. Sometimes it stops when everything seems favorable, and starts again after a while. The hop, bean, and morning-glory, are as quick as any. In a sultry day, and when in full vigor, they commonly sweep round the circle in less than two hours. They move by night as well as by day. This sweeping is the cause of the twining. The stem sweeps round in order that it may reach some neighboring support; as it grows it sweeps a wider and wider space, that is, reaches farther and farther out. When it strikes against any solid body, like the stalk of a neighboring plant, it is stopped; but the portion beyond the contact is free to move as before, and, continuing to move on and to lengthen, it necessarily winds itself round the support, that is, twines." The sudden closure of the leaf of the Venus's-flytrap, as soon as it is touched by an insect, and the excitement of the glandular hairs upon its surface, still more closely resemble the instinctive actions of animals, and we here find the power to distinguish between different foreign bodies, for Darwin has shown that, although the leaf will close upon a small piece of meat, it is not excited by contact with a small piece of glass. In animals we meet with a large class of what are called reflex or automatic actions, and these are perhaps a little nearer to true instinctive actions than most of those performed by plants. Such are the actions of the various organs concerned in digestion, which are passive until the presence of food calls them into action, when they at once begin their work. A little higher are those actions which may be performed or controlled by volition, although they are usually automatic; such as winking to protect the eye from injury, and the act of throwing out the arms when in danger of