Page:Darwin - On the movements and habits of climbing plants.djvu/96

 thick or thin branches were tossed to and fro by the wind, the attached tendrils, had they not been excessively elastic, would instantly have been torn off and the plant thrown prostrate. But as it was, the Bryony safely rode out the gale, like a ship with two anchors down, and with a long range of cable ahead to serve as a spring as she surges to the storm.

With respect to the exciting cause of the spiral contraction, little can be said. After reading Prof. Oliver's interesting paper on the hygroscopic contraction of legumes, I allowed a number of different kinds of tendrils to dry slowly, but no spiral contraction ensued; nor did this occur with the tendrils of the Bryony when placed in water, diluted alcohol, and syrup of sugar. We know that the act of clasping a support leads to a change in the nature of their tissues; and we call this a vital action, and so we must call the spiral contraction. The contraction is not related to the spontaneous revolving power, for it occurs in tendrils, such as those of Lathyrus grandiflorus and Ampelopsis hederacea, which do not revolve. It is not necessarily related to the curling of the tips round a support, as we see in the case of the Ampelopsis and Bignonia capreolata, in which the development of the adherent disks suffices to induce the contraction. Yet it certainly seems to stand in some close relation to the curling or clasping movement due to contact with a support; for not only does it soon follow this act, but the spiral contraction generally begins close to the curled extremity, and travels down towards the base, as if the whole tendril tried to imitate the movement of its extremity. If, however, a tendril be very slack, the whole length seems to become almost simultaneously at first flexuous and then spiral. The spiral contraction of a tendril when unattached cannot serve any of the useful ends just described; it does not occur with many kinds of tendrils which contract when attached; and when it does occur, it supervenes, as we have seen, only after a considerable interval of time. It may almost be likened to certain instinctive or habitual movements performed by animals under circumstances rendering them manifestly useless.

When an uncaught tendril contracts spirally, the spire always runs in the same direction from tip to base. A tendril, on the other hand, which has caught a support by its extremity, invariably becomes twisted in one part in one direction, and in another part in the opposite direction; the oppositely turned spires being