Page:Darwin - On the movements and habits of climbing plants.djvu/35

 of a grain is, these facts are remarkable. But I have reason to believe that even a less weight causes a curvature when acting over a broader surface than can be affected by thin thread. Having noticed that the tail of a suspended string, which accidentally touched a petiole, had caused it to bend, I took two pieces of thin twine, 10 inches in length (weighing 1.64 gr.), and, tying them to a stick, let them hang as nearly perpendicularly downwards as their thinness and flexuous nature, after being stretched, would permit; I then quietly placed their ends so as just to rest on two petioles with their tips hanging about the tenth of an inch beneath; both these petioles certainly became curved in 36 h. One of the ends of string, which just touched the angle between a terminal and lateral sub-petiole, was in 48 h. caught as by a forceps between them. In these cases the pressure, though spread over a wider surface than that touched by the cotton thread, must have been excessively slight.

Clematis vitalba.—My plants in pots were not healthy; so that I dare not trust my observations, which indicated much similarity in habits with C.flammula. I mention this species only because I saw many proofs that the petioles of plants growing naturally are excited to movement by very slight pressure. For instance, I found petioles which had clasped thin withered blades of grass, the soft young leaves of a maple, and the lateral flower-peduncles of the quaking-grass or Briza: the latter are only about as thick as a hair from a man's beard, but they were completely surrounded and clasped. The petioles of a leaf, so young that none of the leaflets had expanded, had partially seized on a twig. The petioles of almost every old leaf, even when unattached to any object, are much convoluted; but this is owing to their having come, whilst young, into contact during several hours with some object subsequently removed. With the several above-described species, cultivated in pots and thus carefully observed, there never was any bending of the petioles without the stimulus of contact. When winter comes on, the blades of the leaves of C. vitalba drop off; but the petioles (as was also observed by Mohl) remain, sometimes during two seasons, attached to the branches; and, being convoluted, they curiously resemble true tendrils, such as those occurring in the allied genus Naravelia. The petioles which have clasped an object become much more woody, stiff, hard, and polished than those which have failed in this their proper purpose.

.—I observed T. tricolorum, T. azureum, T.