Page:Darwin - On the movements and habits of climbing plants.djvu/76

 nearly or quite vertical. This occurred both when the supporting internodes were free and when they were tied up; but was perhaps most conspicuous in the latter case, or when the whole shoot happened to stand in an inclined position. The tendril forms a very acute angle with the extremity of the shoot, which projects above the point where the tendril arises; and the stiffening always occurred as the tendril approached, and had to pass in its revolving course, the point of difficulty—that is, the projecting extremity of the shoot. Unless the tendril had the power of thus acting, it would strike against the extremity of the shoot, and be arrested by it. As soon as all three branches of the tendril have begun to stiffen themselves in this remarkable manner, as if by a process of turgescence, and to rise from an inclined into a vertical position, the revolving movement becomes more rapid; and as soon as the tendril has succeeded in passing the extremity of the shoot, its revolving motion, coinciding with that from gravity, often causes it to fall into its previously inclined position so quickly, that the end of the tendril could be distinctly seen travelling like the minute hand of a gigantic clock.

The tendrils are thin, from 7 to 9 inches in length, with a pair of short lateral branches rising not far from the base. The tip is slightly but permanently curved, so as to act to a limited extent as a hook. The concave side of the tip is highly sensitive to a touch, but not so the convex side, as was likewise observed by Mohl (S. 65) with other species of the family. I repeatedly proved this difference by lightly rubbing four or five times the convex side of one tendril, and only once or twice the concave side of another tendril, and the latter alone curled inwards: in a few hours afterwards, when those which had been rubbed on the concave side had recovered themselves, I reversed the process of rubbing, and always with a similar result. After touching the concave side, the tip becomes sensibly curved in one or two minutes; and subsequently, if the touch has been at all rough, it becomes coiled into a helix. But this helix will, after a time, uncoil itself, and be ready to act again. A loop of thin thread only one-sixteenth of a grain in weight caused a temporary flexure in a tendril. One of my plants had two shoots near each other, and the tendrils were repeatedly drawn across each other, but it is a singular fact that they did not once catch each other. It would appear as if the tendrils had become habituated to the contact of other tendrils, for the pressure thus caused would apparently be greater than that caused by a loop of soft thread weighing only the one-sixteenth