Page:Darwin - On the movements and habits of climbing plants.djvu/78

 length by growth. When a stick, flat on one side, was similarly placed, the tip of the tendril could not curl beyond the flat surface, but coiled itself into a helix, which, turning to one side, lay flat on the little flat surface of wood. In one instance a portion of tendril three-quarters of an inch in length was thus dragged on to the flat surface by the coiling in of the helix. But the tendril thus acquires a very insecure hold, and generally slips off: in one case alone the helix subsequently uncoiled itself, and the tip then passed round and clasped the stick. The formation of a helix on the flat side of a stick apparently shows us that the continued striving of the tip to curl itself closely inwards gives the force which drags the tendril round a smooth cylindrical stick. In this latter case, whilst the tendril was slowly and quite insensibly crawling onwards, I several times observed through a lens that the whole surface was not in close contact with the stick; and I can understand the onward movement only by supposing that it is slightly vermicular, or that the tip alternately straightens itself a little and then again curls inwards, thus dragging itself onwards by an insensibly slow, alternate movement, which may be compared to that of a strong man suspended by the ends of his fingers to a horizontal pole, who works his fingers onwards until he can grasp the pole with the palm of his hand. However this may be, the fact is certain that a tendril which has caught a round stick by its extreme point can work itself onwards until it has passed twice or even thrice round the stick, and has permanently grasped it.

Hanburya Mexicana.—The young internodes and tendrils of this anomalous member of the family revolve in the same manner and at about the same rate with the Echinocystis. The stem does not twine, but can ascend an upright stick by the aid of its tendrils. The concave tip of the tendril is very sensitive; after rapidly coiling into a loop from a single touch, it straightened itself in 50 m. The tendril, when in full action, stands vertically up, with the young projecting extremity of the shoot thrown a little on one side out of the way; but the tendril bears near its base, on the inner side, a short branch, which projects out at right angles, like a spur, with the terminal half bowed a little downwards. Hence, as the main vertical branch of the tendril revolves, the spur, from its position and rigidity, cannot pass over the extremity of the shoot in the same curious manner as do the three branches of the tendril of the Echinocystis by stiffening themselves at the proper point, but is pressed laterally against the young shoot in one part of the revolving course, and in another part is carried only a little