Page:Darwin - On the movements and habits of climbing plants.djvu/41

 no effect; but loops of string weighing .82 and 1.64 grain acted capriciously, sometimes causing a slight curvature; but they were never clasped, like the far lighter loops of thread by the petioles.

In the nine vigorous plants which I observed, it is certain that neither the slight spontaneous movements nor the slight sensitiveness of the flower-peduncles were of any service to the plants in climbing. If any member of the Scrophulariaceæ had been known to have flower-peduncles used for climbing, or had tendrils produced by their modification, I should have thought that this Maurandia still retained a useless or rudimentary vestige of a former habit; but this view cannot be maintained. We are almost compelled to believe that by some correlation of growth the power of movement has been transferred from the young internodes to the young peduncles, and in the same manner sensitiveness from the young petioles to the young peduncles; but this latter supposition is the more improbable, as I could detect no sensitiveness in the young internodes of the Maurandia, though in a closely allied genus, Lophospermum, the young internodes, as we shall see, are sensitive. By whatever means the peduncles of this Maurandia have acquired their power of spontaneous movement and their sensitiveness, the case is interesting for us; for we can see that if these now useless capacities were a little perfected, the flower-peduncles could be made as useful for climbing as are the flower-peduncles of Vitis and Cardiospermum, as will hereafter be described.

Rhodochiton volubile.—A long flexible shoot swept a large circle, following the sun, in 5 h. 30 m.; and, as the day became warmer, a second circle in 4 h. 10 m. The shoots sometimes make a whole or half spire round a vertical stick, then run up for a space straight, and afterwards make spiral turns in an opposite direction. The petioles of very young leaves, about one-tenth of their full size are highly sensitive, and bend towards any side which has been touched; but they do not move quickly: one, after being lightly rubbed, was perceptibly curved in 1 h. 10 m., and became considerably arched in 5 h. 40 m. after the rubbing; some other petioles, after being rubbed, were scarcely curved in 5 h. 30 m., but in 6 h. 30 m. were distinctly curved. A curvature was perceptible in a petiole in between 4 h. 30 m. and 5 h., after the suspension of a little loop of string. A loop of fine cotton thread, weighing one-sixteenth of a grain, not only slowly caused a petiole to bend, but was ultimately firmly clasped by it, so that it could be withdrawn only by