Page:Darwin - On the movements and habits of climbing plants.djvu/52

 caused still further bending; so that ultimately both petioles clasped the stick in opposite directions, and the foot-like tendrils, seizing on each other or on their petioles, fastened the stem to the support with surprising security. Hence this species, differently from the last, uses its tendrils, by the intervention of the spontaneously moving and sensitive petioles, when the stem twines round a thin vertical stick. Both species use their tendrils in the same manner when passing through a thicket. This plant seems to me the most efficient climber which I have examined; and it probably could ascend a polished stem incessantly tossed by heavy storms. To show how important vigorous health is for the action of all the parts, I may mention that when I first examined a plant which was growing pretty well, though not vigorously, I concluded that the tendrils acted only like the hooks on a bramble, and that this was the most feeble and inefficient of all climbers!

Bignonia Tweedyana.—This species is closely allied to, and behaves in all respects like the last; perhaps it twines round a vertical stick rather better. On the same plant, one branch twined in one direction and another in an opposite direction. The internodes in one case made two circles, each in 2 h. 33 m. I was enabled in this species to observe, better than in the two preceding, the spontaneous movements of the petioles: one described three small vertical ellipses in the course of eleven hours, another moved laterally in an irregular spire. Some little time after the stem has twined round an upright stick, and is securely fastened to it by the clasping petioles and tendrils, it emits at the base of its leaves aërial roots, which curve partly round and adhere to the stick; so that this one species of Bignonia combines four different methods of climbing, generally characteristic of distinct plants, namely, twining, leaf-climbing, tendril-climbing, and root-climbing.

In the foregoing three species, when the foot-like tendril has caught any object, it continues to grow and to thicken, and ultimately it becomes wonderfully strong, in the same manner as we have seen with the petioles of leaf-climbers. If the tendril catches nothing, it first slowly bends downwards, and then its power of clasping is lost. Very soon afterwards it disarticulates itself from the petiole, like a leaf in autumn from the stem, and drops off. I have seen this process of disarticulation in no other tendrils, but when uncaught they soon wither away.

Bignonia venusta.—The tendrils are here considerably modified