Page:Darwin - On the movements and habits of climbing plants.djvu/108

 rootlets of a plant which had grown up a plaistered wall. Attached to two sets of rootlets on the same branch, I found very many extremely thin threads of a transparent, not viscid, excessively elastic substance, precisely like caoutchouc. These threads, at one end, proceeded from the bark of the rootlet, and at the other end were firmly attached to transparent particles of silex and other hard substances. There could be no mistake in this observation, for I played with the threads for a long time, under the microscope, drawing them out with the dissecting-needles and letting them spring back again. Yet, as I looked repeatedly at other rootlets, similarly treated, and could never discover these elastic threads, I infer that the branch had probably been slightly moved from the wall at some critical period, whilst the fluid secreted from the rootlets was in the act of drying and of changing its nature through the absorption of its watery parts. The genus Ficus abounds with caoutchouc, and from the facts here given we may infer that this substance, at first in solution and ultimately modified into an unelastic cement, is used by Ficus repens to cement its rootlets to any object which it may ascend. Whether most other plants, which climb by their rootlets, emit any cement I do not know; but the rootlets of the Ivy, placed against glass, barely adhered to it, yet secreted a little yellowish matter. I may add, that the rootlets of Marcgravia dubia can adhere firmly to smooth painted wood.

Vanilla aromatica emits aërial roots a foot in length, which point straight down to the ground. According to Mohl (S. 49), these crawl into crevices, and, when they meet with a thin support, wind round it, like tendrils. A plant which I kept was young, and did not form long roots; but on placing thin sticks in contact with them, they certainly bent, in the course of about a day, a little to that side, and adhered by their rootlets to the wood; but they did not bend quite round the sticks, and afterwards they repursued their downward course. If these rootlets are really sensitive to contact and bend to the touched side, in this case the class of root-climbers blends into that of tendril-bearers. According to Mohl, the rootlets of certain species of Lycopodium likewise act as tendrils.

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Plants become climbers, in order, it may be presumed, to reach the light, and to expose a large surface of leaves to its action and to that of the free air. This is effected by climbers with