Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/274

254 of the exterior surface of the rostellum assume a condition intermediate between that of unaltered membrane and of viscid matter, which has been already alluded to. The actual separation of portions of the rostellum depends in many cases on the excitement from a touch; but how a touch thus acts is at present inexplicable. Such sensitiveness, in the stigma to a touch (and the rostellum, as we know, is a modified stigma), and indeed in almost every other part, is by no means a rare quality in plants.

In Listera and Neottea, if the rostellum is touched, even by a human hair, two points rupture and the loculi containing the viscid matter instantly expel it. Here we have a case towards which as yet no gradation is known. But Dr. Hooker has shown that the rostellum is at first cellular, and that the viscid matter is developed within the cells, as in other Orchids.

The last difference which I will mention in the state of the rostellum of various Orchids is the existence in many Ophreæ of two widely-separated viscid discs, sometimes included in two separate pouches. Here it appears at first sight as if there were two rostella; but there is never more than one medial group of spiral vessels. In the Vandeæ we can see how a single viscid disc and a single pedicel might become divided into two; for in some Stanhopeas the heart-shaped disc shows a trace of a tendency to division; and in Angræcum we have two distinct discs and two pedicels, either standing close together or removed only a little way apart.

It might be thought that a similar gradation from a single rostellum into what appears like two distinct rostella was shown still more plainly in the Ophreæ; for we have the following series,—in Orchis pyramidalis a single disc enclosed in a single pouch—in Aceras two