Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/199

. VII. How then does Nature act? She has endowed these plants with, what must be called for want of a better term, sensitiveness, and with the remarkable power of forcibly ejecting their pollinia even to a considerable distance. Hence, when certain definite points of the flower are touched by an insect, the pollinia are shot forth like an arrow, not barbed however, but having a blunt and excessively adhesive point. The insect, disturbed by so sharp a blow, or after having eaten its fill, flies sooner or later away to a female plant, and, whilst standing in the same position as before, the pollen-bearing end of the arrow is inserted into the stigmatic cavity, and a mass of pollen is left on its viscid surface. Thus, and thus alone, can the five species of Catasetum which I have examined be fertilised.

In many Orchideæ, as in Listera, Spiranthes, and Orchis, the surface of the rostellum is so far sensitive, that, when touched or when exposed to the vapour of chloroform, it ruptures in certain defined lines. So it is in the tribe of the Catasetidæ, but with this remarkable difference, that in Catasetum the rostellum is prolonged into two curved tapering horns, or, as I shall call them, antennæ, which stand over the labellum where insects alight. If these are touched even very lightly, they convey some stimulus to the membrane which surrounds and connects the disc of the pollinium with the adjoining surface, causing it instantly to rupture; and as soon as this happens the disc is suddenly set free. We have also seen in several Vandeæ that the pedicels of the pollinia are fastened flat down in a state of tension, and are highly elastic, so that, when freed, they immediately spring up, apparently for the sake of detaching the pollen-masses from the anther-cells. In the genus Catasetum, on the