Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/291

. IX. South Brazil, the prominences on the labellum of Oncidium gnawed. We are thus enabled to understand the meaning of the various extraordinary crests and projections on the labellum of many Orchids; for they invariably stand in such a position that insects, whilst gnawing them, would be almost sure to touch the viscid discs of the pollinia and thus remove them, afterwards effecting the fertilisation of another flower.

MOVEMENTS OF THE POLLINIA.

The pollinia of many Orchids undergo a movement of depression, after they have been removed from their places of attachment and have been exposed for a few seconds to the air. This is due to the contraction of a portion, sometimes to an exceedingly minute portion, of the exterior surface of the rostellum, which retains a membranous condition. This membrane, as we have seen, is likewise sensitive to a touch, so as to rupture in certain definite lines. In a Maxillaria the middle part of the pedicel, and in Habenaria the whole drum-like pedicel contracts. The point of contraction in all the other cases seen by me, is either close to the surface of attachment of the caudicle to the disc, or at the point where the pedicel is united to the disc; but both the disc and pedicel are parts of the exterior surface of the rostellum. In these remarks I do not refer to the movements which are simply due to the elasticity of the pedicel, as in the Vandeæ.

The long strap-formed disc of Gymnadenia conopsea is well adapted to show the mechanism of the movement of depression. The whole pollinium, both in its upright and depressed (but not closely depressed) position, has been shown (p. 65) in fig. 10. The disc, in its uncontracted condition with the caudicle removed,