Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/277

. IX. has lost its normal function of being fertilised. Its shape is most singular, with the upper end thickened, bent over and produced into two long tapering and sensitive antennæ, each of these being hollow within, like an adder's fang. Behind and between the bases of these antennæ we see the large viscid disc, attached to the pedicel; the latter differs in structure from the underlying portion of the rostellum, and is separated from it by a layer of hyaline tissue, which spontaneously dissolves when the flower is mature. The disc, attached to the surrounding parts by a membrane which ruptures as soon as it is excited by a touch, consists of strong upper tissue, with an underlying elastic cushion, coated with viscid matter; and this again in most Orchids is overlaid by a film of a different nature. What an amount of specialisation of parts do we here behold! Yet in the comparatively few Orchids described in this volume, so many and such plainly-marked gradations in the structure of the rostellum have been described, and such plain facilities for the conversion of the upper pistil into this organ, that, we may well believe, if we could see every Orchid which has ever existed throughout the world, we should find all the gaps in the existing chain, and every gap in in many lost chains, filled up by a series of easy transitions.

We now come to the second great peculiarity in the Orchideæ, namely their pollinia. The anther opens early, and often deposits the naked masses of pollen on the back of the rostellum. This action is prefigured in Canna, a member of a family nearly related to the Orchideæ, in which the pollen is deposited on the pistil, close beneath the stigma. In the state of the pollen there is great diversity: in Cypripedium and Vanilla