Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/163

. V. two great tribes might have been run together; as the distinction drawn from the presence of caudicles does not always hold good. But difficulties of this nature are frequently encountered in the classification of largely developed or so-called natural groups, in which there has been comparatively little extinction.

I will begin with the genus Cattleya, of which I have examined several species. These are fertilised in a very simple manner, different from that in any British Orchid. The rostellum (r, fig. 22, A, B) is a broad, tongue-shaped projection, which arches slightly over the stigma; the upper surface is formed of smooth membrane; the lower surface together with the central portion (originally a mass of cells) consists of a very thick layer of viscid matter. This viscid mass is hardly separated from the viscid matter thickly coating the stigmatic surface which lies close beneath the rostellum. The projecting upper lip of the anther rests on, and opens close over the base of the upper membranous surface of the tongue-shaped rostellum. The anther is kept closed by a spring, at its point of attachment on the top of the column. The pollinia consist of four (or eight in Cattleya crispa) waxy masses, each furnished (see figs. C and D) with a ribbon-like tail, formed of a bundle of highly elastic threads, to which numerous separate pollen-grains are attached. The pollen therefore consists of two kinds, namely, waxy masses and separate though compound grains (each, as usual, consisting of four) united by elastic threads. This latter kind of pollen is identical with that of Epipactis and other Neotteæ. These tails, with their appended pollen-grains, act as caudicles,