Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/37

. I. which I have examined, and which is ranked by several botanists as a distinct genus. The relative position of the parts (fig. 3) is here considerably different from what it is in O. mascula and its allies. There are two quite distinct rounded stigmatic surfaces (s, s, A) placed on each side of the pouch-formed rostellum. This latter organ, instead of standing some height above the nectary, is brought down (see side view B) so as to overhang and partially to close its orifice. The ante-chamber to the nectary, formed by the union of the edges of the labellum to the column, which is large in O. mascula and its allies, is here small. The pouch-formed rostellum is hollowed out on the under side in the middle: it is filled with fluid. The viscid disc is single and of the shape of a saddle (figs. C and E); it carries on its nearly flat top or seat the two caudicles of the pollinia, the ends of which firmly adhere to its upper surface. Before the membrane of the rostellum ruptures, the saddle-formed disc can be clearly seen to be continuous with the rest of the surface. The disc is partially hidden and kept damp (which is of great importance) by the over-folding bases of the two anther-cells. It consists of several layers of minute cells, and is therefore rather thick; it is lined beneath with a layer of highly adhesive matter, which is formed within the rostellum. It corresponds strictly to the two minute, oval, separate discs to which the two caudicles of O. mascula and its allies are attached.

When the flower opens and the rostellum has become symmetrically ruptured, either from a touch or spontaneously (I know not which), the slightest pressure depresses the lip, that is, the lower and bilobed portion of the exterior membrane of the rostellum, which projects into the mouth of the nectary.