Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/115

. IV. stigma is bilobed and projects in front of the column (see s in the side and front views, C, D, fig. 15). On its square summit a single, nearly globular rostellum is seated. The anterior face of the rostellum (r, C, D) projects a little beyond the surface of the upper part of the stigma, and this is of importance. In the early bud the rostellum consists of a friable mass of cells, with the exterior surface rough: these superficial cells undergo a great change during development, and become converted into a soft, smooth, highly elastic membrane or tissue, so excessively tender that it can be penetrated by a human hair; when thus penetrated, or when slightly rubbed, the surface becomes milky and in some degree viscid, so that the pollen-grains adhere to it. In some cases, though I observed this more plainly in Epipactis latifolia, the surface of the rostellum apparently becomes milky and viscid without having been touched. This exterior soft elastic membrane forms a cap to the rostellum, and is internally lined with a layer of much more adhesive matter, which, when exposed to the air, dries in from five to ten minutes. By a slight upward and backward push with any object, the whole cap, with its viscid lining, is removed with the greatest ease; a minute square stump, the basis of the rostellum, being alone left on the summit of the stigma.

In the bud-state the anther stands quite free behind the rostellum and stigma; it opens longitudinally whilst the flower is still unexpanded, and exposes the two oval pollen-masses, which now lie loose in their cells. The pollen consists of spherical granules, cohering in fours, but not affecting each other's shapes: and these compound grains are tied together by fine elastic threads. The threads are collected into bundles extending longitudinally along the middle line of the