Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/187

. VI. any other such case, not that I fully understand the contrivances in any Orchid, for new and admirable ones become apparent, the longer I study even one of our commonest British species.

The thin and elongated rostellum of Acropera projects at right angles to the column (see diagram, fig. 23, p. 150); and the pedicel of the pollinium is of course equally long and much thinner. The disc consists of an extremely small cap, viscid within, which fits on the extremity of the rostellum. The viscid matter sets hard but slowly. The upper sepal forms a hood enclosing and protecting the column. The labellum is an extraordinary organ, baffling all description: it is articulated to the column by a thin strap, so elastic and flexible that a breath of wind sets it vibrating. It hangs downwards; and the retention of this position seems to be of importance, for the footstalk (ovarium) of each flower is curved into a semicircle, so as to compensate for the pendulous habit of the plant. The two upper petals and the lateral lobes of the labellum serve as guides leading into the hood-like upper sepal.

The pollinium, when adhering by its disc to an object, undergoes the common movement of depression; and this seems superfluous, for the stigmatic cavity lies (see diagram, fig. 23) high up at the base of the rectangularly projecting rostellum. But this is a comparatively trifling difficulty; the real difficulty lies in the orifice of the stigmatic chamber being so narrow that the pollen-masses, though consisting of thin sheets, can hardly be forced in. I repeatedly tried, and succeeded only three or four times. Even after leaving them to dry for four hours before a fire, and thus to shrink a little, I rarely succeeded in forcing them into the stigma. I examined quite young flowers and