Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/248

228 polished internally; and this is of much importance, as it prevents insects which have once entered the labellum from escaping through the great opening in the upper surface. In the position in which the flower grows, as here represented, the dorsal surface of the column is uppermost. The stigmatic surface is slightly protuberant, and is not adhesive; it stands nearly parallel to the lower surface of the labellum. With a flower in its natural state, the margin of the dorsal surface of the stigma can be barely distinguished between the edges of the labellum and through the notch in the rudimentary, shield-like anther (a'); but in the drawing (s, fig. A) the margin of the stigma has been brought outside the edges of the depressed labellum, and the toe is a little bent downwards, so that the flower is represented as rather more open than it really is. The edges of the pollen-masses of the two lateral anthers (a) can be seen through the two small orifices or open spaces in the labellum (fig. A) on each side, close to the column. These two orifices are essential for the fertilisation of the flower.

The grains of pollen are coated by and immersed in viscid fluid, which is so glutinous that it can be drawn out into short threads. As the two anthers stand behind and above the lower convex surface (see fig. B) of the stigma, it is impossible that the glutinous pollen can without some mechanical aid get on to this, the efficient surface of the stigma. The economy here shown by Nature in her manner of gaining the same end is surprising. In all the other Orchids seen by me, the stigma is viscid and more or less concave, by which means the dry pollen, transported by means of the viscid matter secreted by the rostellum or modified stigma, is retained. In Cypripedium the pollen is glutinous, and assumes the function of viscidity, which