Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/249

. VIII. in all other Orchids except Vanilla belongs exclusively to the rostellum and the two confluent stigmas. These latter organs, on the other hand, in Cypripedium entirely lose their viscidity, and at the same time become slightly convex, so as more effectually to rub off the glutinous pollen adhering to the body of an insect. Moreover in several of the North American species, as in C. acaule and pubescens, the surface of the stigma is beset, as Professor Asa Gray remarks, "with minute, rigid, sharp-pointed papillæ, all directed forwards, which are excellently adapted to brush off the pollen from an insect's head or back." There is one partial exception to the above rule of the pollen of Cypripedium being viscid while the stigma is not viscid and is not convex; for in C. acaule the pollen is more granular and less viscid, according to Asa Gray, than in the other American species, and in C. acaule alone the stigma is slightly concave and viscid. So that here the exception almost proves the truth of the general rule.

I have never been able to detect nectar within the labellum, and Kurr makes the same remark with respect to C. calceolus. The inner surface of the labellum, however, in those species which I examined, is clothed with hairs, the tips of which secrete little drops of slightly viscid fluid. And these if sweet or nutritious would suffice to attract insects. The fluid when dried forms a brittle crust on the summits of the hairs. Whatever the attraction may be, it is certain that small bees frequently enter the labellum.

Formerly I supposed that insects alighted on the labellum and inserted their proboscides through either