Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/96

76 Professor Gray has seen a butterfly (Nisoniades) from Canada with a pollinium of this species attached to each eye. In the case of P. flava, moths are compelled in a different manner to enter the nectary on one side. A narrow but strong protuberance, rising from the base of the labellum, projects upwards and backwards, so as almost to touch the column ; thus the moth, being forced to go to either side, is almost sure to withdraw one of the viscid discs. P. hyperborea and dilatata have been regarded by some botanists as varieties of the same species; and Professor Asa Gray says that he was formerly tempted to come to the same conclusion; but on closer examination he finds, besides other characters, a remarkable physiological difference, namely, that P. dilatata, like its congeners, requires insect aid and cannot fertilise itself; whilst in P. hyperborea the pollen-amasses commonly fall out of the anther-cells whilst the flower is very young or in bud, and thus the stigma is self-fertilised. Nevertheless, the various structures adapted for crossing are still present.

The genus Bonatea is closely allied to Habenaria, and includes plants having an extraordinary structure. Bonatea speciosa is an inhabitant of the Cape of Good Hope, and has been carefully described by Mr. Trimen; but it is impossible to explain its structure without drawings. It is remarkable from the manner in which the two stigmatic surfaces, as well as the two viscid discs, project far out in front of the flower, and from the complex nature of the labellum, which consists of seven, or probably of nine distinct parts all fused