Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/106

86 Cephalanthera offers only a partial exception to the rule that the flowers of Orchids are generally fertilised by pollen from another plant.

Cephalanthera ensifolia.—According to Delpino, the flowers of this species are visited by insects, as shown by the removal of the pollen-masses. He believes that this is effected by their bodies being first rendered sticky by means of the stigmatic secretion. It is not clear whether the flowers also fertilise themselves. Each pollen-mass is divided into two, instead of being merely sub-divided, so that there are four distinct pollen-masses.

Pogonia ophioglossoides.—The flowers of this plant, an inhabitant of the United States, resemble, as described by Mr. Scudder, those of Cephalanthera in not having a rostellum, and in the pollen-masses not being furnished with caudicles. The pollen consists of powdery grains not united by threads. Self-fertilisation seems to be effectually prevented; and the flowers on distinct plants must intercross, for each plant generally bears only a single flower.

Pterostylis trullifolia and longifolia.—I may here briefly mention some Orchids, inhabitants of Australia and New Zealand, which are included by Lindley in the same family of the Arethuseæ with Cephalanthera and Pogonia, and are remarkable from their labella being extremely sensitive or irritable. Two of the petals and one of the sepals form a hood which encloses the column, as may be seen at A in the accompanying figure of Pterostylis longifolia.

The distal portion of the labellum affords a landing-place for insects, in nearly the same manner as with Cephalanthera; but when this organ is touched it rapidly springs up, carrying with it the touching insect,