Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/147

. IV. fall or be transported by minute crawling insects on to the stigma. By this means self-fertilisation is assured, should larger insects fail to visit the flowers. Moreover, the pollen in this state readily adheres to any object; so that by a slight change in the shape of the flower, which is already less open or more tubular than that of Listera, and by the pollen becoming friable at a still earlier age, its fertilisation would be rendered more and more easy without the aid of the explosive rostellum. Ultimately it would become a superfluity; and then, on the principle that every part which is not brought into action tends to disappear, from causes which I have elsewhere endeavoured to explain, this would happen with the rostellum. We should then see a new species, in the condition of Cephalanthera as far as its means of fertilisation were concerned, but in general structure closely allied to Neottia and Listera.

Mr. Fitzgerald, in the introduction to his 'Australian Orchids,' says that Thelymitra carnea, one of the Neotteæ, invariably fertilises itself by means of the incoherent pollen falling on the stigma. Nevertheless a viscid rostellum, and other structures adapted for cross-fertilisation are present. The flowers seldom expand, and never until they have fertilised themselves; so that they seem tending towards a cleistogene condition. Thelymitra longifolia is likewise fertilised in the bud, according to Mr. Fitzgerald, but the flowers open for about an hour on fine days, and thus cross-fertilisation is at least possible. On the other hand, the species of the allied genus Diuris are said to be wholly dependent on insects for their fertilisation.