Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/143

. IV. generally in large quantity, on all the stigmas which I examined.

I will recapitulate the several special adaptations for the fertilisation of this plant. The anther-cells open early, leaving the pollen-masses free, protected by the summit of the column, and with their tips resting on the concave crest of the rostellum. The rostellum then slowly curves over the stigmatic surface, so that its explosive crest stands at a little distance from the summit of the anther; and this is very necessary, otherwise the summit would be caught by the viscid matter, and the pollen for ever locked up. The curvature of the rostellum over the stigma and over the base of the labellum is excellently adapted to favour an insect striking the crest when it raises its head, after having crawled up the labellum and licked the last drop of nectar. The labellum, as C. K. Sprengel has remarked, becomes narrower where it joins the column beneath the rostellum, so that there is no risk of an insect going too much to either side. The crest of the rostellum is so exquisitely sensitive, that a touch from a very minute insect causes it to rupture at two points, and instantly two drops of viscid fluid are expelled, which coalesce. This viscid fluid sets hard in so wonderfully rapid a manner that it rarely fails to cement the tips of the pollinia, nicely laid on the crest of the rostellum, to the forehead of the touching insect. As soon as the rostellum has exploded it suddenly curves downwards so as to project at right angles over the stigma, protecting it from impregnation at an early age, in the same manner as the stigmas of the young flowers of Spiranthes are protected by the labellum clasping the column. But as the column of Spiranthes after a time moves from the labellum, leaving a free passage for the introduc-