Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/124

104 to the air. The protuberant surface of the rostellum, when gently rubbed upwards, is easily removed, and carries with it a strip of membrane, to the hinder part of which the pollinia are attached. The sloping sides which support the rostellum are not removed at the same time, but remain projecting up like a fork and soon wither. The anther is borne on a broad elongated filament; and a membrane on both sides unites this filament to the edges of the stigma, forming an imperfect cup or clinandrum. The anther-cells open in the bud, and the pollen-masses become attached by their anterior faces, just beneath their summits, to the back of the rostellum. Ultimately the anther opens widely, leaving the pollinia almost naked, but partially protected within the membranous cup or clinandrum. Each pollinium is partially divided lengthways; the pollen-grains cohere in subtriangular packets, including a multitude of compound grains, each consisting of four grains; and these packets are tied together by strong elastic threads, which at their upper ends run together and form a single flattened brown elastic ribbon, of which the truncated extremity adheres to the back of the rostellum.

The surface of the orbicular stigma is remarkably viscid, which is necessary in order that the unusually strong threads connecting the packets of pollen should be ruptured. The labellum is partially divided into two portions; the terminal portion is reflexed, and the basal portion is cup-formed and filled with nectar. The passage between the rostellum and labellum is contracted whilst the flower is young; but when mature the column moves further back from the labellum, so as to allow of insects with the pollinia adhering to their proboscides, to enter the flowers more freely. In many of the specimens received, the pollinia had been