Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/139

. IV. and rendered useless. I once found an injured flower which had been pressed and had exploded before fully expanding, and the anther with the enclosed pollen-masses was permanently glued to the crest of the rostellum. The rostellum, which is naturally somewhat arched over the stigma, quickly bends forwards and downwards at the moment of the explosion, so as then to stand (fig. B) at right angles to the surface of the stigma. The pollinia, if not removed by the touching object which causes the explosion, become fixed to the rostellum, and by its movement are likewise drawn a little forward. If their lower ends are now freed by a needle from the anther-cells, they spring up; but they are not by this movement placed on the stigma. In the course of some hours, or of a day, the rostellum not only slowly recovers its original slightly-arched position, but becomes quite straight and parallel to the stigmatic surface. This backward movement of the rostellum is of service; for if after the explosion it had remained permanently projecting at right angles over the stigma, pollen could not readily have been deposited by insects on the viscid surface of the stigma. When the rostellum is touched so quickly that the pollinia are not removed, they are, as I have just said, drawn a little forward; but by the subsequent backward movement of the rostellum they are pushed back again into their original position.

From the account now given we may safely infer how the fertilisation of this Orchid is effected. Small insects alight on the labellum for the sake of the nectar copiously secreted by it; as they lick this they slowly crawl up its narrowed surface until their heads stand directly beneath the overarching crest of the rostellum; when they raise their heads they touch the crest; this then explodes, and the pollinia are instantly