Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/117

. IV. landing-place for insects, is joined to the basal half by a narrow hinge, and naturally (fig. A) is turned a little upwards, so that its edges pass within the edges of the basal portion. So flexible and elastic is the hinge that the weight of even a fly, as Mr. More informs me, depresses the distal portion; it is represented in fig. B in this state; but when the weight is removed it instantly springs up to its former position (fig. A), and with its curious medial ridges partly closes the entrance into the flower. The basal portion of the labellum forms a cup, which at the proper time is filled with nectar.

Now let us see how all the parts, which I have been obliged to describe in detail, act. When I first examined these flowers I was much perplexed: trying in the same manner as I should have done with a true Orchis, I slightly pushed the protuberant rostellum downwards, and it was easily ruptured; some of the viscid matter was withdrawn, but the pollinia remained in their cells. Reflecting on the structure of the flower, it occurred to me that an insect in entering one in order to suck the nectar, would depress the distal portion of the labellum, and consequently would not touch the rostellum; but that, when within the flower, it would be almost compelled, from the springing up of this distal half of the labellum, to rise a little upwards and back out parallel to the stigma. I then brushed the rostellum lightly upwards and backwards with the end of a feather and other such objects; and it was pretty to see how easily the membranous cap of the rostellum came off, and how well from its elasticity it fitted any object, whatever its shape might be, and how firmly it clung to the object owing to the viscidity of its under surface. Large masses of pollen, adhering by the elastic threads to the cap of the rostellum were at the same time withdrawn.