Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/156

136 other, and afford the only entrance; but the presence of these two minute windows (fig. 20) shows how necessary it is that insects should visit the flower in this case as in that of most other Orchids. How insects perform the act of fertilisation I have failed to understand. At the bottom of the roomy and dark chamber formed by the closed sepals, the minute column stands, and in front of it is the furrowed labellum, with a highly flexible hinge, and on each side the two upper petals; a little tube being thus formed. When therefore a minute insect enters, or which is less probable, a larger insect inserts its proboscis through either window, it has to find by the sense of touch the inner tube in order to reach the nectary at the base of the flower. Within the little tube, formed by the column, labellum, and lateral petals, a broad and hinged rostellum projects at right angles, which can easily be upturned. Its under surface is viscid, and this viscid matter soon sets hard and dry. The minute caudicles of the pollinia, projecting out of the anther-case, rest on the base of the upper membranous surface of the rostellum. The stigmatic cavity when mature is not very deep. After cutting away the sepals I vainly endeavoured, by pushing a bristle into the tubular flower, to remove the pollinia, but by the aid of a bent needle, this was effected without much difficulty. The whole structure of the flower seems as if intended to prevent the flower from being easily fertilised; and this proves that we do not understand its structure. Some small insect had entered one of