Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/77

. II. drawn out of its pouch a movement of depression commences, by which the pollinium if attached to the front of an insect's head would be brought into a proper position for striking the stigma. When a pollen-mass is placed on the stigma and then withdrawn, the elastic threads by which the packets are tied together break, and leave several packets on the viscid surface. In all other Orchids the meaning of these several contrivances is unmistakably clear—namely, the downward movement of the lip of the rostellum when gently pushed—the viscidity of the disc—the depression of the caudicle as soon as the disc is exposed to the air—the rupturing of the elastic threads—and the conspicuousness of the flower. Are we to believe that these adaptations for cross-fertilisation in the Bee Ophrys are absolutely purposeless, as would certainly be the case if this species has always been and will always be self-fertilised? It is, however, just possible that insects, although they have never been seen to visit the flowers, may at rare intervals transport the pollinia from plant to plant, during such seasons as that of 1868, when the pollinia did not all fall out of the anther-cells so as to reach the stigmas. The whole case is perplexing in an unparalleled decree, for we have in the same flower elaborate contrivances for directly opposed objects.

That cross-fertilisation is beneficial to most Orchids, we may infer from the innumerable structures serving for this purpose which they present; and I have elsewhere shown in the case of many other groups of plants that the benefits thus derived are of high importance. On the other hand, self-fertilisation is manifestly advantageous in as far as it ensures a full