Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/171

. VI. tion, at first appearing as a zone of hyaline tissue, is gradually formed, which sets free the disc, as well as the whole upper surface of the rostellum, as far back as the point of attachment of the caudicles. If any object now touches the viscid disc, it, together with the whole back of the rostellum, the caudicles and pollen-masses, can all be readily removed together. In botanical works the whole structure between the disc or viscid surface (generally called the gland) and the balls of pollen is designated as the caudicle; but as these parts play an essential part in the fertilisation of the flower, and as they are fundamentally different in their origin and in their minute structure, I shall call the two elastic ropes, which are developed strictly within the anther-cells, the caudicles; and the portion of the rostellum to which the caudicles are attached (see diagram), and which is not viscid, the pedicel. The viscid portion of the rostellum I shall call, as heretofore, the viscid surface or disc. The whole may be conveniently spoken of as the pollinium.

In the Ophreæ we have (except in O. pyramidalis and a few other species), two separate viscid discs. In the Vandeæ, with the exception of Angræcum, we have only one disc. The disc is naked, or is not enclosed in a pouch. In Habenaria the discs, as we have seen, are separated from the two caudicles by short drum-like pedicels, answering to the single and generally much more largely developed pedicel in the Vandeæ. In the Ophreæ the caudicles of the pollinia, though elastic, are rigid, and serve to place the packets of pollen at the right distance from the insect's head or proboscis, so as to reach the stigma. In the Vandeæ this end is gained by the pedicel of the rostellum. The two caudicles in the Vandeæ are embedded and attached within a deep cleft in the pollen-masses,