Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/201

. VII. the parts are a little separated from each other, in order that the mechanism may be intelligible. In the middle of the roof of the stigmatic chamber, far back (d, in A, fig. 28), the upturned anterior edge of the viscid disc can just be seen. The upper membranous surface of the disc, before it is ruptured, is continuous with the fringed bases of the two antennæ between which it lies. The rostellum projects over the disc and stigmatic chamber (see section C, fig. 29), and is prolonged on each side so as to form the two antennæ; the middle part is covered by the ribbon-like, pedicel (ped.) of the pollinium. The lower end of the pedicel is attached to the disc, and the upper end to the two pollen-masses (p) within the anther-cell. The pedicel in its natural position is held much bowed round the protuberant rostellum; when freed it forcibly straightens itself, and at the same time its lateral edges curl inwards. At an early period of growth, it is continuous with the rostellum, but subsequently becomes separated from it by the solution of a layer of cells.

The pollinium when set free and after it has straightened itself, is represented at D, fig. 29. Its under surface, which lies in contact with the rostellum, is shown at E, with the lateral edges of the pedicel now curled inwards. In this latter view, the clefts in the under sides of the two pollen-masses are shown. Within these clefts, near their bases, a layer of strong extensible tissue is attached, forming the caudicles, by which the pollen-masses are united to the pedicel. The lower end of the pedicel is joined to the disc by a flexible hinge, which occurs in no other genus, so that the pedicel can play backwards and forwards, as far as the upturned end (fig. D) of the disc permits. The disc is large and thick ; it consists of a strong upper