Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/231

. VII. of the pollinium. This gorged condition may perhaps facilitate the rupture of the hinge.

The pollinium does not differ much from that of Catasetum (see fig. 29, D, p. 183); and it lies in like manner curved round the rostellum, which is less protuberant than in that genus. The upper and broad end of the pedicel, however, extends beneath the pollen-masses within the anther; and these are attached by rather weak caudicles to a medial crest on its upper surface.

The viscid surface of the large disc lies in contact with the roof of the stigmatic cavity, so that it cannot be touched by an insect visiting the flower. The anterior end of the disc is furnished with a small dependent curtain (dimly shown in fig. 32); and this, before the act of ejection, is continuously joined on each side to the upper margins of the stigmatic cavity. The pedicel is united to the posterior end of the disc; but when the disc is freed, the lowermost part of the pedicel becomes doubly bent, so that it then appears as if attached by a hinge to the centre of the disc.

The labellum is a highly remarkable structure: it is narrowed at its base into a nearly cylindrical foot-stalk, and its sides are so much reflexed as almost to meet at the back, forming a folded crest on the summit of the flower. After rising up perpendicularly it arches over the apex of the column, against which it is firmly pressed down. The labellum at this point is hollowed out (even in the bud) into a slight cavity, which receives the bent summit of the column. This slight depression manifestly represents the large cavity, with thick fleshy walls, which insects gnaw, on the anterior surface of the labellum in the several species of Catasetum. Here by a singular change of function, the cavity serves to keep the labellum in its proper position on the summit of the column, but is, perhaps,