Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/151

. V. its position it partially protects the organs of fructification (fig. 19). In most of the Orchideæ, the upper sepal and the two upper petals afford protection; but here these two petals and all the sepals are reflexed (as may be seen in the drawing, fig. A), apparently to allow insects freely to visit the flower. The position of the labellum is the more remarkable, because it has been purposely acquired, as shown by the ovarium being spirally twisted. In all Orchids the labellum is properly directed upwards, but assumes its usual position on the lower side of the flower by the twisting of the ovarium; but in Malaxis the twisting has been carried so far that the flower occupies the position which it would have held if the ovarium had not been at all twisted, and which the ripe ovarium afterwards assumes, by a process of gradual untwisting.

When the minute flower is dissected, the column is seen to be longitudinally tripartite; the middle portion of the upper half (see fig. B) is the rostellum. The upper edge of the lower part of the column projects where united to the base of the rostellum, and forms a rather deep fold. This fold is the stigmatic cavity, and may be compared to a waistcoat-pocket. I found pollen-masses which had their broad ends pushed by insects into this pocket; and a bundle of pollen-tubes had here penetrated the stigmatic tissue.

The rostellum, which stands immediately above the stigmatic cavity, is a tall-membranous projection of a whitish colour, formed of square cells, and is covered with a thin layer of viscid matter: it is slightly concave posteriorly, and its crest is surmounted by a minute tongue-shaped mass of viscid matter. The column, with its narrow pocket-like stigma and the