Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/221

. VII. remarkable manner from that of the male Catasetum tridentatum, and it is no wonder that the two plants were formerly ranked as distinct genera.

The pollen-masses offer so curious and good an illustration of a structure in a rudimentary condition, that they are worth description; but I must first recur to the perfect pollen-masses of the male Catasetum. These may be seen at D and E, fig. 29, attached to the pedicel: they consist of a large sheet of cemented or waxy pollen-grains, folded over so as to form a sack, with an open slit along the lower surface, within which at the lower and produced end, a layer of highly elastic tissue, forming the caudicle, is attached; the other end being attached to the pedicel of the rostellum. The exterior grains of pollen are more angular, have thicker walls, and are yellower than the interior grains. In the early bud the two pollen-masses are enveloped in two conjoined membranous sacks, which are soon penetrated by the two produced ends of the pollen-masses and by their caudicles; and afterwards the extremities of the caudicles adhere to the pedicel. Before the flower expands the membranous sacks including the two pollen-masses open; and the pollen-masses are left resting naked on the back of the rostellum.

In Monachanthus, on the other hand, the two membranous sacks containing the rudimentary pollen-masses never open; but they easily separate from each other and from the anther. The tissue of which they are formed is thick and pulpy. Like most rudimentary parts, the pollen-masses vary much in size and form; they are only about one-tenth of the bulk of those of the male; they are flask-shaped (p, fig. 31), with the lower end greatly produced so as almost to penetrate the exterior or membranous sack. There is