Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/230

210 right or left, according to the position of the flower on the spike. The twisted stigmatic surface extends down to the base of the column and is hollowed out into a deep cavity at its upper end. The large viscid disc of the pollinium is lodged in this cavity close beneath the rostellum; and the rostellum is seen in the drawing (pd.) covered by the bowed pedicel.

The anther-case (a in the figure) is elongated and triangular, closely resembling that of Catasetum; but it does not extend up to the apex of the column. The apex consists of a thin flattened filament, which from the analogy of Catasetum I suppose to be the produced filament of the stamen; but it may be a prolongation of some other element of the column. In the bud-state it is straight, but before the flower expands, it becomes much bent by the pressure of the labellum. A group of spiral vessels runs up the column as far as the summit of the anther-case; they are then reflexed and run some way down the anther-case. The point of reflexion forms a short thin hinge by which the top of the anther-case is articulated to the column beneath its bent summit. The hinge, although smaller than a pin's head in size, is of paramount importance; for it is sensitive and conveys the stimulus from a touch to the disc of the pollinium, causing it to separate from its place of attachment. The hinge also serves to guide the pollinium during its ejection. As it has to convey the necessary stimulus to the disc, one may suspect that a portion of the rostellum, which lies in close contact with the filament of the anther, runs up to this point; but I could not here detect any difference in structure on comparing these parts with those of Catasetum. The cellular tissue round the hinge is gorged with fluid, and a large drop exudes when the anther is torn from the column during the ejection