Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/234

214 face of the rostellum. As the two pollen-masses adhere, at first, rather firmly to the anther-case, the latter is torn off by the rebound; and as the thin hinge at the summit of the anther-case does not yield so easily as the basal margin, the pollinium together with the anther-case is instantly swung upwards like a pendulum. But in the course of the upward swing the hinge yields, and the whole body is projected perpendicularly up in the air, an inch or two above and close in front of the terminal part of the labellum. If no object is in the way, as the pollinium falls down, it generally alights and sticks, though not firmly, on the folded crest of the labellum, directly over the column. I witnessed repeatedly all that has been here described.

The curtain of the disc, which, after the pollinium has formed itself into a hoop, lies on the anther-case, is of considerable service in preventing the viscid edge of the disc from adhering to the anther, and thus permanently retaining the pollinium in the form of a hoop. This would have been fatal, as we shall presently see, to a subsequent movement of the pollinium which is necessary for the fertilisation of the flower. In some of my experiments, when the free action of the parts was checked, this did occur, and the pollinium, together with the anther-case, remained permanently glued together in the shape of an irregular hoop.

I have already stated that the minute hinge by which the anther-case is articulated to the column, a little way beneath its bent filamentary apex, is sensitive to a touch. I tried four times and found that I could touch with some force any other part; but when I gently touched this point with the finest needle, instantly the membrane which unites the disc