Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/138

118 seconds the whole drop sets hard, soon assuming a purplish-brown tint. So exquisitely sensitive is the rostellum, that a touch from the thinnest human hair suffices to cause the explosion. It will take place under water. Exposure to the vapour of chloroform for about one minute also caused an explosion; but the vapour of sulphuric ether did not thus act, though one flower was exposed for five, and another for twenty minutes to a strong dose. The rostellum of these two flowers when afterwards touched exploded in the usual manner, so that sensitiveness had not been lost in either case. The viscid fluid when pressed between two plates of glass before it has set hard is seen to be structureless; but it has a reticulated appearance, perhaps caused by the presence of globules of a denser immersed in a thinner fluid. As the pointed tips of the pollinia lie on the crest of the rostellum, they are always caught by the exploded drop: I have never seen this once to fail. So rapid is the explosion and so viscid the fluid, that it is difficult to touch the rostellum with a needle, however quickly this may be done, without removing the pollinia. Hence, if a bunch of flowers be carried home in the hand, some of the sepals or petals will almost certainly touch the rostellum and withdraw the pollinia; and this gives the false appearance of their having been ejected to a distance.

After the anther-cells have opened and the naked pollinia have been left resting on the concave back of the rostellum, this latter organ curves a little forwards, and perhaps the anther also moves a little backwards. This movement is of much importance; if it did not occur, the tip of the anther, within which the pollinia are lodged, would be caught by the exploded viscid matter, and the pollinia would be for ever locked up