Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/41



As soon as the flower opens, if the crest of the rostellum be touched ever so lightly, a large drop of viscid fluid is instantaneously expelled; this, as Dr. Hooker has shown, is formed by the coalescence of two drops proceeding from the two depressed spaces on each side of the crest. A good proof of this fact was afforded by some specimens kept in weak spirits of wine, which had apparently expelled the viscid matter slowly, and in which two separate little spherical balls of viscid matter had hardened and had become attached to the two pollinia. The fluid is at first slightly opaque and milky; but as soon as exposed to the air, in less time than a second, a film is formed over it, and in two or three seconds the whole drop sets hard, and soon assumes 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 causes the explosion. The viscid fluid when pressed between two plates of glass before it has set hard is seen to be structureless; but it assumes a reticulated appearance, perhaps caused by the presence and union of globules of a denser fluid immersed in a thinner fluid. As the pointed tips of the loose pollinia lie on the crest of the rostellum, they are always caught by the exploded drop: I have never once seen this fail. So rapid is the explosion and so viscid the fluid, that it is difficult to touch the rostellum with a needle quickly enough not to catch the pollinia already attached to the partially-hardened drop. Hence, if a bunch of flowers be carried home in the hand, some of the sepals and petals, being shaken, will be almost sure to touch the rostellum and withdraw the pollinia, which will cause the false appearance of their having been violently ejected to a distance.

After the anther-cells have opened and have left the naked pollinia resting on the concave back of the