Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/293

. IX. E, p. 18) the two flaps or sides contract and curl inwards; and this causes the divergence of the pollinia. By a kind of contraction two valleys are likewise formed in front of the caudicles, so that the latter are thrown forwards and downwards, almost in the same way as if trenches were dug in front of two upright poles, and then carried on so as to undermine them. As far as I could perceive, an analogous contraction causes the depression of the pollinia in Orchis mascula. With O. hircina both pollinia are attached to a single rather large square disc, the whole front of which, after exposure to the air, sinks down and is then separated from the hinder part by an abrupt step. By this contraction both pollinia are carried forwards and downwards.

Some pollinia which had been gummed on card for several months, when placed in water, rose up and afterwards underwent the movement of depression. A fresh pollinium, on being alternately damped and exposed to the air, rises and sinks several times alternately. Before I had ascertained these facts, which show that the movement is simply hygrometric, I thought that it was a vital action, and tried vapour of chloroform and of prussic acid, and immersion in laudanum; but these reagents did not check the movement. Nevertheless, there are some difficulties in understanding how the movement can be simply hygrometric. The flaps of the saddle in Orchis pyramidalis (see fig. 3, D, p. 18) curl completely inwards in nine seconds, which is a surprisingly short time for mere evaporation to produce an effect; and the