Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/85

 of a touch; and how this is effected is at present inexplicable. But sensitiveness to touch in the stigma (and the rostellum, as we know, is a modified stigma), and indeed in all the organs of vegetation, is not a very rare attribute of many plants.

In Listera and Neottia, when the rostellum is touched, even by a human hair, two points rupture, and the included viscid matter is instantaneously expelled. Here we have a case towards which as yet no gradation is known. But Dr. Hooker has shown that the structure of the rostellum is at first cellular (as in other Orchids), and that the viscid matter originally developed within these cells is contained, apparently in a state of tension, in the loculi, ready to be expelled as soon as the exterior surface ruptures.

The last and conspicuous difference in the state of the rostellum which I will mention is the existence, in many Ophreæ, of two widely-separated viscid discs, sometimes included in two separate pouches. Here it at first appears as if there were two rostella; but there is never more than one medial group of spiral vessels. In the Vandeæ we can see how a single viscid disc and single pedicel might become divided into two; for in some Stanhopeas the heart-shaped disc shows a trace of a tendency to division; and in Angræcum we have two distinct discs and two pedicels, either standing close together or removed a little way apart.

It might be thought that a similar gradation from a single rostellum into what appears like two distinct rostella was still more plainly shown in the Ophrea; for we have the following series, in Orchis pyramidalis a single disc enclosed in a single pouch; in Aceras two discs touching and affecting each other's shapes, but not actually joined; in Orchis latifolia and maculata two quite distinct discs with the pouch still showing plain traces of division; and, lastly, in Ophrys we have two perfectly distinct pouches, including of course two perfectly distinct discs. But this series does not indicate the former steps by which a single rostellum has become divided into two distinct organs; but shows, on the contrary, how the rostellum, after having been anciently divided into two organs, has now in several cases been reunited into a single organ.

This conclusion is founded on the nature of the little medial crest (sometimes called the rostellate process) between the bases of the anther-cells (see Fig. I., B and D). In both divisions of the Ophreæ(those with naked discs and those with discs enclosed in a pouch), whenever the two discs come into close juxtaposition, the medial crest or process appears. On the other hand, when the two discs stand widely separate, the summit of the rostellum between them is smooth, or nearly smooth. In the Frog Orchis (Peristylus viridis) the overarching summit is bent like the roof of a house; and here we see the first stage of the formation of the folded crest.

In Herminium, however, which has two separate and large discs, a crest, or solid ridge, is rather more plainly developed than might have been expected. In Gymnadenia conopsea, Orchis maculata, and others, the crest consists of a hood of thin membrane; in O. mascula the two sides of the hood have partly adhered; and in O. pyramidalis and in Aceras it has been converted into a solid ridge. These facts are intelligible only on the view, that, whilst the two discs were gradually, during a long line of generations, brought together, the intermediate portion or summit of the rostellum became more and more arched, until a folded crest, and finally a solid ridge, was formed.