Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/275

. IX. discs touching and affecting each other's shapes, but not actually joined—in Orchis latifolia and maculata two quite distinct discs but 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 became divided into two distinct organs; on the contrary, it shows 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 two anther-cells (see fig. 1, B and D, p. 8). In both divisions of the Ophreæ—namely the species having naked discs and those having discs enclosed in a pouch—whenever the two discs come into close juxta-position, this medial crest or process appears. On the other hand, when the two discs stand widely apart, 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 in the formation of the folded crest. In Herminium monorchis, 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