Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/276

256 O. mascula the two sides of the hood partly adhere; and in O. pyramidalis and in Aceras it is converted into a solid ridge. These facts are intelligible only on the view, that, whilst the two discs were gradually brought together, during a long series of generations, the intermediate portion or summit of the rostellum became more and more arched, until a folded-crest, and finally a solid ridge was formed.

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Whether we compare together the state of the rostellum in the various tribes of the Orchideæ, or compare the rostellum with the pistil and stigma of an ordinary flower, the differences are wonderfully great. A simple pistil consists of a cylinder surmounted by a small viscid surface. Now, see what a contrast the rostellum of Catasetum, when dissected from all the other elements of the column, presents; and as I traced all the vessels in this Orchid, the drawing may be trusted as approximately accurate. The whole organ