Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/271

. IX. more viscid matter. In Habenaria chlorantha the exterior surface is highly viscid, but still closely resembles, under the microscope, the exterior membrane of Epipactis. Lastly, in some species of Oncidium, &c., the exterior surface, which is viscid, differs, as far as appearance under the microscope goes, from the underlying viscid layer only in colour; but it must have some essential difference, for I find that, until this very thin exterior layer is disturbed, the underlying matter remains viscid; but, after it has been disturbed, the underlying matter rapidly sets hard. The gradation in the state of the surface of the rostellum is not surprising, for in all cases the surface is cellular in the bud; so that an early condition has only to be retained more or less perfectly.

The nature of the viscid matter differs remarkably in different Orchids: in Listera it sets hard almost instantly, more quickly than plaster of Paris; in Malaxis and Angræcum it remains fluid for several days; but these two states pass into each other by many gradations. In an Oncidium I have observed the viscid matter to dry in a minute and a half; in some species of Orchis in two or three minutes; in Epipactis in ten minutes; in Gymnadenia in two hours; and in Habenaria in over twenty-four hours. After the viscid matter of Listera has set hard, neither water nor weak spirits of wine has any effect on it; whereas that of Habenaria bifolia, after having been dried for several months, when moistened became as adhesive as ever it was. The viscid matter in some species of Orchis, when remoistened, presented an intermediate condition.

One of the most important differences in the state of the rostellum is, whether or not the pollinia are permanently attached to it. I do not allude to those