Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/269

. IX. secure the pollen-masses, but indirectly by means of their attachment to an insect's body.

The viscid matter of the rostellum and of the stigma appear to have nearly the same nature; that of the rostellum generally has the peculiar property of quickly drying or setting hard; that of the stigma, when removed from the plant, apparently dries more quickly than gum-water of about equal density or tenacity. This tendency to dry is the more remarkable, as Gärtner found that drops of the stigmatic secretion from Nicotiana did not dry in two months. The viscid matter of the rostellum in many Orchids when exposed to the air changes colour with remarkable quickness, and becomes brownish-purple; and I have noticed a similar but slower change of colour in the viscid secretion of the stigmas of some Orchids, as of Cephalanthera grandiflora. When the viscid disc of an Orchis, as Bauer and Brown have observed, is placed in water, minute particles are expelled with violence in a peculiar manner; and I have observed exactly the same fact in the layer of viscid matter covering the stigmatic utriculi in an unopened flower of Mormodes ignea.

In order to compare the minute structure of the rostellum and stigma, I examined young flower-buds of Epidendrum cochleatum and floribundum, which, when mature, have a simple rostellum. The posterior parts of both organs were quite similar. The whole of the rostellum at this early age consisted of a mass of nearly orbicular cells, containing spheres of brown matter, which resolve themselves into the viscid fluid. The stigma was covered with a thinner layer of similar cells, and beneath them were the coherent spindle