Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/86

66 the two following species of Habenaria: this was well shown by the state of specimens which had been kept in spirits of wine. This weakness apparently stands in relation to the viscid matter of the discs not setting hard and dry as in Orchis; So that a moth with a pollinium attached to its proboscis might be enabled to visit several flowers without having the whole pollinium dragged off by the first stigma which was struck. The two strap-shaped discs lie close together, and form the arched roof of the entrance into the nectary. They are not protected, as in Orchis, by a lower lip or pouch, so that the structure of the rostellum is simpler. When we come to treat of the homologies of the rostellum we shall see that this difference is due to a small change, namely, to the lower and exterior cells of the rostellum resolving themselves into viscid matter; whereas in Orchis the exterior surface retains its early cellular or membranous condition.

As the two viscid discs form the roof of the mouth of the nectary, and are thus brought down near to the labellum, the two stigmas, instead of being confluent and standing beneath the rostellum, as in most of the species of Orchis, are lateral and separate. These stigmas consist of protuberant, almost horn-shaped, processes on each side of the nectary. That their surfaces are really stigmatic I ascertained by finding them deeply penetrated by a multitude of pollen-tubes. As in the case of Orchis pyramidalis, it is a pretty experiment to push a fine bristle straight into the narrow mouth of the nectary, and to observe how certainly the narrow elongated viscid discs, forming the roof, stick to the bristle. When the bristle is withdrawn, the pollinia adhering to its upper side are withdrawn; and as the discs form the sides of the arched roof, they adhere somewhat to the sides