Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/103

. III. out of seventeen flowers which were examined one day, five had their ridges gnawed, and on the next day, seven out of nine other flowers were in this state. As there was no appearance of slime, I do not believe that they had been attacked by slugs; but whether they had been gnawed by winged insects, which alone would be effectual for cross-fertilisation, I know not. The ridges had a taste like that of the labellum of certain Vandeæ, in which tribe (as we shall hereafter see) this part of the flower is often gnawed by insects. Cephalanthera is the only British Orchid, as far as I have observed, which attracts insects, by thus offering to them solid food.

The early penetration of the stigma by a multitude of pollen-tubes, which were traced far down the stigmatic tissue, apparently gives us another case, like that of the Bee Ophrys, of perpetual self-fertilisation. I was much surprised at this fact, and asked myself: Why does the distal portion of the labellum open for a short period? what is the use of the great mass of pollen above and below that layer of grains, the tubes of which alone penetrate the upper edge of the stigma? The stigma has a large flat viscid surface; and during several years I have almost invariably found masses of pollen adhering to its surface, and the friable pillars by some means broken down. It occurred to me that, although the flowers stand upright, and the pillars are well protected from the wind, yet that the pollen-masses might ultimately topple over from their own weight, and so fall on the stigma, thus completing the act of self-fertilisation. Accordingly, I covered with a net a plant having four buds, and examined the flowers as soon as they had withered; the broad stigmas of three of them were perfectly free from pollen, but a little had fallen on one corner of the fourth. With the exception of