Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/295

. IX. compelling insects to visit the flowers in front. Few persons now doubt the correctness of C. K. Sprengel's view, that the bright and conspicuous colours of flowers serve to attract insects from a distance. Nevertheless some Orchids have singularly inconspicuous and greenish flowers, perhaps in order to escape some danger; but many of these are strongly scented, which would equally well serve to attract insects.

The labellum is by far the most important of the external envelopes of the flower. It not only secretes nectar, but is often modelled into variously shaped receptacles for holding this fluid, or is itself rendered attractive so as to be gnawed by insects. Unless the flowers were by some means rendered attractive, most of the species would be cursed with perpetual sterility. The labellum always stands in front of the rostellum, and its outer portion often serves as a landing-place for the necessary visitors. In Epipactis palustris this part is flexible and elastic, and apparently compels insects in retreating to brush against the rostellum. In Cypripedium the distal portion is folded over like the end of a slipper, and compels insects to crawl out of the flower by one of two special passages. In Pterostylis and a few other Orchids the labellum is irritable, so that when touched it shuts the flower, leaving only a single passage by which an insect can escape. In Spiranthes, when the flower is fully mature, the column moves from the labellum, space being thus left for