Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/310

290 slow movement of the column from the labellum in Spiranthes—the diœcious condition of Catasetum—the fact of some species producing only a single lower, &c.—all render it certain or highly probable that the flowers are habitually fertilised with pollen from a distinct plant.

That cross-fertilisation, to the complete exclusion of self-fertilisation, is the rule with the Orchideæ, cannot be doubted from the facts already given in relation to many species in all the tribes throughout the world. I could almost as soon believe that flowers in general were not adapted for the production of seeds, because there are a few plants which have never been known to yield seed, as that the flowers of the Orchideæ are not as a general rule adapted so as to ensure cross-fertilisation. Nevertheless, some species are regularly or often self-fertilised; and I will now give a list of all the cases hitherto observed by myself and others. In some of these the flowers appear often to be fertilised by insects, but they are capable of fertilising themselves without aid, though in a more or less incomplete manner; so that they do not remain utterly barren if insects fail to visit them. Under this head may be included three British species, namely, Cephalanthera grandiflora, Neottia nidus-avis, and perhaps Listera ovata. In South Africa Disa macrantha often fertilises itself; but Mr. Weale believes that it is likewise cross-fertilised by moths. Three species belonging to the Epidendreæ rarely open their flowers in the West Indies; nevertheless these flowers fertilise themselves, but it is doubtful whether they are fully fertilised, for a large proportion of the seeds spontaneously produced by some members of this tribe in a hothouse were destitute of an embryo. Some species of Dendrobium, judging from their structure and from their