Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/158

138 The labellum in all the species which I have seen, more especially in B. rhizophoræ, is remarkable by being joined to the base of the column, by a very narrow, thin, white strap, which is highly elastic and flexible; it is even highly elastic when stretched, like an india-rubber band. When the flowers of this species were blown by a breath of wind the tongue-like labella all oscillated to and fro in a very odd manner. In some species not seen by me, as in B. barbigerum, the labellum is furnished with a beard of fine hairs, and these are said to cause the labellum to be in almost constant motion from the slightest breath of air. What the use can be of this extreme flexibility and liability to movement in the labellum, I cannot conjecture, unless it be to attract the notice of insects, as the flowers of these species are dull-coloured, small, and inconspicuous, instead of being large, brightly-coloured, and conspicuous or odoriferous, as in so many other Orchids. The labella of some of the species are said to be irritable, but I could not detect a trace of this quality in those examined by me. According to Lindley, the labellum of the allied Megaclinium falcatum spontaneously oscillates up and down.

The last genus of the Malaxeæ which I will mention is Dendrobium, of which one at least of the species, namely D. chrysanthum, is interesting, from being apparently contrived to effect its own fertilisation, if an insect, when visiting the flower, should fail to remove the pollen-masses. The rostellum has an upper and a small lower surface composed of membrane; and between these is a thick mass of milky-white matter which can be easily forced out. This white matter is less viscid than is usual; but when exposed to the air a film forms over it in less than half a minute, and it soon sets into a waxy or cheesy