Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/294

274 movement is apparently due to the drying of the under surface, although this is covered with a thick layer of viscid matter. The edges, however, of the saddle might become slightly dry in the nine seconds. When the saddle-formed disc is placed in spirits of wine it contracts energetically; and this is probably due to the attraction of alcohol for water. When replaced in water it opens again. Whether or not the contraction is wholly hygrometric, the movements are admirably regulated in each species, so that the pollen-masses, when transported by insects from flower to flower, assume a proper position for striking the stigmatic surface.

These various movements would be quite useless, unless the pollinia were attached in a uniform position to the insects which visit the flowers so as to be always directed in the same manner after the movement of depression; and this necessitates that the insects should be forced to visit the flowers of the same species in a uniform manner. Hence I must say a few words on the sepals and petals. Their primary function, no doubt, is to protect the organs of fructification in the bud. After the flower is fully expanded, the upper sepal and two upper petals often continue the same office. We cannot doubt that this protection is of service, when we see in Stelis the sepals so neatly reclosing and reprotecting the flower some time after its expansion; in Masdevallia the sepals are permanently soldered together, with two little windows alone left open; and in the open and exposed flowers of Bolbophyllum, the mouth of the stigmatic chamber after a time closes. Analogous facts with respect to Malaxis, Cephalanthera, &c., could be given. But the hood formed by the upper sepal and two upper petals, besides affording protection, evidently forms a guide,