Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/85

. II. rapidly set hard. Their nectaries are stored with free nectar. With respect to the uncovered condition of the discs, the last species, or Peristylus viridis, is in an almost intermediate condition. The four following species compose a much broken series. In Gymnadenia conopsea the vicid discs are narrow and much elongated, and lie close together; in G. albida they are less elongated, but still approximate; in Habenaria bifolia they are oval and far apart; and, lastly, in H. clorantha they are circular and much farther apart.

Gymnadenia conopsea.—In general appearance this plant resembles pretty closely a true Orchis. The pollinia differ in having naked, narrow, strap-shaped

.



.

discs, which are as long as the caudicles (fig. 10). When the pollinia are exposed to the air the caudicle is depressed in from thirty to sixty seconds; and as the posterior surface of the caudicle is slightly hollowed out, it closely clasps the upper membranous surface of the disc. The mechanism of this movement will be described in the last chapter. The elastic threads by which the packets of pollen are bound together are unusually weak, as is likewise the case with