Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/188

168 almost withered ones, for I imagined that the mouth of the chamber might be of larger size at some period of growth; but the difficulty of insertion remained the same. Now when we observe that the viscid disc is extraordinarily small, and consequently its power of attachment not so firm as with Orchids having a large disc, and that the pedicel is very long and thin, it would seem almost indispensable that the stigmatic chamber should be unusually large for the easy insertion of the pollinium, instead of being much contracted. Moreover, the stigmatic surface, as Dr. Hooker has likewise observed, is singularly little adhesive!

The flowers when ready for fertilisation do not secrete nectar; but this is no difficulty, for as Dr. Crüger has seen humble-bees gnawing the projections on the labellum of the closely allied Gongora maculata, there can be little doubt that the distal cup-shaped part of the labellum of Acropera offers a similar attraction to insects. After numberless trials in many ways, I have found that the pollinia can be removed with certainty only by pushing the rostellum a little upwards with a camel-hair brush, held in such a position that the tip slides along the under side of the rostellum, so as to brush off the little viscid cap on its extremity, into which the hairs enter and are glued fast. I further find that if the brush with a pollinium thus attached to its tip is pushed into and then withdrawn from the stigmatic cavity, the mouth of which is furnished with a sharp ridge, the end of the pedicel