Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/111

. III. in the proper manner. I will mention only two peculiarities in the structure of the flowers: the anterior part of the pollen-masses is semi-waxy and the posterior part somewhat friable; the grains are not cemented together into compound grains, and the single grains are not united by fine elastic threads but by viscid matter; this matter would aid in causing the pollen to adhere to an insect, but I should have thought that such aid was superfluous, as the viscid rostellum is well developed. The other peculiarity is that the labellum, in front of the stigma, and some way beneath it, is furnished with a stiff hinged brush, formed of a series of combs one over the other, which point downwards. This structure would allow an insect to crawl easily into a flower, but would compel it whilst retreating to press close against the column; and in doing so it would remove the pollen-masses, leaving them on the stigma of the next flower which was visited.

The genus Sobralia is allied to Vanilla, and Mr. Cavendish Browne informs me that he saw a large humble-bee enter a flower of S. macrantha in his hothouse, and when it crawled out it had the two large pollen-masses firmly fixed to its back, nearer to the tail than to the head. The bee then looked about, and seeing no other flower re-entered the same one of