Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/110

90 Mr. Fitzgerald describes and figures several other genera, and states with respect to Acianthus fornicatus and exsertus that neither species produce seeds if protected from insects, but are easily fertilised by pollen placed on their stigmas. Mr. Cheeseman has witnessed the fertilisation of Acianthus sinclairii in New Zealand, the flowers of which are incessantly visited by Diptera, without whose aid the pollinia are never removed. Out of eighty-seven flowers borne by fourteen plants, no less than seventy-one matured capsules. This plant according to the same observer exhibits one remarkable peculiarity, namely, that the pollen-masses are attached to the rostellum by means of the exserted pollen-tubes, which serve as a caudicle; and the pollen-masses are thus removed together with the rostellum, which is viscid, when the flowers are visited by insects. The flowers of the allied Cyrtostylis are also much frequented by insects, but the pollinia are not so regularly removed as those of the Acianthus; and with Corysanthes, only five out of 200 flowers produced capsules.

The Vanillidæ according to Lindley form a subtribe of the Arethuseæ. The large tubular flowers of Vanilla aromatica are manifestly adapted to be fertilised by insects; and it is known that when this plant is cultivated in foreign countries, for instance in Bourbon, Tahiti, and the East Indies, it fails to produce its aromatic pods unless artificially fertilised. This fact shows that some insect in its American home is specially adapted for the work; and that the insects of the above-named tropical regions, where the Vanilla flourishes, either do not visit the flowers, though they secrete an abundance of nectar, or do not visit them