Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/296

276 the introduction of the pollen-masses attached to the proboscis of a humble-bee. In Mormodes ignea the labellum is perched on the summit of the column, and here insects alight and touch a sensitive point, causing the ejection of the pollen-masses. The labellum is often deeply channelled, or has guiding ridges, or is pressed closely against the column; and in a multitude of cases it approaches closely enough to render the flower tubular. By these several means insects are forced to brush against the rostellum. We must not, however, suppose that every detail of structure in the labellum is of use: in some instances, as with Sarcanthus, its extraordinary shape seems to be partly due to its development in close apposition to the curiously shaped rostellum.

In Listera ovata the labellum stands far from the column, but its base is narrow, so that insects are led to stand exactly beneath the middle of the rostellum, In other cases, as in Stanhopea, Phalænopsis, Gongora, &c., the labellum is furnished with upturned basal lobes, which manifestly act as lateral guides. In some cases, as in Malaxis, the two upper petals are curled backwards so as to be out of the way; in other cases as in Acropera, Masdevallia, and some Bolbophyllums, these upper petals plainly serve as lateral guides, compelling insects to visit the flowers directly in front of the rostellum. In other cases, wings formed by the margins of the clinandrum or of the column, serve as lateral guides, both in the withdrawal of the pollinia and in their subsequent insertion into the stigmatic cavity. So that there can be no doubt that the petals, sepals and rudimentary anthers do good service in several ways, besides affording protection to the bud.

The final end of the whole flower, with all its parts, is the production of seed; and these are produced by