Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/130

110 summit of the pollinium. The leaves are very brittle, and, when placed on the adhesive stigma, large pieces are easily broken off.

Long before the flower expands, the anther-cells, which are pressed against the back of the rostellum, open in their upper part, so that the included pollinia come into contact with the back of the boat-formed disc. The projecting threads then become firmly attached to rather above the middle part of the back of the disc. The anther-cells afterwards open lower down, and their membranous walls contract and become brown; so that by the time the flower is fully expanded the upper part of the pollinia lie quite naked, with their bases resting in a little cup formed by the withered anther-cell, and laterally protected by the clinandrum. As the pollinia thus lie loose, they are easily removed.

The tubular flowers are elegantly arranged in a spire round the spike, and project from it horizontally (fig. A). The labellum is channelled down the middle, and is furnished with a reflexed and fringed lip, on which bees alight; its basal internal angles are produced into two globular processes, which secrete an abundance of nectar. The nectar is collected (n, fig. B) in a small receptacle in the lower part of the labellum. Owing to the protuberance of the inferior margin of the stigma and of the two lateral inflexed nectaries, the orifice into the nectar-receptacle is much contracted. When the flower first opens the receptacle contains nectar, and at this period the front of the rostellum, which is slightly furrowed, lies close to the channelled labellum; consequently a passage is left, but so narrow that only a fine bristle can be passed down it. In a day or two the column moves a little farther from the labellum, and a wider