Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/108

88 upper part of the column, with their edges meeting in front, as may be seen in fig. B. In this drawing the petal on the near side has been cut away, and the labellum is represented in the position which it assumes after having been touched. As soon as the labellum has thus risen, an imprisoned insect cannot escape except by crawling through the narrow passage formed by the two projecting shields. In thus escaping it can hardly fail to remove the pollinia, as, before coming into contact with them, its body will have been smeared with the viscid matter of the rostellum. On being imprisoned in another flower, and on again escaping by the same passage, it will almost certainly leave at least one of the four pollen-masses on the adhesive stigma, and thus fertilise the flower.

All that I have here said is taken from the admirable description given by Mr. Cheeseman of Pterostylis trullifolia; but I have copied the figure of P. longifolia from Mr. Fitzgerald's great work on the Australian Orchids, as it shows plainly the relation of all the parts.

Mr. Cheeseman placed insects within several flowers of P. trullifolia, and saw them afterwards crawl out, generally with pollinia attached to their backs. He also proved the importance of the irritable labellum by removing it from twelve flowers whilst young, and in this case insects which entered the flowers would not have been compelled to crawl out through the passage; and not one of these flowers produced a capsule. The flowers seem to be frequented exclusively by Diptera; but what attraction they present is not known, as they do not secrete nectar. Mr. Cheeseman believes that hardly a quarter of the flowers produce capsules; notwithstanding that on one occasion he examined 110