Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/176

156 the stigma to be seated at the bottom of a deep cavity, low down in the column, or the anther to be seated higher up, or the pedicel of the rostellum to slope more upwards, &c.—all of which contingencies occur in various species,—in such cases, an insect with a pollinium attached to its head, if it flew to another flower, would not place the pollen-masses on the stigma, unless their position had become greatly changed after attachment.

This change is effected in many Vandeæ in the same manner as is so general with the Ophreæ, namely, by a movement of depression in the pollinium in the course of about half a minute after its removal from the rostellum. I have seen this movement conspicuously displayed, generally causing the pollinium to rotate through about a quarter of a circle, in several species of Oncidium, Odontoglossum, Brassia, Vanda, Aerides, Sarcanthus, Saccolabium, Acropera, and Maxillaria. In Rodriguezia suaveolens the movement of depression is remarkable from its extreme slowness; in Eulophia viridis from its small extent. Mr. Charles Wright, in a letter to Professor Asa Gray, says that he observed in Cuba a pollinium of an Oncidium attached to a humble-bee, and he concluded at first that I was completely mistaken about the movement of depression; but after several hours it moved into the proper position for fertilising the flower. In some of the cases above specified in which the pollinia apparently undergo no movement of depression, I am not sure that there was not a very slight one after a time. In the various Ophreæ the anther-cells are sometimes seated exteriorly and sometimes interiorly with respect to the stigma; and there are corresponding outward and inward movements in the pollinia: but in the Vandeæ the anther-cells always lie, as far as I have seen,