Charles Darwin's Works Volume 6 - Softcover

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1877 Excerpt: ...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 Vandeae in the same manner as is so general with the Ophreae, 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, ia 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 Ophreae 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 Vandeae the anther-cells always lie, as far as I have seen, directly over the stigma, and the movement of the pollinium is always directly downwards. In Calanthe, however, the two stigmas are placed exteriorly to the anther-cells, and the pollinia, as we shall see, are mad...