Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/177

. VI. 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 made to strike them by a peculiar mechanical arrangement of the parts.

In the Ophreæ the seat of contraction, which causes the act of depression, is in the upper surface of the viscid disc, close to the point of attachment of the caudicles: in most of the Vandeæ the seat is likewise in the upper surface of the disc, but at the point where the pedicel is united to it, and therefore at a considerable distance from the point of attachment of the true caudicles. The contraction is hygrometric, but to this subject I shall return in the ninth chapter; therefore the movement does not take place until the pollinium has been removed from the rostellum, and the point of union between the disc and pedicel has been exposed for a few seconds or minutes to the air. If, after the contraction and consequent movement of the pedicel, the whole body be placed into water, the pedicel slowly moves back and resumes its former position with respect to the viscid disc. When taken out of water, it again undergoes the movement of depression. It is of importance to notice these facts, as we thus get a test by which this movement can be distinguished from certain other movements.

In Maxillaria ornithorhyncha, we have a unique case. The pedicel of the rostellum is much elongated, and is entirely covered by the produced front lip of the anther, and is thus kept damp. When removed it bends quickly backwards on itself, at about its central point, and thus becomes only half as long as it was before. When placed in water it resumes its original straight form. If the pedicel had not been in some