Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/173

. VI. for instance, in Phalænopsis and Saccolabium, yet when I inserted their pollinia attached to a rough object into the stigmatic chamber, they did not adhere with sufficient force to prevent their removal from the object. I even left them for some little time in contact with the adhesive surface, as an insect would do whilst feeding; but when I pulled the pollinia straight out of the stigmatic chamber, the caudicles, though they were stretched to a great length, did not rupture, nor did their attachment to the object yield so that the balls of pollen were withdrawn. It then occurred to me that an insect in flying away would not pull the pollinia straight out of the chamber, but would pull at nearly right angles to its orifice. Accordingly I imitated the action of a retreating insect, and dragged the pollinia out of the stigmatic chamber at right angles to its orifice; and now the friction on the caudicles thus caused, together with the adhesiveness of the stigmatic surface, generally sufficed to rupture them; the pollen-masses being left on the stigma. Thus, it seems that the great strength and extensibility of the caudicles, which, until stretched, lie embedded within the pollen-masses, serve to protect the pollen-masses from being accidentally lost by an insect whilst flying about, and yet, by friction being brought into play, allow them at the proper time, to be left adhering to the stigmatic surface; the fertilisation of the flower being thus safely effected.

The discs and pedicels of the pollinia present great diversities in shape, and an apparently exhaustless number of adaptations. Even in species of the same genus, as in Oncidium, these parts difier greatly. I here give a few figures (fig. 24), taken almost at hazard. The pedicel generally consists, as far as I have seen, of a thin ribbon-shaped membrane (fig. A); sometimes