Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/34

14 pollinia of Orchis pyramidalis adhering to the proboscis of a moth, with the stump-like caudicles alone left, all the packets of pollen having been left glued to the stigmas of the successively visited flowers.

One or two other little points must be noticed. The balls of viscid matter within the pouch-formed rostellum are surrounded with fluid; and this is very important, for, as already mentioned, the viscid matter sets hard when exposed to the air for a very short time. I have pulled the balls out of their pouches, and found that they had entirely lost the power of adhesion after a few minutes. Again, the little discs of membrane, the movement of which, as causing the movement of the pollinia, is so absolutely indispensable for the fertilisation of the flower, lie at the upper and back surface of the rostellum, and are closely enfolded and thus kept damp within the bases of the anther-cells; and this is very necessary, as an exposure of about thirty seconds causes the movement of depression to take place; but as long as the disc is kept damp, the pollinia remain ready for action whenever removed by an insect.

Lastly, as I have shown, the pouch, after being depressed, springs up to its former position; and this is likewise of great service; for if this action did not take place, and an insect after depressing the lip failed to remove the two viscid balls, or if it removed one alone, then in the first case both, and in the second case one would be left exposed to the air; consequently one or both would quickly lose all adhesiveness, and the pollinium would be rendered absolutely useless. That with many kinds of Orchids insects often remove only one of the two pollinia at a time is certain; it is even probable that they generally remove only one, for the lower and older