Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/7

 Here again comes into play another pretty adaptation, long ago noticed by Robert Brown. The stigma is very viscid, but not so viscid as when touched to pull the whole pollinium off the insect's head or off the pencil, yet sufficiently viscid to break the elastic threads (Fig. I., F) by which the packets of pollen-grains are tied together, and leave some of them on the stigma. Hence a pollinium attached to an insect or to the pencil can be applied to many stigmas, and will fertilise all. I have seen the pollinia of Orchis pyramidalis adhering to the proboscis of a moth, with the stump-like caudicle alone left, all the packets of pollen having been left glued to the stigmas of the flowers successively visited. One or two little points must still 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 have found that in a few minutes they entirely lost their power of adhesion. Again, the little discs of membrane, the movement of which, as causing the movement of the pollinium, 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 pollinium remains 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 of great service; for if this action did not take place, and an insect after depressing the lip failed to remove either viscid ball, or if it removed one alone, in the first case both, and in the second case one of the viscid balls would be left exposed to the air; consequently they would quickly lose all adhesiveness, and the pollinia would be rendered absolutely useless. That insects often remove one alone of the two pollinia at a time in many kinds of Orchis is certain; it is even probable that they generally remove only one at a time, for the lower and older flowers almost always have both pollinia removed, and the younger flowers close beneath the buds, which will have been seldomer visited, have frequently only one pollinium removed. In a spike of Orchis maculata I found as many as ten flowers, chiefly the upper ones, which had only one pollinium removed; the other pollinium being in place, with the lip of the rostellum well closed up, and all the mechanism perfect for its subsequent removal by some insect.

The description now given of the action of the organs in Orchis mascula applies to O. morio, fusca, maculata, and latifolia, and to Aceras anthropomorpha. These species present slight and apparently coordinated differences in the length of the caudicle, in the direction of the nectary, in the shape and position of the stigma, but they are not worth detailing. In all, the pollinia undergo, after removal from the anther-cells, the curious movement of depression, which is so necessary to place them in a right position on the insect's head to strike the stigmatic surface of another flower. In Aceras the caudicle is unusually short; the nectary consists of two minute rounded depressions; the stigma is transversely elongated; the two viscid discs lie so close together within the rostellum that they affect each other's outline; this is worth notice, as a step towards the two becoming absolutely confluent, as in O. pyramidalis. Nevertheless, in Aceras a single pollinium is sometimes removed by insects, though more rarely than with the other species.