Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/63

 chamber.

After trials made on fifteen flowers of three species, I find that no moderate degree of violence on any part of the flower, excepting the antennæ, produces any effect. But when the right-hand antenna of C. saccatum, or either antenna of the two following species, is touched, the pollinium is instantly ejected. The extreme tip and the whole length of the antennæ are sensitive. In one specimen of C. tridentatum a touch from a bristle sufficed; in five specimens of C. saccatum a gentle touch from a fine needle was necessary; but in four other specimens a slight blow was requisite. In C. tridentatum a stream of air and of cold water from a small pipe did not suffice; nor in any case did a touch from a human hair; so that the anteninnæ are less sensitive than the rostellum of Listera. Such extreme sensitiveness would indeed have been useless to the plant, for we have reason to believe that the flowers are visited by bulky insects.

That the disc does not separate by the simple mechanical movement of the antennæ is almost certain; for the antennæ firmly adhere for a considerable space to the sides of the stigmatic chamber, and are thus immoveably fixed near their bases. The flowers in some cases, when they first arrived, were not sensitive, but after the spikes had stood for a day or two in water they became sensitive. Whether this was owing to fuller maturity or to the absorption of water, I do not know. Two flowers of C. callosum, which were completely torpid, were immersed in tepid water for an hour; and then the antennæ became highly sensitive; this indicates that the cellular tissue of the antennæ must be turgid in order to receive and convey the effects of a touch; and it would lead to the suspicion that a vibration is conveyed along them; if this be so, the vibration must be of some special nature, for the ordinary jars of manifold greater force do not cause the act of rupture. Two flowers placed in hot water, but not so hot as to scald my fingers, spontaneously ejected their pollinia. The loss of a plant, on which I intended to try other experiments, prevented my ascertaining whether drops of vapour of acrid fluids would act. From these latter facts it may be doubted whether it can be a vibration from the gentle touch of a needle which is conveyed along the antenine. In C. tridentatum I found that the antennæ were one inch and one-tenth of an inch in length, and a gentle touch from a bristle on the extreme tip was conveyed, as far as I could perceive, instantaneously throughout this length. I measured the length of several cells in the tissue composing the antennæ, and on a rough average it appeared that the stimulus must travel through no less than from seventy to eighty closed cells.

We may, at least, safely conclude that the antennæ, which are characteristic of the genus Catasetum, are specially adapted to receive and convey the effects of a touch to the disc of the pollinium; causing the membrane to rupture, and the whole pollinium to be ejected by its elasticity. If we required further proof, nature has afforded it in the case of the so-called genus Monachanthus, which, as we shall see, is the female plant of Catasetum tridentatum, and has no pollinia to eject, and here the antennæ are entirely absent.

I have stated that in C. saccatum the right-hand antennæ invariably hangs down, with the tip turned slightly outwards, and that it is almost paralysed, I ground my belief on five trials, in which I violently hit, bent, and pricked this antenna, and produced no effect; but immediately afterwards touching the left-hand antenna with much less force, the pollinium was shot forth. In a sixt case a forcible blow on the right-hand antenna did cause the act of ejection, so that it is not completely paralysed. As this antenna does not guard the labellum, which seems in all Orchids to be the part attractive to insects, its sensitiveness would be useless.

From the large size of the flower, more especially of the viscid disc, and from its wonderful power of adhesion, we may safely infer that the flowers are visited by large insects. The viscid matter sticks so firmly when it has set hard, and the pedicel is so strong (though very thin and only one-twentieth of an inch in breadth at the hinge), that to my surprise it supported for a few seconds a weight of 1262 grains, that is nearly three ounces; and it supported for a considerable time a slightly less weight. When the pollinium is shot forth, the large spike-like anther is generally carried with it. When the disc strikes a flat surface like a table, the momentum from the weight of the anther often carries the pollen-bearing end beyond the disc, and the pollinium is thus affixed in a wrong direction, supposing it to have been attached to an insect's body, for the fertilisation of another flower. The flight is also often rather crooked.