Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/208

188 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 antennæ are less sensitive than the rostellum of Listera. Such extreme sensitiveness would indeed have been useless to the plant, for, as is now known, the flowers are visited by powerful insects.

That the disc does not separate owing to the simple mechanical movement of the antennæ is certain; for they adhere firmly for a considerable space to the sides of the stigmatic chamber, and are thus immovably fixed near their bases. If a vibration is conveyed along them, it must be of some special nature, for ordinary jars of manifold greater strength do not excite the act of rupture. The flowers in some cases, when they first arrived, were not sensitive, but after the cut-off 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 know not. 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 either that the cellular tissue of the antennæ must be turgid in order to receive and convey the effects of a touch, or, as is more probable, heat increases their sensitiveness. Two other flowers placed in hot water, but not so hot as to scald my fingers, spontaneously ejected their pollinia. A plant of C. tridentatwm had been kept for some days in a rather cool house, and the antennæ were consequently in a torpid condition; a flower was cut off and placed in water at a temperature of 100° F. (37.7° C.), and no effect was immediately produced; but when it was