Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/31

. I. touch or the vapour of chloroform causes the exterior membrane of the rostellum to rupture along certain defined lines.

At the same time that the rostellum becomes transversely ruptured in front, it probably (for it was impossible to ascertain this fact from the position of the parts) ruptures behind in two oval lines, thus separating and freeing from the rest of the exterior surface of the rostellum the two little discs of membrane, to which the two caudicles are attached externally, and to which the two balls of viscid matter adhere internally. The line of rupture is thus very complex, but strictly defined.

As the two anther-cells are open longitudinally in front from top to bottom, even before the flower expands, it follows that as soon as the rostellum is properly ruptured from the effects of a slight touch, its lip can be depressed easily, and, the two little discs of membrane being already separate, the two pollinia now lie absolutely free, but are still embedded in their proper places. So that the packets of pollen and the caudicles still lie within the anther-cells; the discs still form part of the rostellum, but are separate; and the balls of viscid matter still lie concealed within the rostellum.

Now let us see in the case of Orchis mascula (fig. 1) how this complex mechanism acts. Suppose an insect to alight on the labellum, which forms a good landing-place, and to push its head into the chamber (see side view, A, or front view, B), at the back of which lies the stigma (s), in order to reach with its proboscis the end of the nectary; or, which does equally well to show the action, push very gently a sharply-pointed common pencil into the nectary. Owing to the pouch-formed rostellum projecting into the gangway of the nectary,