Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/128

108 is endowed with a remarkable kind of irritability; for, if the furrow be touched very gently by a needle, or if a bristle be laid along the furrow, it instantly splits along its whole length, and a little milky adhesive fluid exudes. This action is not mechanical, or due to simple violence. The fissure runs up the whole length of the rostellum, from the stigma beneath to the summit: at the summit the fissure bifurcates, and runs down the back of the rostellum on each side and round the stern of the boat-formed disc. Hence after this splitting action the boat-formed disc lies quite free, but embedded in a fork in the rostellum. The act of splitting apparently never takes place spontaneously. I covered a plant with a net, and after five of the flowers had fully expanded they were kept protected for a week: I then examined their rostella, and not one had split; whereas almost every flower on the surrounding and uncovered spikes, which would almost certainly have been visited and touched by insects, had their rostella fissured, though they had been open for only twenty-four hours. Exposure for two minutes to the vapour of a little chloroform causes the rostellum to split; and this we shall hereafter see is likewise the case with some other Orchids.

When a bristle is laid for two or three seconds in the furrow of the rostellum, and the membrane has consequently become fissured, the viscid matter within the boat-formed disc, which lies close to the surface and indeed slightly exudes, is almost sure to glue the disc longitudinally to the bristle, and both are withdrawn together. When the disc, with the pollinia attached to it, is withdrawn, the two sides of the rostellum (fig. D), which have been described by some botanists as two distinct foliaceous projections, are left sticking up like a fork. This is the common con-