Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/30

10 consists of a mass of polygonal cells, full of brownish matter, which cells soon resolve themselves into two balls of extremely viscid semi-fluid matter, void of structure. These viscid masses are slightly elongated, almost flat on the top, and convex below. They lie quite free within the rostellum (being surrounded by fluid), except at the back, where each viscid ball adheres to a small portion or disc of the exterior membrane of the rostellum. The ends of the two caudicles are strongly attached externally to these two little discs of membrane.

The membrane forming the whole exterior surface of the rostellum is at first continuous; but as soon as the flower opens the slightest touch causes it to rupture transversely in a sinuous line, in front of the anther-cells and of the little crest or fold of membrane (see fig. D) between them. This act of rupturing makes no difference in the shape of the rostellum, but converts the front part into a lip, which can be depressed easily. This lip is represented considerably depressed in fig. D, and its edge is seen, fig. B, in the front view. When the lip is thoroughly depressed, the two balls of viscid matter are exposed. Owing to the elasticity of the hinder part, the lip or pouch, after being pressed down, springs up again and encloses the two viscid balls.

I will not affirm that the rupturing of the exterior membrane of the rostellum never takes place spontaneously; and no doubt the membrane is prepared for rupture by having become very weak along defined lines; but several times I saw the act ensue from an excessively slight touch—so slight that I conclude that the action is not simply mechanical, but, for the want of a better term, may be called vital. We shall hereafter meet with other cases, in which the slightest