BATH FLORA, FERTILISATION, ETC. 223 



below. They lie quite free within the rostellum, being sur- 

 rounded by fluid, except at the back, where each viscid ball firmly 

 adheres to a small portion or disc of the exterior membrane of the 

 rostellum. The ends of the two caudicles are strongly attached 

 to these two little discs of membrane. The membrane forming 

 the whole interior surface of the rostellum is at first continuous ; 

 but as soon as the flower opens the slightest touch causes it to 

 open transversely in a sinuous line in front of the anther-cells and 

 of the little crest or fold of membrane between them (see 

 Fig. 8). This act of rupturing makes no difference in the shape 

 of the rostellum, but converts the front part into a lip, which can 

 easily be depressed. This lip is represented considerably de- 

 pressed in P^ig. 8, and its edge is seen in Fig. 9 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, or 

 hinge, the lip, or pouch, when not pressed down, springs up and 

 encloses the two viscid balls. " I will not aftirm " (writes Darwin) 

 " that the rupturing of the exterior membrane of the rostellum 

 never takes place spontaneously, and no doubt the membrane is 

 prepared for the 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 want of a better term may be 

 called vital. ... At the same time that the rostellum 

 becomes transversely ruptured in fronts it probably (for it was 

 impossible to ascertain this fact from the position of the parts) 

 requires behind it 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 internally the two viscid balls of matter 

 adhere. The line of rupture is thus very complex, but strictly 

 defined. As the two anther cells open longitudinally from top to 

 bottom, even before the flower expands, as soon as the rostellum 

 is properly ruptured from the eff"ects of a slight touch, its lips can 

 be easily depressed, 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 lie in the anther-cells ; the discs still 

 form part of the posterior surface of the rostellunij but are 



