254 GKADATION OF ORGANS. Chap. IX. 



of the exterior surface of the rostellum assume a con- 

 dition intermediate between that of unaltered mem- 

 brane and of viscid matter, which has been already 

 alluded to. The actual separation of portions of the 

 rostellum depends in many cases on the excitement 

 from a touch ; but how a touch thus acts is at present 

 inexplicable. Such sensitiveness in the stigma to a 

 touch (and the rostellum, as we know, is a modified 

 stigma), and indeed in almost every other part, is by 

 no means a rare quality in plants. 



In Listera and Neottea, if the rostellum is touched, 

 even by a human hair, two points rupture and the 

 loculi containing the viscid matter instantly expel it. 

 Here we have a case towards which as yet no gradation 

 is known. But Dr. Hooker has shown that the ros- 

 tellum is at first cellular, and that the viscid matter 

 is developed within the cells, as in other Orchids. 



The last difference which I will mention in the 

 state of the rostellum of various Orchids is the exist- 

 ence in many Ophrese of two widely-separated viscid 

 discs, sometimes included in two separate pouches. 

 Here it appears at first sight as if there were two 

 rostella ; but there is never more than one medial 

 group of spiral vessels. In the Vanclese we can see 

 how a single viscid disc and a single pedicel might 

 become divided into two ; for in some Stanhopeas the 

 heart-shaped disc shows a trace of a tendency to divi- 

 sion; and in Angrsecum we have two distinct discs 

 and two pedicels, either standing close together or 

 removed only a little way apart. 



It might be thought that a similar gradation from a 

 single rostellum into what appears like two distinct ros- 

 tella was shown still more plainly in the Ophrese ; for 

 we have the following series, — in Orchis injramidalis a 

 single disc enclosed in a single pouch — in Aceras two 



