Chap. XX. GRADATION OF OEGANS. 253 



after protruding through a small opeuing iu the mem- 

 branous case in a semi-developed state, they adhere 

 to the rostellum, against which the anther is pressed. 

 So that the adhesion of the caudicle to the back of the 

 rostellum seems to depend solely on the early rupturing 

 of the anther-case, and on a slight protrusion of the 

 caudicles, before they have become fully developed and 

 hardened. 



In all the Orchidese a portion of the rostellum is 

 removed by insects when the poUinia are removed ; for 

 the viscid matter, though conveniently spoken of as a 

 secretion, is in fact part of the rostellum in a modified 

 condition. But in those species which have their 

 caudicles attached at an early period to the rostellum, 

 a membranous or solid portion of its exterior surface in 

 an unmodified condition is likewise removed. In the 

 Vandeaj this portion is sometimes of considerable size 

 (forming the disc and pedicel of the poUinium), and 

 gives to their poUinia their remarkable character; but 

 the differences in the shape and size of the removed 

 portions of the rostellum can be finely graduated to- 

 gether, even within the single tribe of the Vandese ; 

 and still more closely by commencing with the minute 

 oval atom of membrane to which the caudicle of Orchis 

 adheres, passing thence to that of Mahenaria hifoUa, to 

 that of if. chlorantha with its drum-like pedicel, and 

 thence through many forms to the great disc and 

 pedicel of Catasetum. 



In all the cases in which a portion of the exterior 

 surface of the rostellum is removed together with the 

 caudicles of the pollen-masses, definite and often com- 

 plicated lines of separation are formed, so as to allow 

 of the easy separation of the removed portions. But 

 the formation of these lines of separation does not 

 differ much from the process by which certain portions 



