256 GEADATION OF ORGANS. Chap. IX. 



0. mascula the two sides of the hood partly adhere ; 

 and in 0. j^yramidalis and in Aceras it is converted 

 into a solid ridge. These facts are intelligible only 

 on the view, that, whilst the two discs were gradually 

 brought together, during a long series of generations, 

 the intermediate portion or summit of the rostellum 

 became more and more arched, until a folded crest, 

 and finally a solid ridge was formed. 



Rostellum of CATASETUia. 



an. antennas of rostellum. I ped. pedicel of rostellum, to which 



d. viscid disc. I the pollen-masses are attached. 



Whether we compare together the state of the ros- 

 tellum in the various tribes of the Orchidese, or com- 

 pare the rostellum with the pistil and stigma of an 

 ordinary flower, the differences are wonderfully great. 

 A simple pistil consists of a cylinder surmounted by 

 a small viscid surface. Now, see what a contrast the 

 rostellum of Catasetum, when dissected from all the 

 other elements of the column, presents ; and as I 

 traced all the vessels in this Orchid, the drawing may 

 b(^ trusted as approximately accurate. The whole organ 



