Chap. IX. MOVEMENTS OF THE POLLINIA. 271 



South Brazil, the prominences on the labellum of 

 Oncidium gnawed. We are thus enabled to under- 

 stand the meaning of the various extraordinary crests 

 and projections on the labellum of many Orchids ; for 

 they invariably stand in such a position that insects, 

 whilst gnawing them, would be almost sure to touch 

 the viscid discs of the poUinia and thus remove them, 

 afterwards effecting the fertilisation of another flower. 



MOVEMENTS OF THE POLLINIA. 



The poUinia of many Orchids undergo a movement 

 of depression, after they have been removed from their 

 places of attachment and have been exposed for a few 

 seconds to the air. This is due to the contraction of a 

 portion, sometimes to an exceedingly minute portion, 

 of the exterior surface of the rostellum, which retains 

 a membranous condition. This membrane, as we have 

 seen, is likewise sensitive to a touch, so as to rupture 

 in certain definite lines. In a Maxillaria the middle 

 part of the pedicel, and in Habenaria the whole drum- 

 like pedicel contracts. The point of contraction in all 

 the other cases seen by me, is either close to the 

 surface of attachment of the caudicle to the disc, or at 

 the point where the pedicel is united to the disc ; but 

 both the disc and pedicel are parts of the exterior 

 surface of the rostellum. In these remarks I do not 

 refer to the movements which are simply due to the 

 elasticity of the pedicel, as in the Vandese. 



The long strap-formed disc of Gymnadenia conopsea 

 is well adapted to show the mechanism of the move- 

 ment of depression. The whole poUinium, both in its 

 upright and depressed (but not closely depressed) 

 position, has been shown (p. 65) in fig. 10. The disc, 

 in its uncontracted condition with the caudicle removed, 



