Ohap. JX. gradation OF ORGANS. 257 



has lost its normal function of being fertilised. Its 

 shape is most singular, with the upper end thickened, 

 bent over and produced into two long tapering and 

 sensitive antennae, each of these being hollow within, 

 like an adder's fang. Behind and between the bases 

 of these antennae we see the large viscid disc, attached 

 to the pedicel ; the latter differs in structure from the 

 underlying portion of the rostellum, and is separated 

 from it by a layer of hyaline tissue, which spontaneously 

 dissolves when the flower is mature. The disc, attached 

 to the surrounding parts by a membrane which ruptures 

 as soon as it is excited by a touch, consists of strong 

 upper tissue, with an underlying elastic cushion, 

 coated with viscid matter; and this again in most 

 Orchids is overlaid by a film of a differeiifc nature. 

 What an amount of specialisation of paifcs do we 

 here behold ! Yet in the comparatively few Orchids 

 described in this volume, so many and such plainly- 

 marked gradations in the structure of the rostellum 

 have been described, and such plain facilities for the 

 conversion of the upper pistil into this organ, that, we 

 may well believe, if we could see every Orchid which 

 has ever existed throughout the world, we should find 

 all the gaps in the existing chain, and every gap in 

 in many lost chains, filled up by a series of easy 

 transitions. 



We now come to the second great peculiarity in the 

 Orchideae, namely their pollinia. The anther opens 

 early, and often deposits the naked masses of pollen on 

 the back of the rostellum. This action is prefigured 

 in Canna, a member of a family nearly related to the 

 Orchideae, in which the pollen is deposited on the pistil, 

 close beneath the stigma. In the state of the pollen 

 there is great diversity : in Cypripedium and Vanilla 



