xii] 'ADAPTATION' TO WATER LIFE 171 



A consideration of the structure of submerged leaves opens 

 up a series of perplexing theoretical problems. The idea that 

 the submerged type of leaf arises as an adaptive response to 

 the milieu^ proves on examination altogether inadequate. The 

 general form of these leaves seems attributable to poor nutri- 

 tion, while certain characters thinness, lack of differentiation 

 of spongy and palisade parenchyma, and presence of chlorophyll 

 in the epidermis are also common, in some degree, to terres- 

 trial plants growing in the shade, and seem intimately con- 

 nected with lack of sunlight^. We may perhaps suppose that the 

 dimness of the light which reaches a plant living below the 

 surface of the water may be directly responsible for these 

 characters; the green pigment, for instance, may be present 

 in the epidermis simply because the leaf is not exposed to direct 

 sunlight, which in the case of terrestrial plants destroys the 

 chlorophyll in the epidermis as fast as it is formed^. Now 

 there is little doubt that a thin leaf with an epidermis rich in 

 chlorophyll is particularly well adapted for the assimilation of 

 dissolved carbon dioxide; how then are we to account for the 

 singular coincidence that characters arising in this fortuitous 

 and mechanical fashion prove definitely advantageous to the 

 plant.-^ It is perhaps conceivable that it is the very fact that 

 terrestrial plants under conditions of poor illumination tend to 

 develop this type of leaf, which has rendered possible the as- 

 sumption of the submerged habit, and that it is those plants 

 whose leaves happened under such conditions to develop on the 

 lines particularly suited to water life, which have accomplished 

 the transformation into thorough-paced aquatics. 



1 Schenck, H. (1885). 2 Stohr, A. (1879). 



