xii] CHLOROPHYLL AND STOMATES 165 



majority of terrestrial Dicotyledons, though they are generally 

 absent in the case of terrestrial Monocotyledons^. They are 

 usually to be observed only in the lower epidermis of the leaf, 

 but it seems probable that this is due to the destructive action 

 of sunlight upon the chlorophyll in the upper epidermis. In 

 support of this view it may be mentioned that, in diffused light, 

 chlorophyll occurs in the upper epidermis of the leaves of Bellis 

 perennis^ whereas under normal conditions there is chlorophyll 

 only in the lower epidermis. The presence of green plastids 

 in the epidermis of submerged plants may thus be regarded as 

 representing merely the elaboration of a character already 

 existing in terrestrial plants, which finds favourable oppor- 

 tunities for development in the relatively dim illumination 

 which submerged plants receive. 



The statement, frequently made, that stomates are absent 

 from submerged leaves, and from the lower surface of floating 



Fig. ro6. Elodea canadensis, Michx. T.S. leaf; i, intercellular air channels. 



[Schenck, H. (1886).] 



leaves, needs considerable qualification^. It is, indeed, broadly 

 true that stomates are much less frequent in submerged than 

 in terrestrial leaves, and, moreover, in certain water plants, 

 such as Elodea (Fig. 106), Vallisneria^ Thalassia^ and other 

 Hydrocharitaceae which always live entirely submerged, sto- 

 mates never occur ^. Among the Cryptogams, Isoetes lacustris 

 is entirely free from stomates, and GoebeH even found that it 

 failed to produce any when grown for two years as a land plant. 

 Submerged leaves in general are not only poor in stomates but 

 also in hairs; it has been suggested by Mer^ that this like the 



1 Stohr, A. (1879). 



2 Costantin, J. (1885^). See also Porsch, O. (1903} for citations of a 

 large number of cases in which the occurrence of stomates on submerged 

 organs is mentioned in the literature. ^ Solereder, H. (19 13). 



^ Goebel, K. (i 891-1893). 5 Mer, t. (1880I) and (1882I). 



