[ 139 ] 



CHAPTER XI 

 LEAF TYPES AND HETEROPHYLLY IN AQUATICS 



(i) TYPES OF LEAF IN WATER PLANTS 



THE types of leaf characteristic of aquatics excluding 

 those that rise wholly into the air and are thus comparable 

 with the leaves of terrestrial plants fall into two groups: firstly, 

 those which float on the water surface, and thus preserve con- 

 tact on tiie*Ventral side with the atmosphere and on the dorsal 

 side with the water, and secondly, those which have more com- 

 pletely adopted the water life, since they keep up no direct 

 contact with the atmosphere, but live entirely submerged. The 

 general question of the relation of floating leaves to their en- 

 vironment has been discussed, in connexion with the Water- 

 lilies, on pp. 30-32. There is considerable monotony in the out- 

 line and structure of a large proportion of such leaves, associated 

 no doubt with the very definite and uniform physical condi- 

 tions to which they are subject. Submerged leaves, on the other 

 hand, are characterised by much greater variety. With a number 

 of exceptions, they fall mainly into two groups those that 

 present a very thin, entire lamina, generally ribbon-shaped but 

 sometimes broad, and those in which the leaf blade is finely 

 subdivided, either by fenestration or dissection. In both these 

 types of leaf, the ratio of surface to volume is higher than is the 

 case in a normal, terrestrial lamina, and many botanists regard 

 their peculiarities as definite adaptations for obtaining from the 

 water an adequate supply of gases in solution. It is generally 

 assumed that the dissected type of leaf is the more efficient 

 form for the purpose. A Russian writer 1 has recently proved, 

 however, that this assumption is scarcely borne out by a critical 

 examination of the facts. By measurements and calculations 

 1 Uspenskij, E. E. (1913). 



