vi] THE RIVER LETTUCE 83 



and lower leaf-surfaces are covered with minute depressed 

 hairs, which prevent the leaves from being wetted 1 . Fig. 53 

 shows the apical opening, so often found in aquatics, through 

 which water is eliminated from the leaf 2 . Like the Lemnaceae, 

 Pistia represents a type which is singularly successful in the 

 matter of vegetative growth. Its reproduction is so rapid that 

 it sometimes chokes water-channels and proves a serious hin- 

 drance to navigation 3 . 



1 Ito, T. (1899). 2 Minden, M. von (1899). See also p. 267. 



3 This subject is dealt with more fully in Chapter xvn, p. 213. 



62 



