vi] THE RIVER LETTUCE 83 



and lower leaf-surfaces are covered with minute depressed 

 hairs, which prevent the leaves from being wetted^. Fig. ^2 

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

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

 Pisiia 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^. 



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



^ This subject is dealt with more fully in Chapter xvii, p. 213. 



