INFLUENCE OF ENVIRONMENT ON PLANTS 341 



prominent. The raw materials of their food reach them 

 dissolved in the water in which they live, and hence they 

 have no need of the complicated root system with its 

 absorbent root-hairs, which is so characteristic of a plant 

 growing in ordinary soil. Gaseous absorption takes place 

 through the general surface to a large extent, but this 

 direct supply is insufficient for respiration. The ordinary 



FIG. 142. SECTION OF RHIZOME OF Marsilea. 

 co.la., lacunoe in cortex. 



arrangements for aeration, consisting of a network of inter- 

 cellular spaces freely in communication with numerous 

 stomata, are not quite the same in plants surrounded by 

 water. We have seen that many of them have no stomata, 

 the leaves being quite submerged ; others have relatively 

 few on the upper surfaces of the floating leaves. The 

 gaseous interchange between the interior and the exterior 

 is consequently greatly impeded. The large intercellular 



