INFLUENCE OF ENVIRONMENT ON PLANTS 841 
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 
Fra. 142.—Srcrion of Rutzomr oF Marsilea. 
co.la., lacunse 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 
