322 PHYSIOLOGY 



are usually some special substances or structures that repel water ; and 

 so it does not come into contact with the wet and permeable walls of the 

 internal cells. Here then is an arrangement, not found elsewhere in the 

 plant, by which water may leave the body rather freely, yet practically 

 cannot enter it when conditions are reversed. 



It may be assumed that there may enter the cuticle, when wet, amounts 

 of water corresponding to those that evaporate from it when dry. The re- 

 vival of wilted plants after the foliage is sprinkled, however, is due chiefly to 

 checking the evaporation; yet the trifling amount of water entering tends to 

 the same result. 



Entry and exit of gases. The aerial parts facilitate the entry and 

 exit of gases. The external atmosphere communicates freely with the 

 internal atmosphere of the intercellular spaces by way of the stomata. 

 Any oxygen -or carbon dioxid in the air of the intercellular spaces may 

 dissolve in the water of the cell walls and then migrate into the adjacent 

 cells, if the pressure of these solutes is less in the cells than in the internal 

 atmosphere. In like manner either may diffuse into the internal at- 

 mosphere when the reverse conditions exist. The solubility of CO 2 and 

 O 2 in water under like conditions is very unequal, the former being about 

 30 times as soluble at ordinary temperatures as the latter. The rate of 

 diffusion is also unequal. The quantity of each used or produced by the 

 plant likewise differs. These factors all play a part in determining the 

 amount of gas which enters or leaves. As the composition of the internal 

 air fluctuates on account of subtraction or addition of CO 2 or O 2 , a dif- 

 ference is created between the internal and external atmosphere, which 

 leads at once to diffusion through the stomata in a direction determined 

 by the existing inequality in pressure of either gas. 1 Nitrogen, the 

 only other considerable constituent of air, is neither used nor produced; 

 hence practical equilibrium between the N 2 of the air and the N 2 in 

 solution in the plant is early attained, and this equilibrium is scarcely 

 disturbed thereafter. 



In submersed plants the oxygen and carbon dioxid are dissolved 

 in the water and find admission at any permeable surface, like other 

 solutes. 



1 Further discussion of the r61e of these gases will be found in the sections on Photo- 

 synthesis (p. 363) and Respiration (p. 403). 



