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 r&le of these gases will be found in tne sections on Photo 
synthesis (p. 363) and Respiration (p. 403). 
