226 PLANT PHYSIOLOGY 



diameter, which is approximately the proportion existing between 

 the area of the stomata and their distance apart. This means 

 that the diffusion into the leaf is as great when the stomata are 

 open as it would be if no epidermis were present and the carbon 

 dioxide could diffuse in freely to the intercellular air spaces! 

 Analyses of gases in the intercellular spaces have supported this 

 conclusion since they show that the gases here have the same com- 

 position as the outside air. 



Laws of Diffusion. — The rate of diffusion of various gases into 

 the intercellular spaces follows the ordinary law of diffusion, 

 i. e., the velocity varies inversely with the square root of the den- 

 sity. Thus oxygen, which is sixteen times as dense as hydrogen, 

 diffuses only one-fourth as fast under the same temperature and 

 pressure. The diffusion from the intercellular spaces into the 

 cells does not depend upon the density, however, because one is 

 here dealing with dissolved gases. In this case the rate of diffusion 

 depends upon the solubility of the gas in the watery liquid through 

 which it passes. If it were a question of the density, carbon diox- 

 ide would enter the cells more slowly than oxygen or any of the 

 other common gases in the air, because it is the densest of them 

 all; but, since it is a question of solubility, it enters more quickly 

 than the others. Carbon dioxide is very soluble in water and thus, 

 in spite of the small amount present in the air, it is absorbed by 

 the plant in sufficient quantities to produce the luxuriant vegeta- 

 tion with which the land surface is in most places covered. 



Carbon dioxide diffuses from the outside air into the stomata, 

 then, according to the same laws which regulate the diffusion of 

 gases everywhere; and the presence of the leaf cuticle, as shown 

 by Brown and Escombe, is not a hindrance. Diffusion operates 

 freely according to the gas laws, which may be illustrated by a 

 very simple experiment. 



Let us suppose a vessel made of palladium, holding one liter, 

 is placed inside another vessel of glass which holds two liters 

 (Fig. 11). Both vessels contain manometers for measuring the 

 pressure of contained gases. The smaller vessel contains nitrogen 

 and the outer vessel is filled with hydrogen. At the beginning of 

 the experiment both gases are under one atmosphere pressure, 

 so that the mercury in both manometers shows the pressure out- 

 side and inside to be the same. 



Now palladium is permeable to hydrogen and impermeable to 



