Books and Current Literature. 73 



Within the porous clay walls the conditions are reversed, and 

 the larger partial pressure is due to water (or alcohol) vaoor 

 while the smaller one is due to air. Since the porous clav is 

 permeable to air as well as water, we should expect to find 

 diffusion taking place in both directions through the walls, 

 diflfusion of water vapor outward and diffusion of air inward. 

 But the chamber is kept separated with water vapor bv evapor- 

 ation from the surrounding clay, so that the pressure of water 

 vapor remains constant within, and the water vapor which 

 comes off exteriorly is rapidly removed by air currents, so that 

 the pressure of air remains nearly constant without. Since the 

 chamber is open to the outside through the tube, the internal 

 pressure due to air can never rise much above that at the start, 

 for water vapor and air will be pressed out through the water 

 below in the form of bubbles. Water vapor does not move 

 outward through the wet walls as rapidly as air moves inward, 

 and so we have a sort of osmometer with what mav be termed 

 a strong solution of water vapor in air on the inside and an ex- 

 tremely weak solution of the same sort on the outside, the mem- 

 brane being more permeable to the solvent air than to the solute 

 water vapor. Hence the action is a sort of osmosis of gases, a 

 large amount of water vapor being mingled with the air as it 

 exudes from the tube. 



In the plant it appears that air is continually entering 

 through the cuticle and stomata and diffusing through the wet 

 membranes into the air channels, thus producing a higher total 

 gas pressure within than without the plant. Apparently the 

 internal pressure rises faster than it can be decreased by outward 

 currents of gas through any other channels that mav be open 

 and the main outlet occurs through the porous tissue near the 

 junction of blade and petiole. Any condition which accelerates 

 evaporation tends to prevent the accumulation of water vapor 

 in the vicinity of the leaf, thus at the same time tending to main- 

 tain the original high partial pressure of air on the outside. 

 This action is like that of a stirrer in an osmometer, which so 

 mixes the internal solution that it does not become too weak 

 next to the membrane. — Burton E. Livingston. 



