DIFFUSION THROUGH STOMATA. 233 



leaf in sunlight is taken into account, and also the very limited 

 area of the stomatal slits, it is difficult to realise how the neces- 

 sary interchange through these slits can take place. Brown 

 and Escombe* found that a leaf of Catalpa bignioides can 

 absorb from ordinary air, containing 3 parts per 10,000 of 

 carbon dioxide, about -07 c.c. (N.T.P.) of carbon dioxide per 

 square centimetre of leaf surface per hour. On each square 

 centimetre there were 14,500 stomata, and each stoma, when 

 fully open, had an area of '0000618 square mm. Conse- 

 quently the united area of the stomatal openings only amounted 

 to '9% of the whole surface. Hence, if all the absorption took 

 place by diffusion through these openings, diffusion of carbon 

 dioxide through them must have taken place at the rate of 

 about 7 '77 c.c. per square centimetre per hour. With strong 

 caustic soda solution they found that the rate of absorption of 

 carbon dioxide from normal air by a free surface varied from 

 12 c.c. to '177 c.c. per square centimetre per hour. So that a 

 leaf of catalpa in sunlight absorbs carbon dioxide at about half 

 the speed at which it would if covered with a continually re- 

 newed film of caustic soda solution, and. if all absorption occurs 

 through the stomata the carbon dioxide must move about fifty 

 times as fast through the openings as it would if they were 

 filled with a strong solution of caustic soda. 



Brown and Escombe have shown, however, that when an 

 absorbent surface is covered with a diaphragm placed some 

 distance above it the rate of diffusion of a gas from the 

 outside air into the absorbing chamber per unit area increases 

 enormously with a diminution of size of the aperture. This 

 fact is understood by applying the kinetic theory of gases to 

 the problem. The chance of any given molecule of carbon 

 dioxide moving by virtue of its kinetic motion into the cell is 

 proportional to the area of the opening ; but once within the 

 cell its chance of moving out again is less and less as the size 

 of the aperture diminishes. Now the rate of diffusion through 

 an aperture is the difference between the number of molecules 

 which move in and out in a given time. 



The number of molecules, so long as the temperature re- 

 mains constant, moving inwards is solely dependent upon 



* Phil. Trans., vol. 193 (1900), 223. 



