THE NATURE OE PHOTOSYNTHESIS 69 



outside not more than .9 per cent of this leaf area — we arrive at the 

 somewhat remarkable conclusion that during assimilation the absorption 

 [yer unit area of these openings must be from 43 to 64 times as fast 

 as the absorption of a unit area of a freely exposed solution of caustic 

 alkaH. In other words, under the natural conditions of assimilation 

 the stomatal openings, supposing them alone to be operative, must take 

 in carbon dioxide from the air about 50 times as fast as they would 

 do supposing it were possible to fill them with a constantly renewed 

 solution of caustic alkali." 



In order to explain these facts Brown and Escombe carried through 

 an elaborate investigation of the diffusion of gases and liquids under a 

 variety of conditions. They departed somewhat from the conventional 

 method of studying interdiffusivity of gases in which these are at equal 

 pressure so that one gas has to diffuse against an equal and opposite 

 flow of the other, but rather set the conditions so as to study the rate 

 of diffusive flow of carbon dioxide of very low initial pressure down 

 a stationary column of air on its way to an absorbent surface. The 

 absorbent was caustic soda and the surface thereof was 20 to 25 times 

 the cross-section area of the column, thus simulating the conditions extant 

 in the stomata. Their experiments show a variation in the diffusive flow 

 which is inversely proportional to the length of the column and the re- 

 sults may be stated by the following expression : 



k(p — pQA.t 

 ^~ L 



in which O represents the amount of carbon dioxide flowing down the 

 cylinder towards the absorptive surface, A the area of the cross-section, 

 L the length of the column, t the time, p and pi the partial pressure of 

 carbon dioxide in the outer air and at the surface of the absorbent re- 

 spectively and k the diffusivity constant of carbon dioxide and air, i.e. 

 the number of cc. of the gas, measured at the temperature of the experi- 

 ment, which will pass across a section of 1 square cm. in area 



P — Pi 

 when the fall of pressure is 1 atmosphere in 1 cm. — is the 



concentration gradient of the carbon dioxide and must be constant for 

 unit thickness of any two adjacent layers at right angles to the axis of 

 the cylinder. If the alkaline solution is a perfect absorbent, pi at the 

 immediate surface of the absorbent is zero and the concentration gradient 



then becomes ~. If the absorption is not perfect pi will possess a sensible 



value and then the value for k will be lower than it should be. 



Thus Brown and Escombe give the following experimental results : 

 "The diffusivity, K°, of the atmospheric carbon dioxide at 0° C, in 



C. G. S. units, is obtained from the equation : 



