12 PHOTOSYNTHESIS 



air and the intercellular spaces is the same for the two 

 processes. 

 Thus, 



R^tCtranspiration ^- ^HaO^HoO (2.2) 



■^^^tCphotosynthesis -^CO.^COa 



where K represents the respective coefficients of diffusion 

 and A the difference in concentration. Measurements of this 

 ratio, i.e. the water lost per carbon gained, indicate that 

 under field conditions the concentration of carbon dioxide 

 in the intercellular spaces may be nearer the external con- 

 centration than the measurements of Heath and Gabrielson 

 suggest. (See, for example, the papers of Maskell (1928) and 

 of Penman and Schofield (195 1).) 



THE DIFFUSION PATH BEYOND THE INTERCELLULAR SPACES 



Carbon dioxide diffuses in the gaseous state up to the cell 

 wall; subsequently it must diffuse in an aqueous phase 

 through the mesophyll cell wall and to the point where 

 reaction occurs. This hydrodiffusion path may be regarded 

 as extending to a point at which the concentration of CO2 

 would be zero. Although the hydrodiffusion path is only a 

 small distance relative to the air path, since the diffusion of 

 carbon dioxide in water is so much slower than in air, its 

 contribution to the total path is far from negligible. An esti- 

 mate of its magnitude can be made by comparing under 

 identical conditions the rate of water loss (transpiration) 

 with the rate of photosynthesis. Brown and Escombe calcu- 

 lated these two quantities assuming the diffusion path to be 

 that through the stomata alone (i.e. they had no term corre- 

 sponding to jzR/^). The calculated rate of transpiration was 

 6 times greater than the maximum value ever observed and 

 that of photosynthesis 20 times the maximum observed 

 rate. Although their values do not refer to the same environ- 

 mental conditions we may take it that they indicate a total 

 gaseous diffusion path 6 times that of the stomatal path and 

 an additional resistance to carbon dioxide uptake equivalent 

 to some 14 times the stomatal path or about 2 J times the 

 gaseous diffusion path. 



