916 CONCENTRATION FACTORS CHAP. 27 



one quarter open (2 ju). This difference is understandable since at 8000 

 lux the maximum rate of photosynthesis is only 8 mg. COo/cm.- hr., while 

 in 26,000 lux, it rises to > 20 mg. C02/cm.2 hr. Stalfelt concluded that 

 stomatal openings easily may limit the carbon dioxide supply in ordinary 

 air, and therefore also the rate of photosynthesis under natural conditions, 

 particularly in strong light. Like Maskell, she supported this view by cal- 

 culations of the rate of diffusion through the stomata, based on equations 

 of Brown and Escombe (1900). These calculations confirmed that the 

 maximum rate of carbon dioxide flow from ordinary air through wide open 

 stomata is of the same order of magnitude as the maximum rate of photo- 

 synthesis. 



These results, while clearly showing the possible "bottleneck" role of 

 the stomata, do not mean that other parts of the path between atmosphere 

 and chloroplasts do not contribute commensurable — or even greater — 

 terms to the total diffusion resistance. 



In the face of these results, one must disagree with Renner (1910), who thought that 

 the resistance of the stomata represents only a negligible fraction of the total diffusion re- 

 sistance on the carbon dioxide path from the atmosphere to the chloroplasts, as well as 

 with Schroeder (1924), who attempted to prove that the diffusion resistance of the air 

 channels is the rate-limiting influence in the photosynthesis of the higher plants, and in 

 this proof altogether omitted the resistance of the stomata. 



Romell (1927) pointed out that Schroeder neglected, not only the flow resistance 

 of the stomata, but also that of the gas-liquid interface, and of the liquid phase between 

 the cell wall and the chloroplasts. Romell calculated that the gradient of the carbon 

 dioxide concentration in the air channels must be smaller than in the protoplasm (be- 

 tween cell wall and chloroplast), and that both these gradients should be negligible in 

 comparison with the drop of concentration at the phase boundary, caused by the rela- 

 tively small accommodation coefficient of carbon dioxide on water (as calculated from 

 Bohr's measurements of the velocity of escape of carbon dioxide from aqueous solution). 

 The maximum theoretical rate of diffusion, calculated by Romell by taking all these 

 factors into account, proved to be considerably lower than the maximum rate of photo- 

 synthesis that the leaves actually can reach in open air. One is thus led to assume (c/. 

 van der Honert 1930) that the accommodation coefficient of carbon dioxide is larger on 

 the cell wall than on a water-air interface. 



7. Interpretation of Carbon Dioxide Curves 



The preceding pages show that reliable experimental material for analyt- 

 ical interpretation of the carbon dioxide curves of photosynthesis is hardly 

 available at present, and will not be easy to obtain. We have stated that 

 at least two intrinsic kinetic factors could make the rate of photosynthesis 

 a function of the external carbon dioxide pressure: the probable reversi- 

 bility of the primary carbon dioxide absorption (carboxylation) step, and 

 a finite rate of carboxylation. The difficulty is to recognize the workings 

 of these "intrinsic" or chemical factors behind the more incidental, physi- 



