THE ROLE OF CARBON DIOXIDE 345 



apart), for example, the diffusion is 31.4 per cent of that from the open tube, 

 although only 0.70 per cent of the septum surface is occupied by the apertures. 

 Brown and Escombe also calculated the theoretical carbon dioxide absorb- 

 ing capacity of a sunflower leaf, using the following as average values: cross- 

 sectional area of a stomatal pore, 0.0000908 mm.- ; length of stomatal pore, 

 0.014 mm.; and number of stomates per square centimeter, 33,ooo. A zero 

 partial pressure of carbon dioxide in the intercellular spaces was assumed. 

 According to their calculations, when the stomates are fully open carbon 

 dioxide theoretically could diffuse into the leaf at the rate of 2.095 cc. per 

 hour per square centimeter of leaf surface in quiet air. The correspondmg 

 figure for rapidly moving air is 2.578 cubic centimeters. As shown by the 

 figures given in the second paragraph under this sub-topic these values are 

 many times greater than any observed values for the rate of diffusion of 

 carbon dioxide into sunflower leaves exposed to the atmosphere. The dif- 

 fusive capacity of the stomates is clearly more than adequate to account for 

 the necessary rate of entrance of carbon dioxide into leaves even when photo- 

 synthesis is occurring at its maximum rate. 



3. Effects of Variations in the Concentration of Atmospheric Carbon 

 Dioxide upon the Rate of Photosynthesis. — The effect of various concentra- 

 tions of carbon dioxide upon the rate of photosynthesis of wheat plants ex- 

 posed to different light intensities is depicted graphically in Fig. 85. In gen- 

 eral, with increase in the concentration of atmospheric carbon dioxide, there 

 is an increase in the rate of photosynthesis until some other factor, in this 

 example light intensity, becomes limiting. Furthermore, as long as the limit- 

 ing effect of another factor does not become apparent, the rate of photosyn- 

 thesis is approximately proportional to the concentration of carbon dioxide. 

 It should be noted, however, that even the highest light intensity employed 

 in this experiment is very much less than the intensity of full summer sun- 

 light. 



The foregoing statements only apply when plants are exposed to relatively 

 low concentrations of carbon dioxide. Higher concentrations of this gas 

 (i5_25 percent) exert an inhibitory action upon many plant processes includ- 

 ing photosynthesis. The exact concentrations at which this checking effect 

 of carbon dioxide upon photosynthesis becomes apparent in various species 

 does not seem to have been determined. The rate of photosynthesis in prac- 

 tically all species shows an increase with increase in the concentration of 

 carbon dioxide gas up to at least twenty or thirty times the percentage usually 

 present in the atmosphere, if no other factors are limiting. For many species 

 the carbon dioxide concentration can be raised to values considerably in excess 

 of this before any inhibitory effect upon photosynthesis can be detected. 



