INTERACTION OF FACTORS 13 



different conditions may have no real significance. This was 

 recognized by Warburg * who used in his experiments Chlorella, 

 a green alga of the simplest possible morphology, the sphere. 

 With respect to the factors light intensity and concentration 

 of carbon dioxide, he found that when carbon dioxide was in 

 small amount, the intensity of carbon assimilation was in 

 proportion to this amount ; that is, carbon dioxide was a 

 limiting factor. But when the amount of carbon dioxide was 

 progressively increased, the photosynthesis did not corre- 

 spondingly increase and the curve became logarithmic. War- 

 burg's conclusion is that the intensity of photosynthesis is 

 proportional to the concentration of carbon dioxide and to 

 the concentration of a second substance which enters into 

 combination with it ; that is, the intensity of photosynthesis 

 is governed by the rate of a chemical reaction. It will be 

 obvious that the diffusion factor in his experiments is reduced 

 to a minimum. 



The balance of evidence relating to the interaction of 

 the light intensity and carbon dioxide concentration factors 

 favours the view that a factor is strictly limiting only when it 

 is very weak, or " in minimum," to use Harder's terminology ; 

 when, however, their intensity or concentration more nearly 

 approach one another, an increase in either will result in an 

 increased photosynthesis ; that is, the reaction intensity is 

 controlled by both factors. This relates only to the two 

 factors mentioned ; to what extent it applies to combinations 

 of other related factors is uncertain. It is not improbable 

 that the complete process of carbon assimilation consists of 

 separate phases, f the most obvious of which are diffusion, 

 photochemical, and chemical which is independent of light. 

 The intensity of each phase is conditioned by a set of factors 

 which may be peculiar to itself, thus light is effective in the 

 photochemical but not in the chemical, in which stage tem- 

 perature is highly important. Further, the intensity of one 

 phase may be governed by the intensity of another, thus the 

 accumulation of sugar in the leaves of the beet may effect a 



* Warburg : " Biochem. Zeit.," 1919, ioo, 230. 



f See Briggs : " Proc. Roy. Soc," B, 1920, 91, 249. 



