48 PHOTOSYNTHESIS 



becomes small compared with (i?c»+r) and the equation 

 simplifies to that of two straight lines. 



(R-Roo) {LR-L . r-KC)=o 

 i.e. R=Rcc or R-r^-j-^ 



JL/ 



The relationship between rate and external concentration 

 will then be in two parts: (i) a region when the rate of appar- 

 ent assimilation is proportional to the external concentration 

 and (2) a region where the rate is independent of the exter- 

 nal concentration. This type of relationship, which is of the 

 type observed by Blackman et al., represents a limiting case. 

 The slope of the line in the region showing proportionality 



is — so that in this region diffusion is the limiting process 



and the rate of photosynthesis will be determined by all fac- 

 tors affecting the rate of diffusion. In the region where the 

 rate is independent of concentration the rate will be deter- 

 mined by all factors affecting the rate of consumption of 

 carbon dioxide, e.g. light intensity, temperature, and pos- 

 sibly chlorophyll content. Blackman interpreted his results 

 by ascribing the region of proportionality to one limited by a 

 single factor, namely the concentration of carbon dioxide, 

 but analysis clearly shows it to be preferable to refer to the 

 region as limited by a limiting process. Thus the data of 

 Blackman on the one hand and those of Warburg on the 

 other represent two extreme cases; one in which the external 

 concentration is very different from the internal and the 

 other in which the two concentrations are nearly the same. 

 The data of other workers are intermediate between these 

 extremes, and represent variations in the length of diffusion 

 path. There may, in addition, be other 'internal' factors 

 which vary from plant to plant and can be related to the 

 form of relationship between rate of photosynthesis and con- 

 centration of carbon dioxide (see Briggs and Whittingham, 

 1952). This possibility will only be appreciated when further 

 work with a greater variety of plants has been done. 

 In the foregoing discussion the rate of photosynthesis has 



