64 Carbon Assimilation. 



Chapter IV. 



The Factors Influenmig the Intake of Carbon Dioxide. 



A. General Remarks. 



As we have already said in our introductory chapter, carbon 

 assimilation is a complex of processes which probably obey quite 

 different laws. Thus we know that one or more of these processes 

 must be photo-chemical since light is required for carbon assimilation. 

 Consequently one would hardlyexpect toexpress therelation between 

 the amount of carbon dioxide used and the various factors which 

 influence the intake of carbon dioxide in a simple way. 



It is the great merit of P. P. Blackman that many years ago 

 he called attention to the complexity of the processes of carbon 

 assimilation, and showed that it was impossible to construct such 

 a curve as a temperature-assimilation curve without regard to the 

 possible effects of other factors. The result of Blackman's analysis 

 of the intake of carbon dioxide under various conditions is expressed 

 in his principle of limiting factors, and summed up in his work 

 ' Optima and Limiting Pactors ' in Annals of Botany for 1905, a 

 paper with which every student of plant physiology should be well 

 acquainted, as the considerations contained therein are so funda- 

 mental for all biological processes. 



Before P. P. Blackman's publications, investigators dealing 

 with the influence of a factor on any physiological process, spoke 

 of the factor having minimum and maximum values, below and 

 above which the process does not take place and an optimum 

 value at which the process proceeds at its greatest rate. In carbon 

 assimilation there was alleged to be an optimum value of temperature 

 at which assimilation is greatest. Similarly, there was supposed 

 to be an optimum carbon dioxide supply and an optimum illumination 

 for carbon assimilation. The optimum values obtained by different 

 authors did not show any concordance, and Blackman pointed out 

 that the method of experimentation in which, for instance, the 

 influence of carbon dioxide and light are neglected when the effect 

 of temperature is considered, is utterly illogical and cannot be 

 expected to give results of any clear value. 



Blackman states the principle of limiting factors as follows : 

 " When a process is conditioned as to its rapidity by a number of 

 separate factors, the rate of the process is limited by the pace of 

 the ' slowest ' factor," 



