Carbon Assimilation. 131 
Chapter IV. 
The Factors Influencing 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 hardly expect to express the relation 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 
4 Optima and Limiting Factors’ 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. F. 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 1 slowest ’ factor.” 
