Carbon Assimilation. 
133 
The principle of limiting factors is, of course, of general 
application where a process depends on a number of factors. It 
is, indeed, rather an elaboration of the ‘ Law of the Minimum ’ which 
had been applied to agricultural problems by Liebig as far back as 
1843. 
Fig. 5. Scheme to illustrate the action of a limiting factor. (After F. F. 
Blackman). 
The question of the optimum value of a factor requires some 
consideration. 
As regards temperature we have already referred to the van’t 
Hoff rule that for every rise of 10 r, C the rate of a chemical reaction 
is doubled or trebled. If this law were followed throughout it is 
clear that there could be no optimum temperature for assimilation, 
which would increase more and more rapidly with increasing 
temperature. As a matter of fact for several plant processes the 
van’t Hoff rule is followed between say 5°C and 29 n C, but at higher 
temperatures it is quite clear that the rate of metabolic change in 
the organism slows down and the rule does not express the relation 
between temperature and the process. 
To explain this slowing down at high temperatures Blackman 
introduces a ‘ time factor.’ Thus at 25°C and lower temperatures 
the initial assimilation rate is maintained unchanged fora consider¬ 
able time, but at higher temperatures, 30°C and over, although the 
leaf after exposure to light commences to assimilate at a rate given 
by the van’t Hoff rule, this initial rate of assimilation cannot be 
maintained but falls off regularly, and the higher the temperature 
the more rapid the falling off. If then the assimilation at various 
