66 Carbon Assiviilation. 



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. 



lis 



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15 



I 13-^567 



Co-rton (LioxleLft suppt^eii 



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°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''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 for a 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 



