of the physiology, morphology and ecology 

 of both plants and animals. 



The Law of the Minimum must be taken 

 into account in all experimental work, for 

 which it serves both as a precaution and a 

 guide.^ When investigating the effect of 

 an external factor such as temperature, 

 light, etc., on any given process, it is neces- 

 sary to keep all other variable factors con- 

 stant, and then to determine the effect of 

 changes in the factor under consideration. 

 What results might be obtained when this 

 method is used in studying carbon assimila- 

 tion? Suppose the COo supply and the 

 light are kept constant, while the tempera- 

 ture is varied. If the CO, supply is such 

 that it becomes a limiting factor when the 

 temperature rises above 10° C. then the 

 rate of assimilation will rise with the tem- 

 perature up to this point, but will remain 

 constant at all higher temperatures, until 

 the destructive effect of the high tempera- 

 ture is manifested and the curve again falls 

 off. Above 10° C. variations in the tem- 

 perature have no apparent effect under 

 these experimental conditions. But if the 

 CO2 supply is increased so as to permit 

 more rapid assimilation, then the tempera- 

 ture curve can be extended. Negative re- 

 sults from such an experimental method 

 are therefore without significance. It is not 

 enough that the experiment be conducted 

 under constant conditions; the constant 

 factors must not interfere in any way with 

 the carrying out of the process ; that is, they 

 must not be limiting factors. On the other 

 hand, it is a simple matter to determine by 

 the shape of the curve whether any other 

 factor than the one under investigation is 

 9 Cf. B. E. Livingston, loc. cit. 



