254 CHAP. XXVII. SIMULTANEOUS VARIATION OF FACTORS 



change of all factors from clt to CLT will be x X y X z, 

 the product of the partial effects induced by the individual 

 factors. This, in contrast to arithmetical summation, will 

 be designated as the Law of Product or of Multiplication. 



In order to arrive at the above generalisation, it might 

 be thought necessary to undertake a very extensive series 

 of investigations on the effects of different combinations, 

 not only of all the varying factors, but also of different 

 intensities of the same factor. The number of such com- 

 binations would be so numerous that their investigation 

 could hardly be completed within a lifetime. It is, how- 

 ever, possible to arrange a well-thought-out programme of 

 about a dozen different experiments, the results of which 

 will be crucial in the establishment of a photosynthetic 

 law which will be of universal application. I give a list of 

 these experiments under six heads, each including different 

 combinations of increasing complexity. 



i. Two variable factors : C0 2 -concentration and light ; 

 the different combinations are cl, cL, CI and CL. 



ii. Two variable factors : C0 2 -concentration and tem- 

 perature ; the different combinations are ct, cT, Ct and CT. 



hi. Two variable factors : light and C0 2 -concentration ; 

 light carried through half a cycle, l v l 2 , l 3 ; C0 2 -concentra- 

 tion in two stages, c and C. 



iv. Two variable factors : light and temperature ; light 

 of two intensities and temperature carried through a cycle. 



v. Three variable factors : light, C0 2 -concentration and 

 temperature. 



vi. Four variable factors : C0 2 -concentration, light, tem- 

 perature and tonic condition. 



Method of Procedure 



It is desirable that each set of experiments should be 

 carried out with an identical specimen ; for otherwise the 

 observed variations might be ascribed to physiological 

 differences of the specimens. 



