Light. 8 1 



of the experiment. Blacltman actually continues his curves back 

 for 2 hours, not 1| hours, and the value so obtained should therefore 

 represent approximately the initial rate of assimilation. Kanitz's 

 criticism is thus founded on a misconception. 



The further criticism of Blackman's work offered by Kanitz is 

 based on the assumption that Blackman's construction (Fig. 10) 

 and the application of the time law for enzyme action are mutually 

 exclusive. This is evidently not so. Blackman himself realised 

 the resemblance between his results and those obtained for enzyme 

 actions, but preferred to express his results in a non-committal form 

 to making assumptions which were unproved by experiment. 



With the criticisms of Blackman's construction offered by 

 some other continental writers we need not deal, as they amount 

 to no more than the expressions of personal opinion. 



Kanitz points out how from a consideration of Duclaux's curve 

 (Fig. 12) it is obvious that the position of the optimum is no fixed 

 point, but must depend on the quantity of enzyme present, which 

 will also depend on the previous history of the system : on its 

 previous temperature, on the velocity with which it is brought to 

 the optimal temperature, etc. It is interesting in this connection 

 to recall that Miss Matthaei found it necessary, in order to obtain 

 uniform results, to take particular care that all leaves used in her 

 experiments were subjected to the same treatment for the 24 hours 

 between their removal from the tree and the commencement of the 

 experiment. 



It may also be mentioned here that some investigators regard 

 the optimum temperature as the highest temperature which can be 

 maintained continuously without a depression of the function 

 resulting, and recently Miss Leitch (1916) has adopted this idea 

 for the case of growth. It is clear, however, that if it is really 

 Tammann's principle or something strictly analogous to it that is 

 involved in ' time factors ' in carbon assimilation and growth, the 

 position of the optimum, as Kanitz points out, is not a definite fixed 

 point, but depends upon other factors which will only remain 

 constant if the plants are subjected to the same previous history. 



C. Light. 



The effect of light on the intake of carbon dioxide has been 

 indicated in the previous section of this chapter. Reference was 

 there made to Miss Matthaei's experiments in which it was shown 

 that intensity of light may limit the intake of carbon dioxide, in 



F 



