I 
156 Ingvar Jorgensen and Walter Stiles. 
of the experiment. Blackman actually continues his curves back 
for 2 hours, not 1J 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 
