Photosynthesis : a Reply to Criticism. 527 
for their theory to throw doubt upon the result at 1 1 °, they have recourse to 
another suggestion, which I hope to show is equally improbable. ‘ The 
difference (between the result at n° and those at 25*4° and 32*1°) can 
readily be explained as due to differences in the seasonal activity of the 
leaves/ say Brown and Heise. Now the experiment at n° was done on 
Mar. 4, the experiments at 25-4° on Jan. 30 and 31, and the experiments at 
32*1° on Feb. 4 and 5. In discussing the change in the seasonal activity of 
the leaves, Matthaei, who along with Blackman and independently had 
worked with Cherry-laurel leaves for some years, states that the change in 
the seasonal activity of the leaves occurred comparatively suddenly at the 
beginning of April, with the onset of warmer weather, and thus long after 
Mar. 4. She found in 1901 and 1902 a high assimilatory activity through 
March, and from her experience she felt justified in regarding this experiment 
of early March as belonging to the same category, as regards assimilatory 
activity, as the experiments at the beginning of February. Against this 
testimony it will need more than the unsupported assertion of Brown and 
Heise, who are observers in a tropical climate and have never carried out 
a single experiment on a Cherry-laurel leaf, that they can readily explain the 
difference between the result at n° (Mar. 4) and the result at 25*4° (Jan. 31) 
as due to difference in the seasonal activity of the leaves. We may 
conclude that these experiments agree with all the previous ones and 
establish that with sufficient light the values put forward as characteristic of 
— 6°, 0*4°, 9*2°, and n° are limited by the temperature and that the true 
effect of increase of temperature is a rapid rise in assimilation. 
Finally, Brown and Heise consider the experiments at high tempera- 
tures with very intense light and with thermo-electric records of the internal 
temperature of the leaf. From these experiments Matthaei obtained 
a rapidly rising curve. As, however, in the first three experiments both 
light and temperature were increased, it was necessary for Matthaei’s 
hypothesis that she should furnish evidence that the temperature and not 
the light was the controlling factor. This she did by comparing these 
experiments with those of a former series in which in each case a larger 
amount of assimilation was obtained with a less intense light. The inference 
was drawn that the lower values in the present series were due to the 
limiting effect of temperature. Brown and Heise’s criticism is that Matthaei’s 
comparison was made between two sets of leaves plucked at two different 
seasons and therefore differing in assimilatory activity. They say that the 
results in the second series are lower than would be expected from the 
temperature of these experiments, judging from the results of the first series, 
and that the only possible conclusion that can be drawn from a comparison 
of these experiments is that the leaves used in April (the second series) 
were less active than those used in March (the first series). Now I propose 
to show that while there is indeed an error in Matthaei’s reasoning, yet we 
