Photosynthesis : a Reply to Criticism. 529 
shows a marked increase in the rate of assimilation over that shown in 
Expt. 58 with the same light and a temperature of 30*5°. The rate 
of assimilation at 37 * 5 ° fell off very rapidly with successive readings. This 
shows that at this temperature there are complicating side-reactions of 
considerable magnitude, so that it is not to be expected that a photo- 
chemical ratio would hold. The importance of side-reactions will be shown 
in another connexion. Moreover, one experiment under extreme conditions 
cannot be regarded as reliable when we consider the magnitude of the 
experimental error with medium temperatures. For the above reasons we 
have thought it best not to attempt to draw any conclusion from the 
experiment at the temperature of 37*5°. It is interesting to note that 
Expts. 42 and 43 (Matthaei, Table VII), with light intensity of eight 
units and temperature of 38-3°, do not show the decrease in the rate of 
assimilation that is seen in Expt. 59.’ 
Their first sentence recognizes a fact which they are quite unable to 
explain, as they will not admit the obvious solution that temperature, the 
Only factor altered, was limiting in Expt. 58. The fact that assimilation fell 
witl} time in Expt. 59 makes their difficulty greater, for obviously it is 
the higher values which most nearly represent the true assimilatory activity. 
Hypothetical side-reactions, therefore, do not help them. Their statement 
about experimental error is incorrect. The difference of the first readings 
in Expts. 58 and 59 is 0-00080, i. e. five times the probable error, which has 
been shown to be +o-cooi6, so that it cannot be held.to be insignificant. Nor 
is this experiment an isolated one. It is part of a concordant series. Nor, 
indeed, is the temperature condition of 37*5 an ‘extreme 5 condition. After 
casting up this cloud of unjustified innuendo the authors can only say that 
they find it best to leave the experiment out of account altogether. 
The Work of Blackman and Matthaei. 
Blackman and Matthaei ( 1905 ) have shown that the temperature- 
coefficient of Heliantlms is even greater than that for Cherry- laurel and is 
probably about 2*5. They base their temperature-curve upon the results 
of four experiments, in each of which definite proof is given that the 
temperature was the controlling factor. Brown and Heise’s comment on 
these four experiments is as follows : ‘ It is not evident why the changes 
in the rate of assimilation cannot be explained as due to variation in light 
intensity, especially since it is evident from their individual experiments 
that fluctuations in light intensity are accompanied by marked changes in 
assimilation.’ It will scarcely be believed, after this comment, that in each 
of the four experiments quoted Blackman and Matthaei gave specific proof 
that increase of light did not increase the assimilation at that temperature. 
Since, however-, this does not seem to have been made clear to Brown and 
Heise, it will be necessary to study each of the experiments mentioned. 
