535 
Photosynthesis : a Reply to Criticism . 
limiting factor curve drawn in that paper. A critical glance at Brown’s 
Fig. i will show that the so-called improvement in presentation put forward 
by Brown is merely a matter of drawing a line arbitrarily between the 
various experimental values, so as to produce an illusory effect of convex 
curvature, rather than horizontality. His proposed correction of the latter 
part of the curve by allowing for the temperature-coefficient is quite 
illegitimate, for convincing evidence was given that both the values to which 
Brown has applied this correction were already much below the maximal 
values for those temperatures. The values were, in fact, kept low ^y the 
controlling influence of some other factor, presumably light. As, therefore, 
they are not temperature-values at all, they cannot be subjected to a correc- 
tion for temperature-coefficient. Apart from this proposed change, the 
difference between Brown’s curve and that put forward by Blackman and 
Smith is insignificant. The attempt made by Brown to separate the earliest 
part of the curve into two individual parts is seen, when closely examined, to 
involve him in inconsistencies. It is not proposed, however, here to follow 
him into these details of interpretation, for it is not upon such details as 
these that the validity of the theory of limiting factors as applied to carbon 
assimilation depends. This theory does not depend upon any single curve, 
nor can it be overthrown by suggesting, as Brown has done, minor differences 
in the interpretation of any single curve. Thus Matthaei’s supposition that 
in her earliest curve light was the limiting factor at all temperatures higher 
than 3 0 , did not become established from a consideration of the form of that 
particular curve, but from the fact that by subsequent increases of the inten- 
sity of the light she obtained a whole series of similar inflected curves, each 
one higher than the last in correspondence with the increased light. In 
a similar way, though not so completely, Blackman and Smith, in the paper 
under discussion, showed that a light of 6 - o units was limiting the assimila- 
tion in their general curve, because raising the light from 4*2 to about 6- o 
units caused an increase in the rate of assimilation. 
The revival by Brown of the hypothesis that there exists an { optimum 
concentration of carbon dioxide which is indicated by his drawing of 
Blackman and Smith’s curve, has surely very little value in the light of the 
experimental proof that the assimilation corresponding to that optimum can 
be varied as desired by changing the intensity of the light used in the 
experiments. When such a series of curves is obtained, as was obtained by 
Matthaei for Cherry-laurel in increasing intensities of light, any one of these 
curves may perhaps be of the form preferred by Brown, i. e. such as he 
would call an optimum curve. But how can all the curves be optimum 
curves, and what is the value of the idea of an optimum which changes with 
every change of one of the factors conditioning the experiment ? 
It is not claimed that a limiting factor curve always adheres rigidly to 
a typical form with a sharp angle at the point of change of the limiting factor. 
