Photosynthesis : a Reply to Criticism. 531 
these values are in agreement with the van ’t Hoff rule for 4 dark ’ reactions 
and are much higher than photochemical coefficients. Recently Osterhout 
and Haas (1919) have obtained a coefficient of 1 *8 1 for Ulva , which is 
nearer to the values characteristic of ‘ dark ’ reactions than to those usual 
in ‘ light ’ reactions. Even this figure, however, is probably not high 
enough, for in the very brief details given of their experiments in the low 
C0 2 tension of sea-water they provide no evidence against the natural view 
that low C0 2 tension, based on slow dissociation, and diffusion were limiting 
their observed value at 37 0 to something lower than would have been 
obtained had temperature been exerting its maximum effect. 
It was stated in the introduction to this paper that carbon assimilation 
is a complex process involving both ‘ light ’ and ‘ dark ’ reactions, and that 
the 1 dark ’ reactions govern the maximal possibilities of the system. The 
survey since made of the work done on this problem has shown that the 
reliable experimental evidence is all in favour of the idea that the velocity 
of the whole process is determined by some reaction having a high 
temperature-coefficient, and not a low coefficient such as is characteristic 
of pure photochemical reactions. 
Assimilation and Light Intensity. 
In their second paper Brown and Heise (1917 b ) maintain that the 
relation between light and assimilation is not one of direct proportionality, 
as has been held broadly by Blackman and his co-workers. They state 
that the relation is such that from each increase in light intensity there 
results a progressively smaller increase in the velocity of assimilation. 
This contention is based upon the experiments of Matthaei (1904), Pantanelli 
(1903), Reinke (1883), and Timiriazefif (1889). Brown and Heise themselves 
state that Timiriazefif’s results are based upon faulty experimentation. 
They do not, therefore, put these results forward as independent evidence for 
their thesis. There is thus no need for any discussion of Timiriazefif s 
results here. The results of the other workers as interpreted by Brown 
and Heise need, however, some consideration. 
The Work of Matthaei on Cherry -laurel. 
In Fig. 3 of their first paper Brown and Heise have drawn a curve 
supposed to represent the relation of light intensity and carbon dioxide 
assimilation in Cherry-laurel based on Matthaei’s data. It is clear that 
such a curve ought to consist of the results of experiments in which light has 
been definitely proved to be the limiting factor. Now of the ten values 
which make up this curve it can be asserted of only three that there is proof 
that they are light-limited values obtained from normal leaves. The value 
o*oo 22 with unit light is light-limited, as is proved by increasing the light to 
two units, resulting in a large increase of assimilation at the same tempera- 
tures. The value 0*0038 with two units of light is light-limited, as is shown 
