- 1 8 Smith . — The Temperature-coefficient of 
have low coefficients also, however strong the evidence may appear to be 
against this. They therefore, without any misgivings, set out to explain 
away or discredit all measurements of carbon assimilation in plants which 
indicate high temperature-coefficients. There is nothing judicial or scientific 
in their method. Data are adopted or rejected quite apart from the amount 
of evidence supporting them. Sound progress can, however, only be made 
by shaping theories to fit facts and not vice versa. It has not occurred to 
them that though certain stages in the complex chain of reactions of carbon 
assimilation are certainly photochemical, yet other subsequent stages are 
‘ dark reactions \ From this conception it follows that it would be a matter, 
not for a priori dogmatism, but for experimental investigation, whether the 
temperature relations of the total process were those of a ‘ light , or of 
a { dark ’ reaction. 
This view of the complex nature of carbon assimilation has been held 
by teachers and workers for many years now, and it has generally been 
accepted, on the available evidence, that the dark reactions govern the 
maximal possibilities of the system. The attack upon this view that Brown 
and Heise have made consists of a mass of detailed criticism of individual 
measurements of photosynthesis. As few readers are likely to weigh all 
these criticisms for themselves, it seems unfortunately necessary to point out 
in some detail how arbitrary and unsound they are. 
Assimilation and Temperature. 
Brown and Heise first discuss the work of van Amstel ( 1916 ) on 
E lode a. 
Experiments of van Amstel on El ode a. 
** 
Miss van Amstel investigated the output of oxygen by Elodea in 
a current of water laden with C 0 2 at a few temperatures, namely, 24 0 , 36*5°, 
and 40 0 . 1 She set out to measure the temperature-relations for these points, 
but admits that it is a difficult and complicated matter to get critical values 
for high temperatures. In one experiment only (Table V) did she compare 
the effect of the temperatures 24 0 and 36*5° on the same shoot of Elodea , 
and the observed increased rate indicated 1-26 as the value of K 10 . This 
observation is the only low value for a temperature-coefficient of photo- 
synthesis available in the literature, and it is of course eagerly adopted by 
Brown and Heise without any examination of credentials. It must first be 
pointed out in fairness to Miss van Amstel that she expressly disclaims that 
she has succeeded in establishing trustworthy ratios for these high tempera- 
tures. She says on p. 3 : ‘ Thus the experiments, which are to be discussed 
here, will lead to the conclusion that in fact we did not measure the 
influence of temperature on the C 0 2 assimilation itself, but that physical 
1 All temperature figures are in C°. 
