Photosynthesis : a Reply to Criticism . 523 
limbs, one for each intensity of light used by Matthaei. As each horizontal 
limb does cover assimilation over a fair range of temperature, they thus 
reach the desired conclusion that temperature has practically no effect upon 
assimilation. 
We must now consider in some detail the criticisms levelled at the 
rising limbs of each of Matthaei’s four curves, which were carried out with 1, 
1 , 4, and 8 arbitrary units of light respectively. The first of these curves sets 
forth the results of twenty-one experiments ranging from —6° to +33°, 
all in unit intensity of light. The curve is of very definite form, and it is 
clear from it that the results obtained at temperatures below 3 0 increase in 
proportion to increasing temperature, while the remainder of the results show 
the limiting effect of unit light. The eight experiments on the rising part 
of the curves are in direct opposition to Brown and Heise’s contention, and 
they therefore simply rule them out of the discussion by announcing their 
intention of confining their consideration to values obtained between 4-3° 
and 33 0 . This method of arbitrarily choosing a lower limit (3 0 ) which 
will be convenient for their theory is felt by them to be too summary 
a procedure to pass unsupported by any argument. They therefore say, 
‘ The part of the curve below 3 0 shows very much higher temperature- 
coefficients than would be called for according to the van’t Hoff principle. 
At these temperatures many plant processes are just coming into activity. 
Under such conditions it would not be surprising to find that a general ratio 
would not hold for any particular function.’ 
To this it may be replied that results below 3 0 with weak light are 
perfectly consistent with results above 3 0 in somewhat stronger light when 
treated from the point of view of Matthaei, Blackman ( 1905 ), Kanitz ( 1915 ), 
and many others who accept the application of the van ’t Hoff rule. This is, 
therefore, in favour of their outlook and against the view of Brown and Heise, 
who have to assume a sudden discontinuity of principles at this temperature, 
passing from big temperature-coefficients below it to very small ones 
above it. 
Brown and Heise then pass to Matthaei’s second curve, expressing the 
results at different temperatures with light of twice unit intensity. It will be 
seen that the general form of the curve is the same as that for unit light just 
discussed. The important point is that though double light may produce 
increased assimilation, as pointed out by Brown and Heise, yet it only does 
so when the temperature is raised. So long as the temperature remains low , 
no amount of light increase causes increased assimilation. This fact, 
unchallenged, would be fatal to Brown and Heise’s whole contention. They 
deal first of all with Matthaei’s proof that at temp. 0-4° doubling the light 
does not increase the assimilation, while at temperatures 9 0 , n°, and 25 0 the 
assimilation is thereby almost doubled. Their comment is as follows : 
‘ The temperature is below that which we are considering. It may be 
