528 Smith . — The Temperature-coefficient of 
are by no means confined to the barren conclusion suggested by Brown and 
Heise. It is obvious that, the leaves in the second series were less active 
than those in the first series, and Matthaei pointed this out and was fully 
aware of it. This lessened activity implies that the less active leaf requires 
more light to perform the same assimilation as the more active leaf. Now, 
since more light is needed for the same assimilation, comparisons cannot 
safely be made between the more active and the less active leaves. Thus, 
when Matthaei says, as a result of the comparison between the less active 
leaf of Expt. 56 and the more active leaf of Expt. 37, that the former must 
be exposed to nearly twice the light necessary for the same assimilation in 
a normal leaf, we cannot accept her conclusion without further inquiry. 
For while the more active leaf performed 0-0072 assimilation with 8 units 
of light, the less active leaf would certainly require more light than this for 
the same assimilation, and we do not know exactly how much more. It is, 
indeed, probable that by increasing the light to 13 units (i. e. by 65 per cent.) 
a sufficient margin was provided, but it is not certain. It is a practical 
certainty that the margin was insufficient in Expt. 57, for the light used was 
more than three times that used in Expt. 38, and it could scarcely therefore 
have been limiting even for the more sluggish leaf. But even in the case 
of Expt. 56 it is possible to calculate approximately whether the light 
provided was in excess. If the experiments of Matthaei’s Fig. 3, made in 
April with unit light, are compared with those of her Fig. 2, made in 
February with unit light, it is seen that on the average the activity of the 
more sluggish leaf is about 75 per cent, of that of the more active one, 
° °° 2 j = 0-77. It follows that if the more active leaves perform 0-0072 
0-0022 
assimilation with 8 units of light the less active ones will require § x f = 
10-7 units for the same assimilation. Therefore 13 units of light is in excess 
and cannot be the factor limiting the assimilation of Expt. 56 to 0-00705. 
Thus Matthaei’s assumption is justified. 
A similar calculation will show that the margin in Expt. 57 compared 
with Expt. 38 is so large that full allowance can be made for the lessened 
activity of the leaves and yet the light must be in excess and therefore the 
temperature limiting. Though no exact calculation can be made for 
Expt. 58 compared with Expt. 50, yet the probability again is that the light 
is in excess. This is confirmed by the result of Expt. 59, in which, though 
the light is not increased, the assimilation (both the initial and the average 
values) is greater. 
This increase is therefore obviously due to the rise in temperature, show- 
ing that at 30*5° C. the temperature was the limiting factor. In this case 
the proof does not depend upon the comparison of leaves of differing activities 
at all. This fact has been dealt with by Brown and Heise as follows : 
‘ Expt. 59, at a temperature of 37-5° and light intensity of 45 units, 
