524 Smith.— The Temperature-coefficient of 
said, however, that with unit intensity of light the result at 0-4° is very 
similar to those obtained at all higher temperatures, and so is greater than 
would be expected from the more complete results recorded in Table I and 
plotted in Fig. 1. This figure shows an increase in assimilation of consider- 
ably more than 50 per cent, between 0-4° and 3*6°. Doubling the intensity 
of light did, however, increase this apparently too high result in Table II 
for unit intensity of light at 0-4°.’ 
This passage is obscure, and has been rendered more so by carelessness 
in the citation of tables. Table I should evidently be Table V and Table II 
should be Table VI. In Matthaei’s paper these results are incorporated in 
Figs. 2 and 3 respectively. 
The first sentence of their comment is a repetition of their evasion of data 
which do not suit them. The statement that the result in Matthaei’s Fig. 3 
(Fig. 1 of this paper) for 0-4° is greater than would be expected from the 
more complete results in Matthaei’s Fig. 2 is very curious, for comparison 
will show that the values from the two curves are exactly the same. 
Apparently Brown and Heise suppose that the figure for 0-4° should in 
every series of experiments be the same proportion of the result for 3*6° as 
it is in Matthaei’s Fig. 2. A moment’s consideration will show the 
improbability of such a supposition. The value for the higher temperatures 
shown in Matthaei’s Fig. 3 (Fig. 1 of this paper) for unit light is depressed 
because of a lowered seasonal activity of the leaves. This phenomenon is 
perfectly familiar to any one who has worked with either land or water 
plants, and, though its causes are obscure, it is best expressed in Matthaei’s 
words : ‘ It appears as if in the sluggish condition of the leaves more light 
were necessary to do the same amount of work.’ The assimilation for unit 
light is therefore lower in Matthaei’s Fig. 3 than in her Fig. 2. But at 0-4°, 
where, according to all the evidence, light is not the limiting factor, but tem- 
perature, there is no reason why the assimilation should be depressed from 
its usual temperature value. The result at 0*4° and light intensity one unit 
is therefore not too high, but, as has already been remarked, exactly the 
same and confirmatory of the result in Matthaei’s Fig. 2. The result at 
the same temperature and light intensity two units is the same within the 
probable error of the experiment, and this constitutes a proof that a tem- 
perature of 0-4° is the limiting factor, for results of another order altogether 
are obtained when .temperature is not limiting, as is shown by the three 
concordant experiments at 9 0 , n°, and 25 0 . 
It is well established, therefore, that with light intensity of two units 
a temperature of 0*4° is limiting and a fortiori that lower temperatures are 
limiting also. This is important, for it cannot be expected that in every 
fresh series of experiments with more intense light the proof that these 
lower temperatures are limiting should again be repeated. When, therefore, 
Matthaei comes to the experiments with light intensity of four units she is 
