Photosynthesis : a Reply to Criticism. 519 
processes have exerted their limiting influence.’ 5 In spite of this circum- 
stance we are publishing our results now because they may indicate to 
others the probable way to obtain the desired results.’ 
In the results presented only five are at temperatures other than 24 0 : 
three at 36°, one at 40°, one at 45 0 . Of those at 36° only one can be com- 
pared with assimilation at 24 0 because the other two were done on smaller 
and undefined amounts of material. Throughout this work no adequate 
uniformity in the amount of material employed is attained, and the values 
for ‘apparent assimilation’, under as uniform conditions as possible, vary 
from experiment to experiment, 0-152, 0-166, 0-168, 0-186, 0-189. 
Our present interest is therefore entirely concentrated on the case 
where, with constant light (2,482 Hefner candles), constant C 0 2 supply 
(350 c.c. of water containing 0-152 mg. free C 0 2 per litre every 4! to 5 
minutes), the observed output of oxygen was 0-189 mg. per minute at 
24 0 , while it rose to 0-238 mg. at 36*5°, thus indicating a ratio which would 
make K 10 = 1-26. 
For these values to be significant it is absolutely essential that each 
must represent the maximal value possible at that temperature. This can 
only be established by producing evidence that neither light nor C0 2 supply is 
limiting the assimilation to a value below the appropriate specific maximum. 
The value of 0-189 mg. 0 2 we can accept as maximal for 24 0 C., because 
raising the temperature alone considerably increased the value. But can we 
accept 0-258 mg. as maximal for 36-5° ? Van Amstel has given no proof in 
her paper that 0-258 is a maximal value. There is no experiment in which 
the temperature was raised from 36-5° to any higher value, using the same 
shoot. For information as to the possibilities of assimilation at 40° we 
must turn to an experiment on a different shoot from that which furnished 
the value 0-258 mg. This second shoot gave at 40° an average assimila- 
tion of 0-242 mg. Since, however, it had already at 24 0 given a lower 
value than the first shoot, van Amstel obtained, by allowing for its lower 
assimilatory activity, a figure for its assimilation at 40° which was 5 per 
cent, higher than that of the first shoot at 36*5°. There are no data from 
which to calculate the probable error of van Amstel’s experiments. In 
their absence it is safe to say that there is no proof that this rise of 5 per 
cent., obtained in such an indirect way, represents a real rise in assimilatory 
activity due to the higher temperature. Consequently it is unproven that 
0-258 is the maximal value possible at a temperature of 36*5°. 
Van Amstel was of opinion that she had given proof that both C 0 2 
supply and light were more than sufficient for the assimilation possible at 
any of the temperatures used in her experiments. Closer inquiry, however, 
may lead to a different conclusion. With regard to C 0 2 supply, she 
trusted only to alteration of rate of flow ; the concentration of C 0 2 in the 
water-supply was the same in the whole of her experiments, The shoot 
P p 2 
