520 
Smith . — The Temperature-coefficient of 
occupied the axis of the cylindrical vessel through which the water flowed 
and most of the flow would be along the unimpeded track at the periphery 
of the vessel. Now the important thing is the concentration in the more 
stagnant layers at the surface of the leaves and the diffusion gradient main- 
tained from them into the leaf tissue. So when she found in one experiment 
that halving the total flow-rate through the cylinder did not appreciably 
lower the rate of photosynthesis, we cannot conclude that the concentration 
at the surface of the plant was seriously diminished by the reduced flow out 
beyond it. Experiments made at Cambridge by Mr. F. Summers on the 
bubbling of water-plants in a stream of tap-water supersaturated with air 
showed that doubling the rate of flow only increased the bubble-rate by 
15 per cent. Indeed, if the flow is very fast, a further increase of 20 per 
cent, in rate may not increase the bubble-rate at all. Concentration of C 0 2 
is the only significant thing, especially in a water medium where diffusion is 
so sluggish, and it is a pity that the whole of van Amstel’s work was done 
with but one concentration. 
As proof that the light, 2,482 Hefner candles, was sufficient for the full 
assimilation at 36-5° van Amstel furnishes one experiment in which at 
this temperature an increase of light to 3,377 candles did not increase the 
assimilation (van Amstel, Table II). The value of the assimilation in this 
experiment (0-148 mg.) is, however, so widely different from that obtained 
in the later experiment at 36*5° (0-258 mg.) that, unless some explana- 
tion of the discrepancy is forthcoming, the result cannot be considered 
significant. For all these reasons, therefore, the figure 0-258 mg. 0 2 per 
min. cannot be regarded as having been proved to be maximal for a tem- 
perature of 36-5°. 
Further, there is the important point that no account is taken of the 
respiratory gas-exchange going on at the same time as photosynthesis. At 
low temperatures this will be small, but at high temperatures it cannot be 
neglected for exact work. Therefore at 36-5° the real photosynthesis 
value must be appreciably higher than the apparent value actually observed 
by just the amount of oxygen that is circulating round and round in the 
respiration of the moment. 
It appears, therefore, that the temperature-coefficient of 1*26 is not 
sufficiently substantiated. 
The Work of Blackman and Smith. 
Brown and Heise quote experiments made by Blackman and Smith 
( 1911 ) and calculate therefrom three temperature-coefficients for assimilation 
in Elodea. The first, 2*05, between 7 0 and 33 0 , agrees with the calcula- 
tions of that paper. The second, 1-75, between 7 0 and 2i°, and the third, 
1-35, between 13 0 and 2i°, were not calculated in the paper, because there 
was no proof that the figure obtained at 2i° was the real measure of the 
