536 Smith. — The Temper ature- coefficient of Photosynthesis. 
It is conceivable, and is indeed probable, that when, so to speak, two factors 
are close to the limiting value a change in the one not limiting may have 
some appreciable effect on assimilation. This will show itself about the in- 
flexion of the curve where the limiting factor is changing. For example, 
when C0 2 is limiting, increase of temperature may cause a small increase of 
assimilation by increasing the rate of diffusion of the C0 2 . But all minor 
details like these apart, the hypothesis of limiting factors rests broadly on the 
possibility by its means of interpreting simply and logically the greatest 
number of the known facts about the rate of carbon assimilation. 
Finally, Brown reveals his singular lack of appreciation of the com- 
plexity of the system involved in assimilation when he asks how such 
a principle as that of limiting factors can hold in the phenomena of assimi- 
lation when he can see no sign of this principle at work in the action of 
hydrochloric acid upon marble or the solution of a gas in water. 
I am glad to acknowledge the valuable assistance of Dr. F. F. Blackman, 
F.R.S., with whom I have discussed many of the points raised in this paper. 
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