B lackman. — Optima and Limiting Factors . 285 
representing two hours, and plot out the falling series of readings obtained 
at 30*5, 37-5, and 40-5° C. in curves starting from the initial values indicated 
by method I. 
It then becomes at once obvious that the calculated initial value and 
the observed subsequent values fall into one fairly harmonious curve for 
each temperature. We thus attain a graphic demonstration that both 
methods indicate practically identical initial values, and this affords, it 
seems to me, satisfactory evidence for a preliminary acceptance of the 
theory that such values actually occur, though it is not possible to measure 
them. 
At low temperatures — up to 25 0 C. — the assimilation-values keep up to 
the initial value for a long period, but at these higher temperatures the 
fall continues and in time brings the assimilation down to zero, and that 
the more rapidly the higher the temperature. The points at which the 
respective curves reach zero on the base line are partly outside the limits 
of the diagram, but the various curves have been continued by freehand 
beyond the actual estimations in the way that is indicated by other 
experiments. 
Extending the schema that we have arrived at to higher temperatures 
than 40-5° C., we find at 45 0 C. a still more rapid fall of the assimilation, for 
which, however, we have no suitable numerical data, and this declines to 
zero in a comparatively short time. This is represented in the diagram by 
the curve starting from F. 
Finally, to conclude the series we ought to find a temperature at 
which the earliest estimation that could be actually made would give no 
measurable assimilation. The lowest temperature to give this result might 
be called the ‘ extinction temperature,’ and here we should hypothecate 
that, for the first few seconds after attaining it, each chloroplast would give 
a higher assimilation rate than at any lower temperature, but that the rate 
would immediately fall, and that so rapidly that it would become nil almost 
at once (say in 100 seconds, for the accepted specific extinction temperature 
would of course have to be arbitrarily defined in time-units). 
In estimating carbon-assimilation in the presence of over-balancing 
respiration, it is almost impossible that this temperature should be deter- 
mined directly, but we are accumulating evidence as to the location of it. 
I have placed it at 48° C. in the diagram. We thus arrive at a complete 
theoretical schema of the primary relation between the rate of the photo- 
synthetic process, different temperatures, and time. 
Now, without going into details again, it may be stated that for cherry- 
laurel leaves the process of respiration, although it contrasts powerfully 
with assimilation as to its metabolic significance, also shows quite the same 
kind of relation to temperature. 
I therefore venture to suggest that, making suitable changes in the 
x 2 
