456 Dr. F. F. Blackman and Miss G. L. C. Matthaei. [Apr. 11, 
to about 18 parts in 10,000, would appear to be indicated. As a matter of 
fact, if this were done the temperature would then become the limiting factor 
and each leaf would stop at the values given in column 3. The waste wouid 
then be the difference between 0:0530 and the values in this column. 
The leaf at 10° C. is, however, already limited by temperature, and cannot, 
therefore, avail itself of even half the CO, that might diffuse into it. 
At hours remote from noon the solar radiation falling on a leaf will be 
much less intense, and the average for all the daylight hours of a year will be 
very much below 0°0530. 
This consideration is of interest in relation to the theory that the reduction 
of carbon-dioxide by plants in the carboniferous epoch must have been much 
more energetic than at the present time to account for the deposition of such 
enormous masses of carbonaceous matter in the earth. 
If the illumination was the same as at the present time, there would hardly 
have been enough energy for more than threefold intensity of total CO:- 
reduction, however rich the atmosphere might have been in this gas. 
The general lesson to be learned from these limitations of assimilation in 
nature would seem to be that the biclogical advantage which plants gain 
when dwelling in the brightest habitats, is not increased assimilation, but 
probably increased warmth. This may be the explanation of the fact noted 
by Wiesner in his studies on “ Lichtgenuss,” that shade plants are abundant 
in the tropics, but gradually fail in northern latitudes, and are absent in 
arctic vegetation. 
VIII. —Conelusions. 
There are three conspicuous factors which control the amount of assimila- 
tion of carbon dioxide that a leaf can perform: (1) the intensity of the 
illumination, (2) the temperature of the leaf, and (3) the pressure of the 
CO, in the surrounding air. If the illumination is feeble, though the other 
factors are favourable, then the amount of photosynthesis will be kept down 
and light will be a “limiting factor” to the process. So similarly with the 
CO,-supply, and also, as has been shown in the previous paper of this series, 
with the temperature. For each temperature there is a definite amount of 
assimilation that a leaf can perform and no more. For a given plant these 
amounts are very constant; at high temperatures the high rate of assimila- 
tion cannot be maintained for long, and a “time factor” comes in to 
complicate the relation. 
The present work is an attempt to interpret the quantitative variations of 
carbon-dioxide assimilation, under natural or semi-natural conditions, in terms 
of these three chief factors. 
When an increase of assimilation follows an increase in the sun’s radiation 
