
1905.] On the Physiological Processes of Green Leaves. 87 
This excess of photosynthetic radiation in weakened sunlight is a fact 
of the highest importance to the plant, since it enables the leaf to carry out 
unimpaired the main function of assimilation in diffuse light, and in sunlight 
of a considerable degree of obliquity.* It has also an important bearing on the 
“economic coefficient” of the leaf, for it is manifest that this must vary very 
much according to the intensity of insolation. The “ coefficient” will be at a 
maximum when the insolation is just sufficient to produce the maximal rate 
of assimilation for a given partial pressure of carbon dioxide in the air, and 
will decrease with increased incideut radiation. The accuracy of this deduc- 
tion is shown by the detailed results which follow, especially those obtained 
by experiments with revolving sectors. 
A few words still remain to be said about the dissipation of the superfluous 
energy absorbed by the leaf. We have already seen that this is brought 
about by the two processes of water vaporisation (transpiration) and thermal 
emission. The relative part taken by each of these processes will vary greatly 
according to the nature of the plant and the surrounding conditions. In the 
case of plants well adapted to water-conduction and provided with abundant 
stomata, the transpiratory “safety-valve” no doubt plays the more important 
part, and the temperature-gradient between the leaf and its environment need 
never be large in non-saturated air. For instance, in the case of a leaf 
transpiring at the moderate rate of 500 grainmes per square metre per hour, 
it can readily be shown that the vaporisation of this amount of water will 
absorb about 0°5 calorie per square centimetre per minute. We have never 
observed a higher maximal value for solar radiation at Kew than about 
1:0 calorie per square centimetre per minute, and for observations extending 
over several hours it has seldom exceeded 0°5 calorie. 
solar radiation ought to be approximately determinable by the use of the revolving radial sectors 
described in Part IT, p. 54. 
In the particular case cited above no effect on assimilation was produced until the solar radiation 
of 0°5 calorie per square centimetre per minute had been reduced by the screening to one-twelfth 
of its original value, 7.e., to 0°041 calorie. The observed rate of assimilation at this point was 
2°07 c.c. of carbon dioxide per square decimetre per hour, or 0°00034 c.c. per square centimetre, 
per minute, which must mark the stage at which practically the whole of the rays capable of 
producing photosynthesis were utilised for that purpose. From what has been said before it 
follows that this amount of assimilated CO, corresponds to an absorption of energy of 
0°00034 x 5°02 = 0:0017 calorie per square centimetre per minute. Since no selective absorption 
has taken place in the screening process, the proportion of photosynthetic rays in the reduced 
0:0017 x 100 
aC GANe sa 41 per cent., also represents the percentage of 
photosynthesizing energy in the original unscreened solar radiation. 
* It follows that the limiting factor in assimilation in ordinary sunlight of even a low degree 
of intensity is the high degree of dilution of the atmospheric carbon dioxide. 
radiation falling on the leaf, 7.e., 
