86 Dr. H. T. Brown and Mr. F. Escombe. [Jan. 9, 
When a leaf is exposed to full sunshine the radiant energy which is 
utilised for the photosynthetic process represents only a very small part of 
the total incident radiation. If we restrict the term “economic coefficient ” 
to the ratio of these two values, the full radiation falling on the leaf being 
taken as 100, it is evident that the leaf is an extremely wasteful trans- 
former of energy, since it receives a very large amount of superfluous energy 
which does not contribute to the main function of the leaf, and has to be 
dissipated by some means. 
If for the purpose of argument we assume that photosynthetic work is 
confined to that portion of the solar spectrum corresponding to the principal 
absorption-band of chlorophyll, lying between the lines B and C, some idea 
may be obtained of the maximal theoretical efficiency of a leaf exposed under 
the most favourable conditions to full sunshine, provided we know the relation 
of the energy in this restricted portion of the spectrum to the total energy. 
The question has already been discussed by one of us from this point of 
view,* and the conclusion was reached that the maximal “economic 
coefficient” for full sunshine would probably be about 6°5 per cent. if the 
leaf were in a position to sift out and utilise the whole of the particular grade 
of energy useful for photosynthetic work. But this implies a set of conditions 
which can never exist in nature owing to the limits imposed on the assimilatory 
process by the high state of dilution of the “atmospheric carbon dioxide.” 
That the photosynthetic rays, even in sunlight of very moderate intensity, 
are in excess of the power of the leaf to utilise them has been shown by the 
experiments described in Part il, p. 54, in the first place by the increased 
assimilatory effect produced under constant or practically constant illumination 
by increasing the carbon dioxide in the surrounding air, and secondly by 
observing the rate of photosynthesis in air of fixed carbon dioxide-content when 
the leaf is submitted to solar radiation of varying and known relative intensity. 
It was found, for instance, when solar radiation of an average intensity of 
about 0-5 calorie per square centimetre per minute was reduced to about 
one-third of this intensity by passing through a thin canvas screen, forming 
an artificial “cloud,” that it still contained an excess of photosynthetic rays 
over and above what was necessary to produce maximal assimilation in 
ordinary air; for by means of the revolving-sector-method the intensity of 
the radiation could be still further reduced to one-quarter, that is to say, to 
one-twelfth of the original amount, before there was any sensible diminution 
in the assimilatory power of a leaf submitted to its influence.t 
* See ‘ Pres. Address,’ section B, ‘ Brit. Assoc. Rep.,’ 1899, p. 681. 
+ In passing it may be noted that the ratio which the photosynthetic radiation bears to the total 
