104 Dr. H. T. Brown and Mr. F. Escombe. [Jan. 9, 
minute, are rearranged in a more convenient form in Table IX, where the 
total amount of energy received by the leaf-lamina is represented by 100, and 
is accounted for by the two modes of internal work in the leaf, the energy 
lost by transmission through the lamina, and that lost by re-radiation and the 
cooling action of the surrounding air. . 
The percentage of energy used in photosynthesis, w, may be regarded as the 
true “economic coefficient” cf the leaf—that is to say, the percentage of the 
total incident energy which, under the particular conditions of each experi- 
ment, the mechanism of the leaf-lamina has been able to transform into useful 
photosynthetic work contributing to the production of new material for the 
plant. | 
The experiments recorded in Tables VIII and IX were carried out on 
different days on which the insolation and atmospheric conditions varied, so 
that they are not comparable with each other except in a general way, even 
where leaves of the same species of plant were employed. 
In the second series of experiments, the details of which are given in 
Tables X and XI, simultaneous duplicate experiments were made on similar 
leaves for which all the external conditions were the same except the degree 
of insolation represented by R. Whilst one leaf, A, was exposed to the full 
sunshine, the other, B, was exposed behind revolving sectors which cut off 
a definite proportion of the solar radiation. Thus, as far as regards the 
conditions which were under control, there was only one variable introduced, 
that of R, and the two members of each pair of experiments are therefore 
capable of being compared with each other from this point of view. It must 
not, however, be lost sight of that variations in the value of R tend to intro- 
duce secondary effects on the internal work of transpiration, not only by 
altering the temperature of the leaf, but also indirectly by altering the degree 
of opening of the stomata. Although, therefore, such experiments as those 
recorded in Tables X and XI admit, under the controlled conditions of 
incident radiation, of a much closer comparison than those of Tables VIII 
and IX, still it is practically impossible to produce conditions under which 
only one factor at a time shall vary, owing to the mutual dependence of the 
various factors one on the other, a fact which has already been fully discussed 
in Section (1), Part III. 
The experiments of Series A of Tables X and XI were all carried out under 
the full available insolation, but, notwithstanding this fact, it will be noted 
that the condition of the stomata and the excess of partial pressure of water- 
vapour between the interspaces of the leat and the surrounding air were so 
favourable that the energy required for the internal work of transpiration so 
