
a 
1905.] On the Physiological Processes of Green Leaves. 99 
Column (14) gives a measure of the fia solar radiation incident on the 
leaf, a value which has been denoted by R in the general thermal equations. 
Column (15) gives the coefficient of absorption of the leaf for solar radiation, 
this coefficient being denoted by a. 
Column (16) gives the proportion of incident solar radiation which is 
absorbed by the leaf, and which must manifestly be equivalent to the incident 
radiation multiplied by the coefficient of absorption, 7.¢., to Ra. 
The amount of energy ET aT through the leaf will, of course, be 
represented by R—Ra. 
Column (17) gives the amount of energy used up by the endothermic 
process of photosynthesis. 
As already explained in Section (1) Part III (p. 77), these values are 
deducible from the volume or mass of carbon dioxide assimilated by the leaf 
as givenin Column (12). The volume of carbon dioxide assimilated per square 
centimetre of leaf-lamina per minute, when multiplied by 5:02, gives a measure 
of the energy w used up in photosynthesis, expressed in calories for the same 
unit-area and unit-time. 
Column (18) in the same manner gives a measure of the amount of energy, 
W, expended in the vaporization of water by the leaf. The values are deduced 
from the experimental results of Column (13), giving the water transpired from 
a given area of leaf in a given time. 
The numbers are obtained by multiplying the grammes of water transpired 
by one square centimetre of the leaf-lamina per minute by the value for the 
latent heat of water, which at 20° is 592°6 calories. 
Column (19) gives a measure of the expenditure of energy for the total 
internal work of the leaf, both for protosynthesis and water vaporization—i.c., 
W-+w. The values given are the sum of those of Columns (17) and (18). 
Column (20) gives the actual amount of energy lost per unit-area of leaf- 
lamina and unit-time owing to re-radiation and the conductive and convective 
properties of the surrounding air. This is the difference between Ra and 
W-+w, when Ra is the greater. It is the only portion of the incident 
radiation which can produce any rise of temperature in the leaf. 
When Ra, the incident radiation falling on the leaf and absorbed by it, is 
less than W +w, the energy used up in internal work, it is manifest that the 
leaf must draw upon its surroundings for the balance. Where this condition 
of things exists it is recorded in Column (21). The leaf is then lower in 
temperature than its surroundings. 
The results tabulated in Columns (14) to (21) of Table VIII, giving the 
actual loss and gain of energy by the insolated leaf per square centimetre per 
H 2 
