
1905.] On the Physiological Processes of Green Leaves. 97 
(e) The Determination of the Thermal Emissivity (e) of the Leaf-lamina. 
For a full consideration of the experimental methods adopted for a deter- 
nuination of the thermal emissivity of a leaf-lamina reference must be made 
to a separate communication on the subject.* 
The mean value of the emissivity for the four kinds of leaf examined was 
for, “still air” conditions, 0°0145 calorie per square centimetre of leaf surface 
per minute for a temperature excess of 1° C. In moving air the emissivity 
increases by 0:000172 calorie per square centimetre per minute for every 
metre per minute increase in the velocity of the air moving over the surface 
of the leaf-lamina. 
(f) Details of Experiments on Leaves submitted to Solar Radiation of known 
Intensity, showing the mode of Disposal of the Incident Energy under 
Defined Conditions. 
The data of these experiments have been brought together in Tables VIII, 
IX, X,and XI. Their general arrangement will be understood from what has 
been said in Section (1) Part III on the thermal relations of a leaf to its 
surroundings, but some detailed explanation of the headings of the columns is 
desirable at this point. 
Tables VIII and IX contain the results of a number of experiments on 
leaves of different species of plants submitted to solar radiation of known 
intensity, whilst a direct determination was made of the internal work of 
photosynthesis and of transpiration. 
In order that the assimilatory experiments should be carried on under 
favourable conditions of temperature, the full solar radiation was, in most 
cases, modified by passing it through a thin canvas screen before it reached 
- the leaf. 
In Table VIII Columns (1) and (2) require no comment. 
Column (3) headed “ percentage of sunshine,” indicates the proportion of the 
time occupied by the experiment, during which the unobstructed solar 
radiation was sufficiently intense to be recorded on the Campbell-Stokes 
burning recorder. 
Column (6) gives the average partial pressure of the water-vapour in the 
air as deduced from the reading of the wet and dry bulb thermometers as 
given in Columns (4) and (5). 
Column (7) gives the average humidity of the air when that of saturated 
air = 100. | 
* See Brown and Wilson, infra, p. 122. 
VOL. LXXVI.—B. H 
