98 Dr. H. T. Brown and Mr. F.. Escombe. [Jan. 9, 
Column (8) gives the average velocity of the wind during the experiment in 
kilometres per hour. 
Column (9) headed “ difference of temperature between leaf and air” 
requires some explanation. The values are deduced, in the first place, from 
the difference between Ra, the energy absorbed by the leaf, and W+v, the 
energy used up in the internai work; and, secondly, from the known thermal 
emissivity of the leaf in an air-current of the mean velocity of the wind during 
the experiment. If the emissivity of the leaf per square centimetre per 
minute for a temperature-excess of 1° be represented by ¢, then the temperature 
difference between the leaf and the surrounding air will be represented by 
SS If Ra exceeds W+w the temperature of the leaf will be above 
that of its surroundings, whereas if Ra is less than W + w the leaf temperature 
will be the lower. 
The values of Column (10), headed “estimated temperature of the leaf,’ 
were obtained from the dry bulb temperatures by adding or subtracting the 
values of Column (9). 
Column (11) gives the excess of the partial pressure of the water-vapour in 
the air-spaces of the experimental leaf over that of the outside atmosphere. 
It is, in fact, the differential partial pressure, measured in millimetres of 
mercury, between the point of saturation corresponding to the leaf temperature 
as given in Column (10) and the partial pressure of the water-vapour of the 
outer alr as given in Column (6). It is a measure of the “ diffusion-gradient ” 
of the water-vapour existing between the interspaces of the leaf and the 
external air. Assuming all other conditions to be identical the rate 
of transpiration should be proportional to the values given in this 
column. 
Column (12) gives the rate of photosynthesis determined directly in the 
manner already fully described. The results are given in terms of cubic 
centimetres of atmospheric carbon dioxide utilised by one square decimetre of 
the leaf in one hour. 
Column (13) gives the rate of transpiration as directly determined, the 
results being expressed in grammes of water transpired by one square 
decimetre of leaf-lamina in one hour. 
Columns (14) to (21) include the results obtained by means of the 
Callendar’s radiometer and give an account of the manner in which the 
incident radiation has been utilized by the leaf. The values are here given in 
terms of water-gramme-units (calories) per square centimetre of leaf-lamina 
per minute. 
