74 Dr. H. T. Brown and Mr. F. Escombe. | Jan. 9, 
As a concrete example of a leaf transpirmg in an enclosure under the 
conditions just postulated, we will assume that the ieaf is losing water steadily 
at the rate of 0°5 gramme per square decimetre per hour, or 0:0000833 gramme 
per square centimetre per minute. The heat required to vaporize this last- 
mentioned amount of water at 20° C. is 0:0000833 x 592°6 = 0:04938 water- 
gramme-units (calories), and, excluding the small disturbance of the respiratory 
process, this must represent, according to the theory of exchanges, the amount 
of energy entering and leaving a square centimetre of the leaf-lamina in one 
minute, when the steady thermal state has been attained. If the leaf has a 
thermal emissivity of 0°015 calorie per square centimetre of leaf surface per 
minute for a 1° C. temperature gradient, the temperature difference, 0—@,, 
0:04938 Og 
Ix0015 = 1°64 502 
when the leaf is transpiring at the rate of 0°5 gramme per decimetre per hour, 
between the leaf-lainina and its surroundings will be 
under “still air” conditions.* 
Case B.—Thermal Relations of a Leaf to its Surroundings when ut is 
Receiving drrect Solar Radiation. 
The experimental data which are required for an investigation of the 
energetics of a leaf receiving direct solar radiation, in addition to those already 
made use of in Case A (see p. 70), are as follows: (1) The total amount of 
solar radiant energy incident on a given area of the leaf in a given time ; 
(2) the amount of this radiant energy which is absorbed by the leat 
(coefficient of absorption); (3) a measure of the internal work of the leaf due 
to (a) water-vaporization, and (0) photosynthesis ; and (4) the influence which 
air currents of definite velocity exercise on the thermal emissivity of the leaf 
surface. 
The total solar radiation falling on the leaf was measured by means of a 
Callendar’s radiometer which was connected with a Callendar’s self-recorder. 
The radiometer was calibrated in water-gramme-units and was placed beside 
the leaf under experiment with similar orientation. The integration of the 
thermal curve recorded on the drum was performed by an attached planimeter, 
the reading of which gave a measure of the average solar radiation falling on 
unit-area of the leaf in unit-time for the whole period during which the 
experiment lasted. 
The coefficient of absorption of the leaf for solar radiation was determined by 
means of the same instrument in the manner described in detail later on. It 
* The influence of movement in the air in increasing the thermal emissivity and in decreasing 
the temperature difference @—6,, will be considered later. 
