
1905.] On the Physiological Processes of Green Leaves. 83 
The water transpired from the leaf under free air conditions amounted to 
1-259 grammes per square decimetre per hour, or 0:000209 gramme per square 
centimetre per minute. The amount of internal work of vaporization, W, is 
therefore 0:000209 x 592°6=0°1243 calorie per square centimetre per minute. 
The total amount of internal work W-+w performed by the leaf is there- 
fore equivalent to 0:1243+0-0017 = 0:1260 calorie per square centimetre of 
leaf-lamina per minute. 
In this instance Ra, the solar radiation absorbed by the leaf, exceeds 
W +, the sum of the internal work of the leaf, by 0°1762 —0°1260 = 0-0502, 
which represents the value of 7 in calories in the equation 
Ra = (W+w)+7. 
This value for 7 of 0:0502 calorie per square centimetre per minute repre- 
sents the only part of the solar radiation which can have had any heating effect 
on the leaf. From the value of 7 we can determine the mean temperature- 
difference between the exposed leaf and its surroundings 0,,—6, if we know 
the thermal emissivity of the leaf, for 0,—0 = 7/2e. 
The thermal emissivity of a leaf of this nature, for “still air” conditions, is 
approximately 0015 calorie per square centimetre of leaf swrface per minute for 
a temperature excess of 1°, and the emissivity increases by 000017 calorie 
per square centimetre per minute for an increased air speed of 1 metre per 
minute. Hence, since the averaye velocity of the wind in this case was 
25°7 kilometres per hour, or 428 metres per minute, the corrected “ emissivity ” 
becomes 0:0150+0:00017 x 428 = 0:0577 calorie per square centimetre of 
surface per minute for a temperature excess of 1°. Hence the temperature 
excess of the leaf-lamina above its surroundings will be 
Since the average temperature of the air during the experiment was 
16°9 C., that of the leaf was about 17°3 C. 
_ We are now in a position to state with a fair approach to accuracy the 
manner in which the leaf has disposed of the energy incident upon it, and to 
obtain some idea of the “economic coefficient” of the leaf under these average 
conditions. If we denote R, the total energy incident on unit-area of the 
leaf in unit-time, by 100, then the disposal of this radiation will be accounted 
for in the following manner :— 
