1905.| Onthe T hermel Enussivity of a Green Leaf. 135 

be evident that this mode of dissipating the excess of radiant energy falling 
on the leaf must be of great importance, especially in those cases where the 
transpiratory work performed is small. Let us consider a somewhat extreme 
case in which the leaf is receiving solar radiation amounting to 1 calorie per 
square centimetre per minute, and let us assume that the absorption coefficient 
of the leaf for this radiation is 0°75, and that the leaf possesses a thermal 
emissivity equal to that of the Leriodendron, wu¢., 00119 calorie per square 
centimetre per minute for a temperature excessof 1° C. The total emissivity 
of the two surfaces of the leaf will, of course, be double this amount, 7c... 
0:0238 calorie per square centimetre per minute for a 1° excess. 
If we assume transpiration to be entirely in abeyance, the temperature to: 
which the leaf will be raised above the surrounding air when the emission exactly 
balances the absorbed radiation will be represented by ae == oh On 
an excess which would speedily prove fatal to the leaf, even supposing the 
surrounding air to have a temperature as low as 20° C. If, however, we: 
suppose the air to be in gentle movement at the rate of about 8°5 kilometres. 
per hour (141 metres per minute), the emissivity of the leaf, counting both 
sides, becomes 0°0361 x 2=0:0722 calorie per square centimetre per minute: 
for 1° of excess, and the leaf therefore cannot rise in temperature above the. 
Ay al= 
; 0°75 : penal 
surrounding air more than 00729 =10°5 C., even when there is no dissipation: 
a 
of the absorbed energy by transpiration. 
The importance of facts such as these in connection with the life-history 
of the xerophytic plants is considerable, and we are in a better position 
to give quantitative expression to these and similar problems connected with. 
the energetics of the plant, now we have the means of determining the: 
thermal emissivity of plant surfaces and the wastage of energy due to this. 
cause. 
We have shown above how it is possible to determine the thermal! 
emissivity of a leaf both for “still air” conditions and for any given 
velocity of an air-current, provided we know the weight of water transpired 
per unit-area and unit-time, and also the temperature difference involved.. 
Since, however, these three values representing thermal emissivity, water 
transpired, and temperature difference, are interdependent, it follows that when 
any two are known the third is calculable. Their relations may be conveniently 
generalized as follows :— 
Let Q represent the amount in grammes of the water transpired per square: 
centimetre of leaf-lamina per minute ; 

