88 Dr. H. T. Brown and Mr. F. Escombe. [Jan. 9, 
Where transpiration is at a minimum, especially in the extreme case of 
xerophytic plants, the dissipation of the absorbed radiant energy by thermal 
emission becomes all important, and we are now able for the first time to 
apply certain numerical values to the dissipation of energy from this cause, 
which are of some interest. | 
Since the emissivity of a leaf surface approximates to 0-015 calorie per square 
centimetre of surface for a temperature excess of 1°, a rise in temperature of 
the leaf of only 10° will, even under s¢z// air conditions, give a dissipation of 
energy equivalent to 10x2x0015=0°3 calorie per square centimetre per 
minute, and in a gentle breeze of only 10 kilometres per hour, this emission 
will be increased to 0'864 calorie per square centimetre per minute. Hence 
in the thermal emissivity of a plant we have a vary potent factor for keeping 
down the temperature, even in the absence of transpiration. 
Hitherto we have been regarding only those cases in which the incidence of 
solar radiation is normal to the leaf surface, conditions which seldom occur in 
nature, especially when radiation is powerful, as in the tropics or at high 
altitudes, where the self-adjusting mechanism of the plant, and its habit, are 
such as seldom to allow the leaf to be placed in a position favourable for 
receiving the full solar radiation.” 
Since the intensity of the radiation received on unit-area varies with the sine 
of the angle of inerdence, a radiation equal to 1 calorie per square centimetre 
per minute will be reduced to 0°707 calorie at an obliquity of 45°, and 
to 05 calorie, or one-half of its normal intensity, at an obliquity of 30°.F 
Section (2).—Lxperimental. 
(a) Determination of R, the total incident Radiation. 
The total solar radiant energy incident on a leaf in a given time is very 
conveniently measured by the Callendar’s radiometer and recorder already. 
referred to in Part I (see p. 39). This instrument is particularly well adapted 
to experiments of this kind, since it receives and records the sky-radiation as 
well as that of direct sunlight, and the area covered by the receiving coils of 
the differential thermometer is comparable in magnitude with the areas of the 
* On this point cf. A. J. Ewart on the “ Effects of Tropical Insolation” (‘Annals of Bot.,’ 
vol. 11, p. 439), who states from observation that “no tropical plant places or allows its leaves to be 
in such a position that the upper surfaces are at right angles to the sun’s incident rays when at the 
zenith.” 
+ With increasing obliquity there will also be less penetration of the radiations owing to 
increased reflection. In the case of light, whilst the reflection from the surface of glass is about 
2°5 per cent. for normal incidence, it is 34 per cent. for an obliquity of 50°, and as much as 
29°9 per cent. at an obliquity of 15°. 
