70 _ Dr. H. T. Brown and Mr. F. Escombe. [Jan. 9, 
consider those physical and chemical changes occurring within the leaf which 
are attended by any sensible evolution or absorption of energy. 
Of these there are only three which need be taken into serious account ; 
(1) the vaporization of water; (2) the photosynthesis of carbohydrates; and 
(3) the chemical changes attendant on respiration. 
The first two changes are endothermic in character, but are as a rule of very 
dissimilar value as regards the actual thermal disturbances induced, (1) under 
ordinary circumstances being very large compared with (2). 
The respiratory changes (3) are exothernve in their final result. 
By suitable means the loss or gain of heat due to each of the above- 
mentioned changes can be determined in water-gramme units, and referred to 
the total amount of energy received by the leaf from its surroundings, either 
through radiation or the convective and conductive action of the air. In 
order, however, to complete our knowledge of the thermal relations of leaf 
and environment, we must know something of the absorptive power of the 
leaf-lamina for the particular form of radiant energy incident upon it, and 
also its thermal emissivity, using that term in its wide sense to include the 
loss or gain of heat due to radiation and air-convection and conduction which 
unit-area of the leaf will experience in unit-time, with unit difference of 
temperature between itself and its surroundings. In some cases we also 
require to know the weight of unit-area of the lamina and its approximate 
specific heat. Our investigations, described in detail later on, have led 
us to fairly accurate methods for the determination of all these factors. 
For some time the story could not be made complete, owing to difficulties 
in determining the thermal emissivity of a leaf. These difficulties have now 
been surmounted by the adoption of a comparatively simple method based on 
the differential self-cooling of leaves which are transpiring at unequal rates. 
The method is fully described in a separate communication. * 
As a preliminary to the study of the thermal interchanges between a leaf 
and its surroundings, it will be convenient to consider two distinct sets 
of general conditions ; (A) when the leaf is shaded from direct solar radiation 
or any other form of radiation which can produce photosynthesis ; and (B) 
when the leaf is receiving solar radiation. 
Case A.—The Thermal Relations of a Leaf to its Surroundings when it is 
shielded from Solar Radiation. 
In order as far as possible to simplify the problem, we will assume in this 
case that a detached leaf, freely supplied with water, is placed in an enclosure, 
* See Brown and Wilson, injra, p. 122. 
