72 Dr. H. T. Brown and Mr. F. Escombe. [Jan. 9, 
with the temperature and with the age and species of the plant. At about 
20° C., we have found healthy mature leaves of Helianthus annuus evolve 
about 0°70 c.c. of carbon dioxide per square decimetre of leaf-lamina per 
hour, or 0°000116 cc. per square centimetre per minute. This corresponds to 
1°55 x 10-* gramme of dextrose, or to an evolution of heat of 1:55 x 107*x 
3760=0:000582 calorie per square centimetre of leaf-lamina per minute. 
Since the weight of one square centimetre of the leaf-lamina of Helianthus 
is about 0:°020 gramme, and its specific heat is about 0°879 (water=1-0) the. 
above amount of heat of respiration would be sufficient to raise the temperature 
of the leaf at the rate of 0°:033 C. per minute, provided no simultaneous loss of 
heat were taking place by radiation, ar-convection, or internal work of vaporiza- . 
t20n. 
But all these sources of loss of heat become operative directly the leaf 
temperature rises above 0, that of its surroundings, and the leaf can only 
come into thermal equilibrium with its surroundings at some temperature 6, 
higher than 6, at which point the heat of respiration produced in unit-time 
just balances the sum of the thermal losses due to radiation, convection, and 
water vaporization. 
Before we can estimate the actual rise of temperature of the leaf under the 
conditions postulated, we must be in a position to determine the rate of loss 
of energy due to each one of these causes. We will assume first of all that 
transpiration is still in abeyance, and that the loss of the evolved energy of 
respiration is due merely to radiation, air-convection, and air-conduction. 
From the results given in another paper* we know that the thermal 
emissivity of the leaf per square centimetre of leaf-surface for “still air,” con- 
ditions and for a temperature-excess in the leaf over its surroundings of 1°C., 
is about 0°015 calorie per square centimetre per minute. This “rate of cool- 
ing” has, of course, to be doubled for the two sides of the leaf, so that the 
emissivity of a square centimetre of leaf-lamina under the above conditions 
amounts to 0°030 calorie per minute per 1° C. excess. 
Since the heat of respiration has been shown to be 0:000582 calorie per 
minute per square centimetre, the temperature which the leaf will attain 
when it is in thermal equilibrium with its surroundings will be given by 
0:000582 
~~ 0:030 _ — 0° 0 019 C, 
This is the maximal excess temperature above its surroundings to which 
the leaf can be raised under “ still air” conditions when it is respiring 0°7 c.c. 
of carbon dioxide per square decimetre per hour, provided transpiration still 
dividing this number by the emissivity-factor, 7. 
* Brown and Wilson, infra, p. 122. 
