312 BOTANICAL GAZETTE [APRIL 
evaporation for the day may be due largely to an increase in the 
rate of water loss itself, as this may be brought about by a rise in 
the air temperature and especially by absorption of solar energy. 
This supposition has the appearance of a paradox, since the 
acceleration of a process is postulated as causing a retardation of 
the same process, but such phenomena are not infrequent where 
disturbances in the equilibrium of a system are dealt with, and 
_the suggestion seems worthy of careful theoretical and experimental 
‘consideration. 
- On purely a-priori grounds it is readily understood from the 
principles of thermodynamics, that a rise in the temperature of the 
air which bathes the leaves (such as normally progresses through- 
out the forenoon and well into the afternoon) should produce an 
ever-increasing vapor pressure of water within the internal atmos- 
phere of the foliar tissues and over the cuticular surfaces. Such 
an increase in vapor pressure within would usually be adjusted 
pari passu with the rise in temperature, by the ejection of an 
increasing amount of water vapor through the stomatal openings, 
and if the temperature were to remain constant for a time the rate 
of water loss during such a period would be somewhat greater than 
at the lower temperature, the slightly greater rate being due to the 
more rapid vaporization and diffusion at higher temperatures. 
But the rise in temperature of air and of leaf are not our main 
consideration with reference to openly exposed green foliage upon 
. asunny day. Air and leaf.do continuously increase in temperature 
from early morning till some time in the afternoon, but in the 
meantime the radiant energy of the sunshine is ever being absorbed 
to a greater or less extent by the green leaf tissue, so that this 
tissue should tend to become warmer than the air. By far the 
larger portion of this absorbed energy (a negligible portion dis- 
appears through photosynthetic changes) must operate to increase 
the vapor tension of the water films which surround the foliar air 
spaces or are held in the epidermal walls. Evaporation from these 
films must thus be increased, the latter remaining nearly constant 
in temperature. The accelerated evaporation within the leaf 
should produce in its turn an increase in the partial gas pressure 
due to water vapor in the internal atmosphere, and hence, the 
