VOLUME LII NUMBER 6 
guises 
BOTANICAL CAZETTE 
DECEMBER 1011 
LIGHT INTENSITY AND TRANSPIRATION*™ 
BurRTON EDWARD LIVINGSTON 
(WITH ONE FIGURE) 
Introduction 
While it is well known that light intensity plays an important 
role in the determination of water loss from plants, our knowledge 
of this matter is purely qualitative. The present paper deals 
with an attempt to find some simple means of physically determin- 
ing the intensity of solar radiation with reference to its effect on 
plant transpiration. While the results to be here brought forward 
do not possess that degree of completeness that we are wont to 
expect in the fields of physics and chemistry, yet they emphasize 
the quantitative aspect in the study of the external factors which 
influence plants in the open, and they seem to place in the hands 
of ecologists of a quantitative turn of mind a somewhat ready means 
of approximating the physical magnitude of one of the most im- 
portant, and at the same time most baffling, of the environmental 
conditions with which they have to deal. 
The total amount of transpirational water loss from a plant, 
for any given period, may be considered as a summation of the 
effects of the evaporating power of the air and of the radiant energy 
absorbed throughout the period, modified by certain secondary 
‘Botanical Contribution from the Johns Hopkins University, No. 21. 
* The pressing need for methods of evaluating the various external factors which 
affect plants has been emphasized in a paper read before the Botanical Society of 
merica in 1908. See Plant World 12:41-46. 1909; and Amer. Nat. 43: 369-378. 
1909. 
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