A METHOD FOR THE QUANTITATIVE DETERMINA- 
TION OF TRANSPIRATION IN PLANTS 
Gero. F. FREEMAN 
(WITH ONE FIGURE) 
In plant-breeding work, which has engaged my attention for several 
years, a pressing need has been felt for some means of measuring the 
drought-resisting quality of individual plants. Morphological char- 
acters, such as small leaves, small and few stomata, pubescence, and 
thick epidermis, are characters which may be assumed to be correlated. 
with drought resistance in that they tend to reduce transpiration 
and are characteristic of xerophytic plants in general. Moreover, it 
may be assumed that, other characters of two plants being equal, the 
one having the lower rate of transpiration per unit of leaf surface 
would be more suited to the drier portions of the plains region or to 
withstanding long periods of drought in the more humid districts. 
Some direct method of measuring the transpiration of plants growing 
in the field would be of great value therefore to the breeder in 
selecting for the quality of drought resistance. 
At the beginning of our work in alfalfa breeding at the Kansas Ex- 
periment Station, no method was known to me which seemed to meet 
all of the requirements for securing data concerning transpiration for 
plant-breeding work. It was desired to use the plants selected as 
mother plants. Therefore they could not be taken up and planted, 
and the rate of loss of moisture determined by weight. Furthermore, 
on account of the very long tap roots of alfalfa, it would be impracti- 
cable to use a pot large enough to accommodate them without such 
pruning as would endanger the life of the plant, or at least make 
them little other than cut stems. 
Although it is known that the cut stems of the plant when placed in 
water do not transpire normally, an attempt was first made to see if 
the differences sought could be revealed by this method. As a 
preliminary study and in order to ascertain the effectiveness of the 
potometer method in determining these individual differences in the 
Botanical Gazette, vol. 46] {118 
