22 



BURTON EDWARD LIVINGSTON 



lower than that of the upper (0.026). When water loss is most 

 retarded in both cases, the transpiring power of the upper foUar 

 surface of Datura is about 1.9 times as great as that of the lower. 

 This point may be emphasized in another way by noting that, 

 from 22 hours of the first day to 3 hours of the second, the tran- 

 spiring power of the upper leaf surface exceeds that of the lower 

 surface in actual magnitude, while the converse holds for the 

 daylight hours. The diurnal march of the indices will be con- 

 sidered below. 



In table 5 are given the ratios of relative transpiration, derived 

 from the absolute losses of the two plants of Datura and from those 

 of the two atmometers. The average ratios with reference to 

 each instrument are also given. All have been reduced to a 

 basis of the datum of hour 8 as unity, this datum representing 

 the lowest value in each series. Since there was considerable 

 variation in the actual time periods of this experiment, all losses 

 were first computed to hourly rates for hourly periods, and it is 

 from these corrected rates that the data of the table have been 

 derived. 



The ranges of variation of the ratios of relative transpiration, 

 as given in table 5, are to be compared with the range of variation 

 of the indices of transpiring power derived from the hygrometric 

 tests. This comparison may best be accomplished by the follow- 

 ing summary: 



Range of ratio of relative transpiration for Datura 



By white atmometer 



Plant 1 

 Plant 2, 



Average, 



1 to 11.5 

 1 to 18.7 

 1 to 14.1 



By brown atmometer 



Plant 1 

 Plant 2. 



Average 



1 to 9.2 

 1 to 15.8 

 1 to 12.5 



Range of index of transpiring power 



By hygrometric test 



1 to 17,6 



