19 2o] FREEMAN— EVAPORATION AND TRANSPIRATION 207 



were very thick, green, and luxuriant, being markedly different 

 from those growing on the dry mesa a few hundred yards distant. 

 A single experiment comparing the orchard greasewood with that 

 of the mesa gave a standard dewpoint 2 higher for the former. 

 This indicates that its leaves had undergone a modification which 

 favored a greater transpiration rate. 



As an example of the use of this method in the study of the 

 comparative water loss from different varieties of a single species, 

 a set of eight experiments extending over a period of about 

 two weeks may be given. These were upon two pure lines of 

 alfalfa nos. 91 and 17. The results are collected in table VI, 

 which has the same arrangment of material as table V. The 

 dewpoint of the escaping air was greater for the cylinder cov- 

 ering branches of race no. 91 three times and less four times, 

 but when calculated to the same standard dewpoint as used in 

 table V, it was greater from race no. 91 for six out of the seven 

 experiments made. These results, which would appear hopelessly 

 confusing at first, when calculated to the standard dewpoint, seem 

 to indicate with a fair degree of definiteness that the leaves of race 

 no. 91 offered less hindrance to evaporation than did those of 

 race no. 17. Here again it was the thicker leaves which gave the 

 greater transpiration rate, since the average weight per 100 sq. cm. 

 of race no. 91 was 2 .57 gm., whereas that of race no. 17 was only 

 2 .29 gm. 



In comparing tables V and VI it will be noted that the differ- 

 ences in the standard dewpoints of the same species or variety for 

 different days was often as great as or greater than the differences 

 between the species or varieties being compared on the same day. 

 In spite of this, however, the differences between the species or 

 varieties were so constant that one cannot but believe they were 

 real and significant. The differences in the standard dewpoints 

 for different days must be sought therefore in some common factor 



which has influenced both types alike. 



Such a factor, as shown in table IV, is the dewpoint of the 

 external air. In examining tables V and VI it will be noted that 

 whereas high standard dewpoints are perhaps more frequently 

 than otherwise accompanied by high external dewpoints, this is 



