208 The Plant World. 



of the installation of the instrument have been presented in 

 a paper on ' ' Evaporation and Plant Development, ' ' (Plant 

 World IJ: 269-276. 1907), and need not here be repeated. 

 At the end of the season the porous cups were returned to Tucson 

 and there standardized, the readings obtained by the observers 

 being afteiward corrected according to the coefficients thus ob- 

 tained. 



The results of this series of observations have been pub- 

 lished in the author's article upon "Evaporation and Centers 

 of Plant Distribution,'" {Plant World, 11: 106-112. 1908), on 

 page 110 of which will be found a table showing the 'Relative 

 evaporating powei of the air at 16 stations in the United States, 

 June 3 to Sept. 30, 1907," together with the names of the observ- 

 ers who so kindly co-operated, and thus made this work possible 

 by attending to the instruments. 



These results were unsatisfactory and inadequate to our 

 purpose, one reason for this being the smallness of the number 

 of stations, so that anything like an evajwration chart of the 

 country could not be attempted therefrom. An attempt was, 

 however, made (in the last article cited), to draw tentative 

 comparisons between the intensities of evaporation for the re- 

 gions occupied by three of the vegetation centers, and the corre- 

 sponding rain-evaporation ratios for the entire year as these 

 were derived by Transeau (1905). Since typographical errors 

 rendered the conclusions fiom this comparison obscure, they 

 may be repeated here. We may take, from Transeau 's map, 

 the annual rain-evaporation ratios: 1.25, 1.05, and 0.20 as 

 representing approximately the regions occupied by the north- 

 eastern conifer center, the deciduous forest center, and the south- 

 western desert center, respectively. Considering the first 

 ratio as unity, this series of ratios becomes 1.00, 0.84, 0.16. 

 From our own data we derive the generalization that the dif- 

 ferent evaporating powers of the air for the summer of 1907, 

 for these three centers should be about 1.00, 1.16, and 2.86, 

 respectively. The reciprocals of these numbers give us 1.00, 

 0.86, 0.35, to compare directly with 1.00, 0.84, 0.16, from Tran- 

 seau. The agreement between 0.86 and 0.84 is surprisingly 

 good. The discrepancy between 0.35 and 0.16 means little 



