220 The Plant World. 



(5) Westward from a line drawn from Winnipeg, Man., 

 to Corpus Christi, Tex., the rate rapidly rises. The highest 

 evaporation intensity is exhibited by western Texas and New 

 Mexico, and this area is extended northwestward to western 

 Washington, and westward to include the desert of southern 

 California. 



(6) vSan Diego appears to have about the same rate as 

 New York state. 



From the information here brought forwaid, the evapora- 

 tion rates for a single summer, any attempt to relate plant 

 distribution to this factor in any but a verv superficial war 

 would Vje imfounded, but it is aj^parent that the northwestern 

 and northeastern conifer vegetational centers are characterized 

 by average weekly rates of less than 150 cc. from our standard 

 instrument, — i erhaps about 100 cc. would be a truer limit for 

 the northeastern center. The well-known southern prolongation 

 of the northeastern center in the eastern mountains is clearlv 

 paralleled on the evaporation chart. The deciduous forest of 

 the middle east occupies a region with over 100 cc, often over 

 150 and even 200 cc, as the mean weekly summer rate. The 

 southeastern conifer center occupies the southern part of the 

 evaporation area which is characterized bv rates of from 100 cc. 

 to 200 cc. Summer evaporation alone can not explain its exist- 

 ence as a distinct vegetational type. The geneially cultivated 

 area of Oregon and California appears to have summer rates of 

 150 cc. to 200 cc. ]:ier week, and the great arid region is clearly 

 indicated by our atmometric data. The piairie region appears 

 to be climatically a potential deciduous forest, as is suggested bv 

 the success which has attended tree planting in Iowa, eastern 

 Nebraska, eastern Kansas, etc., as well as by other considera- 

 tions. The grassland represents the transition from the evapor- 

 ation conditions of the deciduous forest center to those of the 

 arid region. 



It will be interesting to apply the method used in 1907 to 

 the evaporation rates as exhibited by the northeastern conifer, 

 the deciduous and the desert regions for the summer of 1908. 

 Inspection of the map leads us to set the average summer (1908) 

 eva])oration rate for the northeastern conifer area at about 100, 

 that for the deciduous forest at about 1 75, and that for the desert 



