CORRELATION OF DISTRIBUTIONAL FEATURES. 495 



former is based on the precipitation data of the frostless season and the 

 latter on those of the entire year. 



Evaporation (plate 53, fig. 24). — The daily mean evaporation for 

 the frostless season reaches its highest values in the Desert region, 

 although some of the stations situated in this vegetation have readings 

 so low as to give the Desert an amplitude of evaporation conditions 

 which is half that for the entire country. The Semidesert and Grass- 

 land have limits and amplitudes which are closely similar. The 

 Grassland Deciduous-Forest Transition exhibits a minimum which is 

 the same as that of the Grassland, but it has a much smaller amplitude 

 of evaporation conditions, nowhere reaching values as high as the 

 lowest ones for the Desert. The Deciduous Forest has a much greater 

 amphtude than the transition region between it and the Grassland, and 

 its highest values are as great as the minimum values for the Desert, 

 corresponding to the southward prolongation of the Deciduous Forest 

 into Texas. The lowest evaporation values are found in the North- 

 western Hygrophytic Evergreen Forest, and relatively low minimum 

 values are found in the Southwestern Mesophytic Evergreen Forest 

 and in the eastern section of the Northern Mesophytic Evergreen 

 Forest. The maximum values of the three areas just mentioned are 

 remarkably similar. The Umits and amplitude of evaporation condi- 

 tions for the western section of the Northern Mesophytic Forest are 

 higher than for any of the other evergreen forest areas, and closely 

 similar to the Semidesert and Grassland values. 



Moisture ratio (plate 59, fig. 25.) — The data here considered are those 

 derived from the precipitation index for the frostless season and the 30 



Moisture Ratio. Norm«i. rr/E, F. S. 



VEGETATION £6 4.48 



DESERT ^ I 



Semidesert 

 Grassland 



Grasslano-Deciouous-ForestTiunsition 

 Deciduous Forest 



Northwestern hvgropmytic Evfsgreen Forest 

 Southeastern Mesophytic Evergreen Forest 

 Northern Mesophytic Evergresn Forest 'Westi 

 Northern Mesophytic Evergreen Forest iEast) 



Humidity. Normal Mean, F. S. 



Vegetation 03.6 



Desert 



Semidesert 



Grassland 



Grassland-Oeciduous-ForestTransition 



Deciduous Forest 



Northwestern HYGROPMrTic Evergreen Forest 

 Southeastern mesophytic Evergreen Forest 

 Northern Mesophytic Evergreen Forest iwest' 

 Northern Mesophttic Evergreen Forest ieasti 



Fig. 25. 



