CORRELATION OF DISTRIBUTIONAL FEATURES. 497 



the same as the overlapping of the Grassland and Deciduous Forest. 

 The total amplitude of humidity conditions in the western section of 

 the Northern Mesophytic Evergreen Forest is half that of the entire 

 United States. The other three evergreen-forest areas have ampli- 

 tudes and limits which are extremely similar, and fall within the 

 range for the western section of the northern forest. The range of 

 humidity conditions which appears to favor the evergreen type of 

 forest also falls within the range for the Deciduous Forest. This 

 indicates that the forest regions of the United States as a whole are to 

 be found under very similar humidity conditions (about 70 to 80 per 

 cent) for the frostless season, and that the diversified types of forest 

 comprised in the western section of the Northern Mesophytic Ever- 

 green Forest extend into regions of both lower and higher humidity, 

 overlapping slightly with the highest values of the Desert Region. 



Normal daily duration of sunshine (plate 69, fig. 26). — The sunshine 

 conditions of the frostless season are imperfectly known for the Desert 

 region, where they might be expected to possess their highest values. 

 Much lower minimum values are found in the Desert than in the 

 Semidesert, and in the latter region are the highest known values. 

 The sunshine conditions are very similar throughout the Grassland, 

 Grassland-Deciduous Forest, and Deciduous Forest regions, reaching 

 the lowest value for the United States in the first-named of these 

 regions. The sunshine conditions are very dissimilar in the four ever- 

 green forest areas, reaching a wide amplitude in the Southeastern 

 Mesophytic and western section of the Northern Mesophytic Ever- 

 green Forests. 



SuNSHiNC. Normal Daily Duration, r. S. 



VtGETATION 



1127 



DiSERT (Z 



SEMI-DESERT C 



Grassland I 



Grassland-Deciduous-ForestTransition C 



Deciduous Forest F 



Northwestern HYGROPHVTiC Evergreen Forest C 



Southeastern Mesophytic Evergreen Forest C 



Northern mesophytic Evergreen Forest (Westi [| 

 Northern Mesophytic Evergreen Forest iEasti 



MoisTunE-TeMPCRATufic Index. Normal P/E x T, F. S. 



Vegetation 197 



Desert WM 



semi-desert i i 



Grassland 



Grassland- DECiOoous-FOftESTTRANStnoN 

 Deciduous Forest 



Northwestern Hyghophttic Evergreen Forest 

 Southeastern Mesophytic Evergreen Forest 

 Northern mesophytic Evergreen Forest cWest. 

 Northern Mesophytic Evergreen Forest 'Easti 



Fig. 26. 



