496 CORRELATION OF DISTRIBUTIONAL FEATURES. 



days preceding its commencement, and from the evaporation data for 

 the frostless season. The differentiation of the values of this ratio for 

 the vegetational areas is restricted by reason of the extremely high 

 ratios for the Northwestern Hygrophytic Evergreen Forest. The col- 

 lective amplitudes of the other vegetations cover less than half the 

 total range for the United States. The narrow amplitudes of this 

 climatic feature for all the vegetations except the Hygrophytic Forest 

 are an indication of the importance of the moisture ratio as an expression 

 of the conditions which are critical in determining the distribution of 

 the vegetation of the United States as we have charted it. 



The five vegetations in group A exhibit progressively higher limits 

 for their values of the moisture ratio. The evergreen forest areas of 

 group B are very dissimilar. The western and eastern sections of the 

 Northern Mesophytic Evergreen Forest stand well apart, the former 

 overlapping with the Northwestern Hygrophytic Evergreen Forest, 

 just as these vegetations merge into one another in their actual occur- 

 rence. The Southeastern Mesophytic Evergreen Forest possesses a 

 greater amplitude than the eastern section of the Northern Mesophytic 

 Evergreen Forest, but embraces almost the whole scale of values found 

 in the latter vegetation. The amplitude of the western section of the 

 Northern Evergreen Mesophytic Forest is covered by that of the 

 Semidesert, again emphasizing the relative aridity of this forest 

 region. The amplitude of the eastern section of the Northern Meso- 

 phytic Evergreen Forest is also covered by that of the Deciduous 

 Forest, which is accordant with the overlapping and intermixture of 

 these two forests. 



Normal mean relative humidity (plate 65, fig. 25). — The relative 

 humidity values for the Desert are so low that they have the effect of 

 restricting the differentiation of the values for the remainder of the 

 country, just as the Hygrophytic Forest does with respect to the 

 moisture ratios. The lowest humidity is to be expected in the Desert, 

 but the highest readings would be looked for in the Northwestern 

 Hygrophytic Evergreen Forest rather than in the western section of the 

 Northern Mesophytic Forest, as is the actual case for our data. This 

 is doubtless due to the unfortunate circumstance that there are no 

 humidity records available for localities in the most humid portions of 

 the coast or mountains of Washington and Oregon. The maximum 

 value is recorded for Eureka, California, which is situated in the red- 

 wood type of mesophytic forest. The Semidesert greatly exceeds the 

 Desert in the range of its humidities, as would be expected in the con- 

 trasting of arid coastal regions with an arid interior region. The blocks 

 showing the range and amplitude of humidity for the Grassland, Grass- 

 land Deciduous-Forest, and Deciduous Forest overlap in a manner 

 which is quite analogous to the occurrence of these vegetations. The 

 transition region possesses an amplitude of humidities which is nearly 



