498 CORRELATION OF DISTRIBUTIONAL FEATURES. 



Normal moisture-temperature index for the growing-season (plate 72, fig. 

 26). — This composite climatic feature is remarkable in the fact that 

 it possesses a steep gradient of change on passing from the central to 

 the southeastern part of the United States, and possesses a relative 

 uniformity over the western third of the country. The highest values 

 are consequently to be found in the Southeastern Mesophytic Ever- 

 green Forest and the second highest in the Deciduous Forest, both of 

 which areas show wide amplitudes of this condition. The lowest value 

 is found in the Desert, which is closely approached by the Grassland 

 and the western section of the Northern Mesophytic Evergreen Forest. 

 The last-named vegetation, by reason of its western position, possesses 

 a very narrow amplitude of the conditions expressed by this index. 

 The distribution of the various values of the moisture-temperature 

 index is such as to give closely similar limits and amplitudes to such 

 dissimilar vegetations as Desert, Grassland, Northwestern Hygro- 

 phytic Evergreen Forest, and the eastern section of the Northern 

 Mesophytic Evergreen Forest, with the Deciduous Forest overlapping 

 into this range of conditions. 



III. CONDITIONS THAT PROBABLY DETERMINE THE GENERAL 



VEGETATIONAL AREAS. 



1. OBSERVATIONS FROM THE CHARTS. 



The foregoing review of the comparative ranges and intensities of 12 

 of the leading climatic conditions for the 9 vegetational areas of the 

 United States has served to throw some light on the question as to 

 which of these conditions are most important in controlling the limits 

 of the vegetations. The wide amplitude of all of the temperature 

 conditions ha':; indicated that in the case of many of the vegetations, 

 and particularly those with a wide north-and-south extent, there may 

 be found parallel series of temperature conditions in two cr more 

 vegetations. The moisture-temperature index also fails to exhibit 

 differences between the several vegetational areas such as to give it 

 importance as representing a controlling factor. The precipitation 

 and evaporation data fall much more nearly into groups of intensities 

 and amplitudes which show dissimilarity throughout the series of 

 vegetational areas. The moisture ratio and the relative humidity also 

 show striking differences between the various vegetations. 



Figures 27 to 35 give, in diagrammatic form, the limits and ampli- 

 tudes of 17 selected climatic factors for each of the 9 generalized vege- 

 tational areas. These diagrams make it possible to view the correla- 

 tion of climate and vegetation from a different angle to that employed 

 in the immediately preceding pages. Here it is possible to see a 

 diagrammatic picture of the climate of each of the vegetations, to note 

 whether each of the conditions ranges through a series of values which 



