CORRELATION OF DISTRIBUTIONAL FEATURES. 517 



particularly wide amplitudes and a pronounced tendency to range 

 through about the same values, except in the case of the length of 

 frostless season and the normal daily mean of the coldest 14 days of the 

 year, in both of which conditions the Semidesert departs from the 

 conditions of the other vegetations of the group. A comparison of the 

 extremes of the temperature conditions for the vegetations of group 

 B (the evergreen forests) shows a much greater dissimilarity, indicating 

 that temperature conditions play a more important role in differen- 

 tiating our evergreen-forest areas than they do in differentiating the 

 other vegetations of the country. This applies to the moisture-tem- 

 perature index, as well as to the purely temperature conditions. 



It is in the vegetations of group A that the moisture conditions show 

 their greatest differentiation and appear to exert their strongest con- 

 trol. This is shown with diagrammatical clearness by the smoothed 

 data for the mean total precipitation of the year (fig. 24), and it is also 

 shown by the moisture ratio and by relative humidity (fig. 25). There 

 is a much less marked differentiation of moisture conditions among 

 the evergreen-forest areas, as will be seen in the close agreement of the 

 extremes of daily mean evaporation for the Southeastern Mesophytic 

 Evergreen Forest and for the eastern section of the Northern Meso- 

 phytic Evergreen Forest (fig. 24), and also in the similarity of humidity 

 conditions for the above forests and for the Northwestern Hygrophytic 

 Evergreen Forest (fig. 25). 



The preceding pages have brought out the fact that the moisture 

 ratio tt/E is the most important single expression of climatic 

 conditions with respect to the vegetation as a whole. The nar- 

 row amplitude of this condition in all of the vegetations except the 

 Northwestern Hygrophytic Evergreen Forest, and the distinctness of 

 its extremes for all of the vegetations give an indication of its impor- 

 tance which is well borne out by a comparison of the climatic and 

 vegetational lines of plate 2 and plate 59. The isoclimatic line for the 

 ratio value 0.110 closely follows the limit of the Southeastern Meso- 

 phytic Evergreen Forest from Alabama to New Jersey, and then 

 swings westward in such a manner as to approximate closely the 

 southern limit of the eastern section of the Northern Mesophytic 

 Evergreen Forest, failing to dip with the vegetation along the Alle- 

 ghenies (where there are no data for this climatic map), but closely 

 following the vegetational line to Minnesota. The similarity of the 

 conditions expressed by the moisture ratio for the evergreen forests of 

 the Southeastern and Northeastern States is indicated in figure 25, 

 but no indication is there given of the closeness with which the inner 

 limits of these vegetations follow a single isochmatic line. The line for 

 the value 0.80 is an equally close approximation of the inner limit of 

 the Northwestern Hygrophytic Evergreen Forest. The line separating 

 the Grassland and the Grassland Deciduous-Forest Transition is 

 closely followed by the isoclimatic line of 0.60 in the South and by that 



