CORRELATION OF DISTRIBUTIONAL FEATURES. 493 



Normal daily mean temperature of coldest 14 days of the year (plate 43, 

 fig. 23). — The daily mean temperature of the coldest period of the 

 year is a climatic factor which varies greatly in the various vegeta- 

 tional areas. The lowest value, 0°, is found in the northernmost part 

 of the Grassland area, and minimum values of 5° are found in the Grass- 

 land Deciduous-Forest Transition and in the eastern section of the 

 Northern Mesophytic Evergreen Forest. The highest mean tempera- 

 ture of this coldest period, 69°, is found in the Southeastern Mesophytic 

 Evergreen Forest, and the nearest maximum values approaching it in 

 other vegetations are found in the Desert and Semidesert, where 

 maxima of 54° occur. 



There is apparently no temperature factor which exhibits greater 

 diversity in its amplitude and extremes in the several vegetational 

 areas than does the mean temperature of the coldest fortnight. The 

 normal da,ily mean of the hottest six weeks, for which we have used the 

 data elaborated by Merriam, fails to show such a diversity for the 

 vegetational areas (see tables 32 to 40). 



Normal daily mean precipitation offrostless season (plate 46, fig. 23). — 

 This climatic index exhibits a well-graduated series of differences in the 

 9 vegetational areas, from its lowest values for the Desert (0.009 inch) 

 and the Semidesert (0.017 inch) to its highest value for the North- 

 western Hygrophytic Evergreen Forest (0.199 inch). Here again the 

 Evergreen Forest areas of group B exhibit greater differences than 

 do the areas of group A. Pronounced similarities exist between the 

 Desert and Semidesert, and between the Grassland Deciduous-Forest 

 Transition and the Deciduous Forest. Similarities which are less 



TCMrCHATUKC. NONM*L D«ILT McikN. COIBCST 14 OJkr* OF VCDK 



Vegetation <| 



OeSERT 



Semidesert 

 Grassland 



Grassland-Deciduous-ForestTransition 

 Deciduous Forest 



Nortxwestern HYGROPHmc Evergreen Forest 

 Southeastern MESOPHrric Evergreen Forest 



'NORTHERN MESOPHYTIC EVERGREEN FOREST (WEST) 

 NORTHERN MESOPHYTIC EVERGREEN FOREST (EASTI 



69 



'rccipitation. Normal Dailv Mean. F. S. 



VEGETATION .009 lif 



Desert 



semi-desert 



Grassland 



Grassland-Oeciou<ius-ForestTrans<tion 



Deciduous Forest 



Northwestern Hygrophytic Evergreen Forest 

 Southeastern mesophytic Evergreen Forest 

 Northern mesophytic Evergreen Forest West) 

 Nortmern mesophytic Evergreen Forest <East) 



Fig. 23. 



