494 CORRELATION OF DISTRIBUTIONAL FEATURES. 



marked but more surprising exist between the Semidesert and the 

 western section of the Northern Mesophytic Evergreen Forest, and 

 between the Grassland and the eastern section of the Northern Meso- 

 phytic Evergreen Forest. These incongruities probably find their 

 explanation in the differences in the seasonal distribution of rainfall 

 and in the compensating influences of other conditions. The western 

 section of the Northern Mesophytic Evergreen Forest (which is much 

 drier than the eastern section) does not include markedly higher 

 values than does the Desert, a fact which must be considered in con- 

 nection with the temperature and evaporation differences between 

 these two vegetations. 



Mean total annual precipitation (plate 52, fig. 24). — The graph repre- 

 senting these data is given a conventionalized appearance from the 

 fact that we have used the map prepared by Gannett and have not 

 drawn the extremes from the readings of individual stations, as in the 

 case of most of the other climatic conditions. This graph bears a 

 generic resemblance to the one showing the normal daily mean pre- 

 cipitation for the frostless season. Owing to the smoothed nature of 

 the data on which it is based, it shows an even more pronounced gra- 

 dation between the several vegetational areas. No new features are 

 brought out in this figure, as compared with the one just discussed, 

 and indeed some of the indications of the latter are partially con- 

 tradicted by this one. Owing to the fact that the data from plate 52 

 are not the readings of individual stations, it is impossible to tell in 

 how far the differences between this figure and the preceding are due 

 to this circumstance and in how far they are due to the fact that the 



vegetation 



SemiDesert 

 Grassland 



Grassland-Deciduous-ForestTransition 

 Deciduous Forest 



Northwestern Hygrophttic Evergreen Forest 

 Southeastern Mesophytic Evergreen Forest 

 Northern Mesophytic Evergreen Forest iwesti 

 Northern Mesophytic Evergreen Forest (East) 



PRtCIPITATIOK. Mt»N TOTHL. YCAR 

 90 



Evaporation. Dailv Mean. 1887-B, F. S. 



Vegetation ■.052 



Desert 1 IJBBI^H 



Semi-Desert I ^^JWBBBWWB^^^BBBBHI 



Grassland I ZMBMBII^WWHBHBM 



grasslano-deciduous-foresttransinon 1 ^ 



Deciduous Forest [^ZZZIHHM^HHH^^^^HHIBHZZZ 



Northwestern HYCROPHrric Evergreen Forest WB^ M^Wi^^^W 



Southeastern Mesophytic Evergreen Forest I ~MBH^ 



NORTHERN Mesophytic Evergreen Forest 'Westi I ^M— BM— 



NOSTHER'I ME'^orHYIIC EVERGREEN FOREST 'EAST) I ■■B^^l^— ~~ 



Fig. 24. 



