572 



CORRELATION OF DISTRIBUTIONAL FEATURES. 



Pdrietaria pennsylvanica, and, to a less extent, Arenaria lateriflora 

 (plate 30), generally agree in tbeir distributional area with the area 

 of the very cool and cool temperature provinces, based on the length of 

 the average frostless-season. 



On plate 31, Daucus pusillus is shown to occupy an area that corre- 

 sponds, in a very satisfactory manner, with an area composed of the 

 very warm and warm climatic provinces, taken with the warmer half 

 of the medium province, as shown on the frostless-season chart. 



Other correspondences are suggested by our charts, but thesr are 

 the most satisfactory. The generalization is at once suggested that the 

 length of the period of the average frostless season (plate 34) exhibits 

 much more striking correlations to vegetational areas than does the 

 chart based on physiological summations (plate 40). The results of 

 these comparisons are shown graphically by the following scheme : 



Temperature provinces based on frostless season. 



3. MOISTURE PROVINCES. 



For these comparisons with the vegetation charts we have employed 

 four charts showing moisture provinces: (1) mean daily normal pre- 

 cipitation (P, plate 46) ; (2) mean daily evaporation, 1887-88 {E, plate 

 53) ; (3) precipitation-evaporation ratio (P/E, plate 57) ; and (4) mean 

 normal relative humidity (H, plate 65). The main cases of agreement 

 brought out by these comparisons are given below. 



The generalized vegetation chart of plate 2 shows many coordina- 

 tions with the charts of moisture provinces. The best correlation 

 occurs with the moisture-ratio chart (P/E), which alone will be con- 

 sidered here, although a study of the other moisture charts is well worth 

 while in this connection. Desert occupies approximately the arid 

 province. Northwestern Evergreen Forest occupies about the humid 

 and semihumid provinces, western subdivision. Western Evergreen 

 Forest occupies the western and northern subdivisions of the semiarid 

 and semihumid provinces. Grassland occupies most of the eastern 

 subdivision of the semiarid province, extending eastward approximately 

 to the line joining western Hudson Bay with western Gulf of Mexico. 

 Deciduous-Forest Grassland Transition (the so-called prairie type of 

 vegetation) occupies the more arid portion of the eastern subdivision 

 of the semihumid province, merging imperceptibly into the next type. 



