CORRELATION OF DISTRIBUTIONAL FEATURES. 509 



and but little in excess of the corresponding amplitude for the Grass- 

 land. The wide amplitude of the temperature and precipitation con- 

 ditions, taken in themselves, might be held to indicate that there is not 

 a close correlation between the distribution of this vegetation and that 

 of any of the important controlling physical conditions. Such a view 

 would have in its support the fact that this vegetation is one that is 

 well known to be closely correlated in its distribution with the extent 

 of the Atlantic Coastal Plain and with the series of soils typical of that 

 physiographic province. However, the types of vegetation which seem 

 to be strictly controlled by topographic features and by soils in the 

 northern part of the Coastal Plain are not so controlled in the southern 

 part and in the lower Mississippi Valley, where the Coastal Plain is 

 not so sharply defined. In spite of the wide ampUtude of many other 

 conditions, the moisture ratios are of a significance with respect to this 

 region which must not be omitted from consideration. 



In the Southeastern Mesophytic Evergreen Forest all of the leading 

 temperature conditions of the country reach their maximuni values, 

 except the number of cold days, which, reciprocally, reaches its mini- 

 mum value. Five of the six temperature conditions represented in 

 figure 33 range in this vegetation through more than half of the ampli- 

 tude found in the United States as a whole, due to the far northward 

 extension of the region along the Atlantic coast. All of these condi- 

 tions overlap with those of the Deciduous Forest region, which is quite 

 to be expected in view of the overlapping and admixture of the vege- 

 tations themselves. A transition region between the Deciduous Forest 

 and the Southeastern Evergreen Forest has been outlined in the 

 detailed map of vegetation (plate 1), and there is considerable evidence 

 (some of which will be discussed) that portions of the Southeastern 

 Evergreen area are of such a character climatically as to support a 

 deciduous forest. 



The amphtude of the normal daily mean precipitation is nearly 

 equal to that of the Deciduous Forest, but the minimum and maxi- 

 mum values are somewhat higher. The number of days in the longest 

 normal rainy period of the frostless season exceeds even the great 

 amplitude exhibited by this condition in the Deciduous Forest, and 

 reaches, at Cape Hatteras, the highest value for the United States. 

 The number of days in the longest normal dry period exceeds the 

 amphtude of this condition for any of the other forest regions, and is 

 even greater than that for the Grassland and the Desert, although the 

 actual range of this condition is from the lowest value for the country, 

 no days at Cape Hatteras, to 182 days at Key West. 



The amplitude of evaporation conditions for the Southeastern Ever- 

 green Forest is narrow, and the values are relatively low, being ahnost 

 equal, as already shown, to the evaporation values for the eastern 

 section of the Northern Evergreen Forest. The amplitudes of the 



