546 CORRELATION OF DISTRIBUTIONAL FEATURES. 



amplitudes of temperature conditions for the United States, but 

 reaches areas with the maximum number of cold days. One of the 

 narrowest amplitudes among the moisture conditions is found in the 

 case of the number of days in the longest normal dry period, for which 

 the extreme values are 9 and 88 days. A previous allusion was made to 

 the western limit of this tree as showing the manner in which wholly 

 distinct conditions cooperate in controlling the ranges of plants. It 

 will be seen by a comparison of plate 52 (dry days) with plate 18 (dis- 

 tribution of Quercus macrotarpa) that the longest dry period encoun- 

 tered at the western edge of this tree in Oklahoma is less than 50 days, 

 whereas the maximum number encountered near the Canadian 

 boundary is 88 days. The potence of this moisture condition is evi- 

 dently modified by the differences in temperature conditions which are 

 encountered along the western limit of the tree. The values of the 

 moisture ratio (tt/E) encountered in Oklahoma are about 0.60 and 

 those encountered in Montana are about 0.40. An abiUty on the part 

 of Quercus macrocarpa to withstand the same values for these two con- 

 ditions in the latitude of Oklahoma that it does in Montana would 

 carry the tree to the eastern borders of New Mexico with respect to the 

 moisture ratio, and well into the borders of that State with respect to 

 the longest dry period. These two conditions, as modified in their 

 influence by temperature conditions, may be regarded as setting the 

 limit of the westernmost occurrences of this oak, which (like the 

 western limits of so many deciduous trees) are to be found in alluvial 

 bottoms characterized by moisture conditions which are higher than 

 those of the adjacent upland. 



The southern limit of Quercus macrocarpa corresponds roughly with 

 the isotherm of 120 hot days (see plate 36), and this condition, possibly 

 in conjunction with closely related conditions, may be regarded as 

 probably controlling the southern edge of the distributional area. The 

 mean temperature of the hottest 6 weeks is apparently one of the most 

 important of these related conditions, as the isotherm of 78.8° hes near 

 the southern limit (see fig. 45). 



Ilex opaca (fig. 55).— The occurrence of this Ilex is rather closely 

 confined to the Atlantic Coastal Plain throughout all but a small part 

 of its range, where it extends into the Piedmont and mountain sections 

 of Georgia, Alabama, and Tennessee. This range is characterized by 

 wide amplitudes of the temperature conditions, reaching the maximum 

 values in all cases except the number of cold days. With respect to the 

 latter condition, the amplitude is relatively narrow and the maxinium 

 is 55 days, encountered only at the northernmost attenuated limit of 

 occurrence, in Massachusetts. 



The moisture conditions for the area of Ilex are nearly those of the 

 Southeastern Mesophytic Evergreen Forest, the amplitudes being 



