CORRELATION OF DISTRIBUTIONAL FEATURES. 



547 



wide in all cases except the imperfectly determined mean annual pre- 

 cipitation. Although this plant encounters a wide range of the condi- 

 tions expressed in the moisture-temperature index, there is still a close 

 correspondence between its limit and the isoclimatic line of 11,000 for 

 this index. At the localities where it occurs in southern Illinois and 

 Indiana it encounters index values of only 7,000, and at its northern 

 limit in Massachusetts it encounters its minimum value of 5,193. 



Numerous isoclimatic lines have such a direction as to indicate that 

 there are belts of relatively similar climatic conditions extending 

 parallel to the Atlantic coast for long distances (see plates 46, 50, 53, 

 59, 65, and 72). The conditions of the southern part of the Missis- 

 sippi Valley, which is technically a part of the Coastal Plain, are almost 

 always different from those of the coastal strip of this physiographic 

 province. Numerous plants occur throughout the Coastal Plain from 

 New Jersey or Virginia to Georgia or Mississippi, but fail to range 

 coextensively with it in the southern Mississippi Valley, and terminate 

 their distribution before reaching the mouth of the Rio Grande. Ilex 

 opaca, Pinus tceda, Ilea virginica, and Quercus falcata are all examples 

 of this type of distribution. In all of these cases we undoubtedly have 

 to do with three sets of limiting conditions; those operating in the 

 Atlantic coast region, those in the Mississippi Valley, and those deter- 

 mining the extreme southern limit in Texas. In at least the first of 

 these regions we have to reckon with the modifications of climatic 

 conditions which are due to the soil. 



TCMPCIMTUKC 

 0«Tt IN i.OltMAI. FROSTLEtm S»SON <F. S.I 



Hot Oavb. F. S. 

 Coio Oats. F. S. 



pMVSIOLOCrCAL SUMMATION. F. S. 



•feiiMAL Daily Mean, colocst 14 oats or YcAii 

 Normal Daily Mean, Yca* 



Pkcciwtatioh 

 NoMMAL Daily Mcam, F. S. 

 Oats in longest NormaI. Rainy Pehioo, F. S. 

 Days in lonoest Normal Dry Pcatoo, F. S. 

 Mean Total. Year 



Evaporation 

 Daily Mean. 1807*8, P. S. 



Moisture Ratios 

 Normal P/e. F. S. 

 Normal rr/E, F. S; 

 Normal P/E, YEiili. 



Humidity 

 Normal Mean. F. S. 



SunswNE 

 Normal Daily Duration. T. S. 



Moisture-Temperatore Inmccs 

 Normal P/E x T, F. S.^ Pnysioloqical Metmod 



FiQ. 55. Climatic extremes for Ilex opaca. 



