CONCLUSION. 583 



<The determination of the ratio of precipitation to evaporation, 

 which was first applied to distributional problems by Transeau, has 

 been made for the entire United States, and has been derived by three 

 methods. So great is the importance of the "moisture ratio," as this 

 has been designated, that it is greatly to be hoped that our maps 

 showing the distribution of the ratios may soon be redrawn upon the 

 basis of much fuller evaporation data. A further attempt has been 

 made to secure composite expressions of groups of important climatic 

 conditions by determining the products of the moisture ratios and the 

 summations of temperature. A cartographic method of approaching 

 the same end has been employed for the determination of the areas 

 included between the isocUmatic lines for the moisture ratio and those 

 for the physiological summation of temperature. The result is a series 

 of climatic provinces which are based upon the two expressions of 

 climatic conditions that are probably the most fundamental ones 

 dealt with in this work. 



The correlation of climate and vegetation has been carried out in 

 three ways: The maximum and minimum values of each climatic 

 condition have been determined for each vegetational area or for the 

 distributional area of each species. A comparison has been made 

 between the amplitude of the conditions in each botanical area and the 

 amplitude in the United States as a whole, in order to discover how 

 small or how great a portion of the whole range of climatic conditions 

 is occupied by the vegetation or plant in question. Comparisons have 

 been made between the positions of isoclimatic lines and the hues drawn 

 to show the limits of botanical areas, for the purpose of discovering 

 close correspondences. The detailed results of these methods of 

 correlation are given in the preceding pages; they do not lend them- 

 selves to being smnmarized. 



The parallelism that exists between the distribution of many of the 

 closely related climatic conditions makes it difficult in some cases to 

 determine which of the several aspects of a given condition is of the 

 greatest importance in controUing a particular plant or vegetation. 

 The methods used rarely fail, however, to demonstrate whether it is 

 the temperature group of conditions or the moisture group that pos- 

 sesses the greater importance. 



With respect to the generalized vegetation areas of the United 

 States, one of the most clear-cut evidences of a fundamental correla- 

 tion exists in the correspondence between the position of the vegeta- 

 tional boundaries and the position of the isoclimatic lines expressing 

 certain values of the moisture ratio for the average frostless season. 

 The composite character of the moisture ratio, and the fact that it is 

 derived from such an important group of climatic conditions, give it a 

 value of the first rank in deahng with the physical conditions determin- 

 ing the distribution of vegetation. 



