CONCLUSION. 



The work presented in this publication has fallen under three heads : 

 (1) giving the facts as to the distribution of certain types of vegetation 

 and certain species of plants of the United States; (2) giving the 

 data to show the intensities of the leading climatic conditions in the 

 United States; (3) correlating these two bodies of facts in such a man- 

 ner as to learn the exact range of conditions under which each plant 

 or vegetation lives with respect to each of the climatic elements. 



The botanical facts lead to the subdivision of the vegetation into a 

 small number of natural areas, delimited on a purely vegetational 

 basis, to the outlining of regions in which particular ecological types 

 are most abundant, and to the presentation of the distributional areas of 

 a number of important species in the vegetation. The climatological 

 data have been selected or elaborated with respect to the conditions 

 which are of most importance to plants, with the aim, wherever possible, 

 of devising new expressions for the cUmatic conditions that might be 

 suited to the botanical problems in hand. The correlation of the dis- 

 tribution of plants with the distribution of various numerical values or 

 indices df the several climatic conditions has been carried out with the 

 full realization that such correlations do not carry conclusive proof of 

 the existence of causal connection. It is only by careful elimination of 

 possibilities and by comparison of results, that these correlations 

 can be used as more than a source of suggestions. 



The existence of a causal relation between the climatic conditions and 

 the vegetation of any given region is so well known as to have become 

 practically axiomatic. A relation between climate and the distribution 

 of the conmaon species which dominate the principal vegetations is 

 likewise well-estabUshed fact. But the relations between cUmate and 

 the distribution of the generaUty of individual species is indirect and is 

 obscured by many considerations. 



In an investigation of the role that is played by the various climatic 

 conditions in determining the optimum activity of a plant or the 

 limitation of its distribution, it is necessary to bear in mind that the 

 conditions operate collectively and that their influences are often 

 interdependent. The role of each condition changes with the changed 

 values of the other conditions. In attempting to determine the relative 

 importance of several climatic conditions as determinants of a given 

 distributional phenomenon, it is seldom possible to do more than 

 speak in general terms. It may be possible to state, for example, that 

 temperature conditions are more important than moisture conditions 

 in a given case, without its being possible to determine, on the same evi- 

 dence, which of the several aspects of temperature is most important. 



The problem of the role of climatic conditions in determining plant 

 distribution is essentially a physiological one, since it rests, in ultimate 

 analysis, upon the influence exerted by environmental conditions on the 



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