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INTRODUCTION. 



The physiological point of view has been constantly held before us, 

 as has been said, in planning and carrying out the complicated com- 

 parisons and correlations with which the present publication deals. 

 Part I shows how the numerous vegetation features employed by us 

 were derived, emphasis being placed on the physiological character- 

 istics of the plants considered. Part II deals with the principles and 

 methods by which the climatic features that we have used were selected, 

 and shows how the requisite numerical computations were made and 

 how the maps were prepared. This selection had to be based, as 

 has already been indicated, upon two different kinds of circumstances : 

 the physiological importance of the climatic features (as these are 

 known to influence plant activity in general) and the availability of 

 climatic data suited to our purposes. These matters will be set forth 

 under three general headings : (I) general influence of the environment 

 on plant life; (II) chief environmental conditions and the general 

 nature of their effects upon plants; and (III) climatic conditions of 

 the United States. The first two of these sections are mainly physio- 

 logical-ecological in nature and are general in their scope; the last 

 is mainly climatological-ecological and deals with the actual climatic 

 data employed in our researches. 



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