INTRODUCTION. 



After presenting in the preceding pages the data which have been 

 selected to exhibit the principal features of plant distribution in the 

 United States, together with climatological data that have been 

 elaborated for use, we are now in a position to proceed to the final 

 stage of our investigation and to discuss the correlation of features of 

 plant distribution with various intensities of the climatic conditions. 

 All that has gone before has been, in a large measure, preparatory to 

 this phase of our work. 



We have sought primarily to discover the amplitude or extreme 

 range of the climatic differences that may be found within the distri- 

 butional area of each vegetation, each species, or each group of species. 

 Such amplitudes, as shown by the climatological figures that we have 

 used, are those of the average year, or the average frostless season. 

 Even where we have used such data as the number of cold days, the 

 number of days in the longest rainy period, or the number of days in 

 the longest dry period, we have been concerned with the average or 

 normal operation of these extreme factors. Our work disregards, in 

 other words, the actual absolute extremes by which every region is 

 visited with respect to every element of the climate, except in the 

 single instance in which we have employed data as to the absolute 

 minimum temperature. The individual climatic extremes of a given 

 year may be of great importance to plants, and may change distribu- 

 tional limits or influence migratory movements. For perennial plants, 

 however, it is probably the average conditions of decades and groups 

 of decades that determine the mean distributional limits, and it is the 

 average conditions that should be taken into account in a preHminary 

 and general investigation of this character. 



In addition to ascertaining the amplitude of each climatic index for 

 each plant area, we have endeavored, in as many cases as possible, to 

 discover evidence that would show which of the various climatic condi- 

 tions appears to be most influential in controlling the distribution of 

 a given vegetation or plant, and what particular intensities of such 

 main controls appear to be the critical points. It requires little 

 experience with such problems to come to a realization that the various 

 parts of the boundary of a plant or vegetational area are not controlled 

 by the same factor or group of factors. In the case of plants which 

 have a distribution of east-and-west extension it is obvious that their 

 northern and southern boundaries are determined by different inten- 

 sities of some condition, or possibly by entirely distinct sets of con- 

 ditions. In the case of plants which have extensive and irregular 

 distributional boundaries it may be possible to state what the limiting 

 conditions are for certain portions of this boundary without being 

 able to discover how far the given controls extend their domination. 



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