CLIMATIC CONDITIONS OF THE UNITED STATES. 151 



of the sand dunes of the Atlantic coast, of the Great Lake region, and of 

 the Southwest are very similar. Furthermore, the pine forest of the 

 Southeast is characteristic only of sandy soils and has the same physio- 

 logical character as have the sandy pine plains of New Jersey, Michigan, 

 Wisconsin, etc. The heavier soils of these regions all bear a very 

 different type of vegetation. 



It has long been the practice of ecological and agricultural writers, 

 in discussing any given region, to present rather elaborate tabulations 

 of meteorological data as a sort of description of the region con- 

 sidered, and then to turn to the discussion of the vegetational phe- 

 nomena in hand, usually without any attempt to correlate the two sets 

 of descriptive data. There seems to have been no doubt that there 

 is some sort of relation between vegetation and the usual sets of 

 climatological observations, but each author has contented himself 

 with presenting the results of such observations, apparently with a 

 faith that someone in the future might be able to interpret them. 

 It has appeared to us high time that some serious attempt were set 

 on foot to develop promising methods for such interpretations. 



It has therefore been necessary, throughout our studies of environ- 

 ment, to choose and devise methods for handling the climatic data 

 that are at hand, so as to derive from them as much information as 

 possible about their probable influence upon plants. In many cases 

 this choice of method has been based mainly upon general physio- 

 logical theory rather than upon actual knowledge, since, as has been 

 pointed out, adequate results of actual tests of the influence of climatic 

 conditions upon plants are not to be looked for until very special 

 facilities for this work have become available. One of the ideal aims 

 that we have held in view during the years which these studies have 

 occupied is the ultimate attainment of what may be termed environ- 

 mental formulas, which might express the minimum, optimum, and 

 maximum for each of the effective environmental conditions for any 

 given plant-form or vegetation type. It is clear that the time is not 

 yet ripe for the estabhshment of more than tentative and general 

 suggestions in this direction, but several such suggestions will appear 

 in following pages. It is here to be emphasized, however, that such 

 formulas are the legitimate end of such investigations as these.^ 



In the following sections we shall bring out the results that we have 

 been able to obtain by various methods of treatment of the available 

 climatological data. Most of these data have been obtained by the 

 U. S. Weather Bureau for other purposes, commercial and political, as 

 well as meteorological, as it appears. Our sources will be given in their 

 proper places. ^ __^ 



1 The idea of such formulas is not new; it has received attention, but from a different stand- 

 point, from some of the phenologists. Simple formulas by which two factors are combined to 

 give a single climatic index have been brought forward by Transeau, by Shreve, and by Livmgston. 

 These will be considered in another place. 



