PREFACE. XIII 



tion. Much less has it been possible to discover quantitative relations 

 between vegetation characters on the one hand and environmental 

 conditions on the other. Before such relations can be looked for it is 

 obvious that environmental conditions must be described in more or 

 less quantitative terms, and similarly quantitative descriptions of the 

 corresponding vegetational forms must also be available. The present 

 publication is a first attempt to bring these two kinds of descriptive 

 knowledge together for the geographic area of the United States. Be- 

 cause of the newness of the point of view, if not of the subject, but little 

 detailed discussion of the reasons for the actual quantitative relations 

 that exist between plants and their surroundings is here attempted. 

 We have generally been content to point out the kinds of observations 

 that appear to be needed and to bring together such observations and 

 descriptive deductions as we have been able to obtain, both with 

 reference to the vegetation and with reference to those of the environ- 

 mental conditions that are measurable, and for which measurements 

 are at hand. 



It is obvious at once that the subterranean conditions of plant 

 habitats have not yet received enough attention from the present 

 point of view to make even a tentative description of these conditions 

 possible; the soil studies that have been made are either not sufficiently 

 quantitative or else they deal with features that are not directly related 

 to plants, or the relation of which to plants is not yet clear. This 

 being the case, the main environmental conditions that thus far lend 

 themselves to quantitative study, albeit in a very superficial way, are 

 those that are effective above the soil surface. These features com- 

 prise those conditions that are generally termed climatic. Therefore 

 our study has dealt almost wholly with climatic features, and the rela- 

 tionships between vegetation and climate are the main relationships 

 with which we have been constrained to deal. It is almost certain 

 that the causal relationships between plants and their environments 

 can not be satisfactorily discussed in the majority of cases until sub- 

 terranean conditions are given at least as thorough treatment as we 

 have been able to give to the aerial conditions, so that any apparently 

 definite conclusions that seem to emerge from our comparisons must 

 be held tentatively until suitable methods for the quantitative study 

 of soil conditions have been devised and generally applied. 



Another aspect of the causal relations that obtain, or have obtained, 

 between plants and the environmental complexes of their habitats 

 brings what has been termed the historic factor into prominence, and 

 this factor involves conditions of the remote past, both aerial and sub- 

 terranean. With this aspect we do not deal seriously in the present 

 pubhcation. 



On the whole, then, our aim has not been to discover true causal 

 relationships between the two categories of observations here con- 



