4 THE VEGETATION OF THE UNITED STATES. 



While the determination and dehneation of chmatic conditions is 

 capable of being given mathematical precision in a number of different 

 ways (as will be shown in Part II), the classification and geographical 

 delimitation of vegetation requires a preliminary discussion of the 

 point of view from which such work may be done and of the concep- 

 tions on which it may be based. The term "vegetation," meaning the 

 total plant population of an area viewed from the anatomical and 

 physiological rather than the taxonomic and fioristic standpoint, is 

 easy of definition and clear in its meaning. So complex, however, are 

 the natural plant assemblages of almost every locality that an attempt 

 to classify them, even in the most preliminary manner, requires at once 

 the use of personal judgment as to criteria of classification and as to 

 the relative weight to be accorded to characteristics that are totally 

 distinct in kind. 



Several 'workers have devised methods for giving mathematical 

 definiteness to the description of plant assemblages in terms of floristics, 

 by count or estimation of the number of individuals of each species 

 involved. To give this same assemblage, however, a definition which 

 might serve to compare adequately the physiological characteristics 

 of its component individuals with those of the individuals in another 

 near or distant assemblage, is at present very far from possible. We 

 are merely able to define such an assemblage as being "salt marsh," 

 "arctic tundra," "coniferous forest," or the like. These categories are 

 sufficiently definite in their meaning to give us a mental picture of the 

 size, gross anatomy, density of stand, seasonal activities, and other 

 features of the plants concerned; they are not adequate, however, for 

 a strict comparison of the salt marsh and the arctic tundra in physio- 

 logical terms, nor for the comparison of two salt marshes that are 

 widely distant. 



The study of vegetation as such has been, on the whole, greatly 

 obscured by the fact that it has never been completely divorced from 

 the study of the flora. Too much emphasis can not be laid, at the 

 present time, on the radical distinctness of the work of physiological 

 plant geography, on the one hand, which attempts to relate the occur- 

 rence and distribution of species as physiological entities, to the factors 

 of environment, and the work of floristic plant geography, or phytogeo- 

 graphy, on the other hand— which attempts to reveal the geological 

 history, the movements, and vicissitudes of species as phylogenetic 

 entities. The floristic flavor which plant geography and ecology have 

 always possessed may be largely accounted for by the fact that all 

 plant-geographical interest has sprung historically out of floristics, 

 and by the fact that we are in the position of not being able to men- 

 tion a plant of particular identity without using its technical Latin 

 name, which is solely an abbreviated expression for denoting the place 

 we beheve it to occupy in the phylogenetic scheme. No one will deny 



