26 THE VEGETATION OF THE UNITED STATES. 



VI. DELIMITATION OF VEGETATIONAL AREAS. 



The botanical areas that have formed a basis for the correlations 

 discussed in the following pages have been outlined in such manner as 

 to show either the distribution of particular types of vegetation or 

 else the ranges of individual species or groups of species. The deter- 

 mining of the distributional area of a given species is a relatively simple 

 matter, depending for its accuracy only on the exploration that has 

 been carried out and the records of occurrence that are available. 

 The delimitation of vegetational areas, however, demands a careful 

 scrutiny of criteria and methods such as we have attempted to give 

 above. We have endeavored primarily to classify and map the vege- 

 tation of the United States upon a basis which is purely vegetational, 

 without regard to floristics, climate, topography, or other features, 

 however closely these may seem to be associated with the vegetation. 



The effort to observe this requirement, for the sake of the logical 

 soundness of our work, is nevertheless far from removing all of the 

 difficulties which beset an attempt to classify and map the vegetation 

 of a large area. It is difficult for the worker to avoid a subjective 

 treatment of his material and to escape the bias which his own particu- 

 lar experiences or field of observation may have given him. A more 

 tangible set of difficulties arises in deciding where to draw the lines 

 of demarcation in subdividing a set of intergrading vegetations, and 

 where, on the map, to place the lines of separation between vegeta- 

 tions that merge into each other over areas of great extent. This 

 difficulty has been met by drawing lines on the chart through all 

 transitional regions, these lines being so drawn as to be regarded as 

 connecting the points that exhibit the same stage in transition, after 

 the manner of isotherms and other isoclimatic lines. 



After a series of vegetational areas has been distinguished and 

 delimited, and each has possibly been subdivided, we must refrain 

 from regarding these divisions or subdivisions as of coordinate value, 

 for there is no means of putting the degree of their relationship to a 

 test. The subdivisions of the forest areas and those of the desert areas 

 appear, on the printed page, to be of the same rank in classification, 

 but we have no actual knowledge upon which we can base such a 

 supposition. 



We have already seen that the features of outward configuration 

 which are considered in distinguishing growth-forms have to do, to 

 a predominant extent, with the water-relations of plants. When we 

 examine into the other features which we use in distinguishing vegeta- 

 tions, such as the height of the dominant plants, the density of stand, 

 and the simplicity or complexity of the stand, we are impressed with 

 the fact that these features stand also in dependence upon water- 

 relations. 



