32 THE VEGETATION OF THE UNITED STATES. 



II. LEADING VEGETATION TYPES OF THE UNITED STATES AND 



THEIR GEOGRAPHICAL AREAS. 



In subdividing the vegetation of the United States we have used 

 primarily the old distinction of desert, grassland, and forest. The 

 distinguishing of these formations involves practically all of the criteria 

 that have been discussed in previous pages. Our ultimate units have 

 resulted from a subdivision of the desert and the forest, but we have 

 not attempted to subdivide the grassland, partly because of Jack of 

 descriptive literature for all parts of the grassland region, and partly 

 because of the extreme complexity exhibited by this region, particu- 

 larly in its central portion. 



The desert areas have fallen into two groups which may be designated 

 as Continental Desert and Coastal Desert. The latter include the 

 semidesert regions of coastal California and of extreme southern Texas, 

 each of them regions in which truly desert areas are found together 

 with areas which scarcely merit this designation. The Continental 

 Desert falls naturally into two regions, one of which is dominated by 

 sclerophyllous shrubs and the other by a mixture of such shrubs and 

 succulent or semisucculent plants. The sclerophyllous desert has been 

 subdivided into the Great Basin Desert and the California Desert, 

 while the succulent desert has been subdivided into the Arizona and 

 Texas areas. The two bodies of sclerophyllous desert are adjacent and 

 merge into each other, whereas the succulent deserts are separated by 

 regions of dissimilar character. 



The Grassland area is bordered on the southwest by the Desert- 

 Grassland Transition and on the east by the Grassland-Deciduous 

 Forest Transition. 



The forested portion of the United States has been subdivided into 

 the Deciduous Forest and 4 areas of Evergreen Needle-Leaved Forest. 

 Two of these evergreen-forest areas are mesophytic in character, one 

 of them xerophytic, and one of them hygrophytic. We have included 

 in the Northern Mesophytic Evergreen Forest all of the needle-leaved 

 forests of the northeastern States as well as of the Rocky Mountain 

 and Pacific regions, with the exception of the extreme Northwest, 

 Although this large forest region exhibits marked differences in its 

 floristic make-up and in minor features of its physiognomy and 

 ecological characteristics, there are nevertheless more reasons for 

 maintaining it as a single area than for separating it into minor sub- 

 divisions, so far as the purposes of our work are concerned. The 

 Southeastern Mesophytic Evergreen Forest, the Western Xerophytic 

 Forest, and the Northwestern Hygrophytic Evergreen Forest are all 

 regions of distinctive character which are neither similar to the 

 Northern Mesophytic Evergreen Forest nor to each other. Two 

 transitional areas have also been outlined, connecting the Deciduous 

 Forest with the Evergreen Forests to the north and to the south of it. 



