72 CARNEGIE INSTITUTION OF WASHINGTON. 



which hinit the great areas of forest, grassland, and desert appear to be 

 best expressed in the ratio of precipitation to evaporation. 



A Map of the Vegetation of the United States from a Purely Vegetational 

 Standpoint, by Forrest Shreve. 



A map showing the principal vegetational areas of the United States 

 has been in preparation for several years in connection with the work 

 on correlation of vegetation and clhnate which is described elsewhere. 

 Repeated revisions of the map have been made whenever further 

 materials were secured regarding it, and the latest of these has been 

 prepared for publication. 



The aim of this map has been to distin^ish the various types of 

 vegetation in the United States solely upon the basis of the collective 

 physiological behavior and anatomical character of the dominant 

 plants concerned. No weight whatever has been given to floristic 

 relationships nor to any of the climatic, physiographic, and geological 

 factors which are known to influence the distribution of vegetation. 

 This departure from the method by which former maps of the plant 

 Ufe of the United States have been made has necessitated the use of 

 original sources of information throughout its preparation. The 

 importance of using purely vegetational data has resided in the fact 

 that the primary purpose of the map was to serve as a foundation for 

 correlations of the distribution of vegetation and the distribution of 

 climatic conditions. 



The chief criterion by which the plant formations of the United 

 States have been distinguished is the character of the growth-forms 

 comprised in these formations. The differentiation of growth-forms 

 among plants is largely an expression of their adjustment to a particular 

 set of water- relations. The form, structure, and foliar organs of every 

 plant betray its water requirements, while no such visible criteria 

 indicate its temperature requirements. It is therefore impossible at 

 the present time to characterize large units of vegetation without 

 neglecting the temperature requirements of the plants, an extended 

 knowledge of which awaits further experimental work. It has been 

 impossible in the preparation of this map to escape these hmitations. 



It has been found necessary, as a result of personal exploration, to 

 subdivide the desert portion of the United States. The regions in the 

 Great Basin and southern California are characterized almost solely 

 by microphyllous shrubs, while the deserts of Arizona, New Mexico, 

 and western Texas abound in succulent and semi-succulent plants. 

 The coastal portions of CaUfornia and of Texas are to be regarded as 

 semi-desert areas, differing widely from the continental areas just 

 mentioned. There is also a well-marked transition area between the 

 succulent deserts of Arizona, New Mexico, and Texas and the western 



