DISTRIBUTION OF VEGETATION IN UNITED STATES. 45 



numerous herbaceous species, usually less highly specialized than those 

 growing in rocky soil or crevices. On the highest mountains of the 

 United States the vegetation is often limited to mosses and lichens 

 or is locally absent. 



Swamps and Marshes.— These terms comprise an extremely varied 

 series of communities, partially dominated by trees and partially by 

 coarse grasses, sedges, or other palustrine plants. The words "swamp" 

 and "marsh" are both somewhat objectionable for use in the present 

 connection because of their intrinsic reference to the nature of the 

 habitat. The distinction between swamp and marsh has so long been 

 drawn in popular speech and scientific writing, however, that the 

 words are used here as terms descriptive of the vegetation rather than 

 designations that imply the feature of the environment which deter- 

 mines the vegetation. The areas of swamp and marsh are so intri- 

 cately interwoven that no effort has been made to separate them. 



The greatest development of swamps and marshes is to be found 

 along the shores of the Atlantic Coastal Plain, although there are 

 smaller areas of marsh on the Pacific Coast and scattered areas of 

 swamp throughout the glaciated region. 



The saline marshes are dominated by nearly pure stands of halo- 

 phytic grasses, while the fresh or brackish marshes are inhabited 

 by very diverse populations of herbaceous perennials and annuals. 

 The swamps of the Southeastern States are composed of a particularly 

 rich assemblage of deciduous broad-leaved trees {Nyssa, Acer, Mag- 

 nolia, and Quercus) or of nearly pure stands of the deciduous needle- 

 leaved bald cypress {Taxodium distichum). 



Although the map of the vegetation of the United States which has 

 just been described (plate 1) is not so detailed as might be possible or 

 desirable, it was found, nevertheless, as has been noted, that many of 

 its smallest areas and the sinuosities of many of its major boundaries 

 would be meaningless when brought into comparison with the rela- 

 tively small number of stations from which we were able to secure 

 climatic data. We have therefore prepared a generahzed map of the 

 vegetation of the United States, shown as plate 2, which was executed 

 with special reference to the number of stations represented in our 

 accumulations of climatological data. The detailed map has been 

 published and described for the sake of giving the basis upon which 

 this more generalized map has been drawn. The number of areas has 

 thus been reduced from 18 to 9 by a reduction of the four desert areas 

 to one, by a consideration of the two semidesert areas as one, by the 

 elimination of the transition areas from desert to grassland and from 

 deciduous forest to the forest areas to the north and south of it, and 

 by disregarding the alpine summits and swamps and marshes. We 

 have chosen to separate our study of climatic correlations for the 

 eastern and western portions of the Northern Mesophytic Evergreen 

 Forest. Although these two areas are ecologically alike, it seemed 



