30 THE VEGETATION OF THE UNITED STATES. 



Three maps have also been prepared showing the ecological dis- 

 tribution of individual species of plants (plates 8, 9, 10). On these 

 maps an effort is made to show the features of distribution in such a 

 way as to indicate the regions of greatest abundance and greatest 

 catholicity of habitat, the regions of frequent occurrence, the regions 

 of rare occurrence, and the extreme geographical limits of the species. 

 Maps of this character afford a picture of the distribution of a single 

 species which is similar to the picture afforded by the maps of cumu- 

 lative distribution of groups of related growth-forms. 



As a basis for the correlation of climatic conditions with the ranges 

 of individual species, 70 plants were selected, the distributional areas 

 of which are shown by groups on plates 13 to 33. Some of the species 

 selected for this purpose were chosen because they are common and 

 dominant elements in important vegetations of wide extent. Others 

 were selected because their geographical ranges seem to be typical of 

 those exhibited by a large number of species. Certain other species 

 were selected because of the interest which attaches, from our point 

 of view, to the chara(?ber and particular direction of their distribution. 

 Several of these plants extend across the continent from the Atlantic 

 to the Pacific, either in the northern or in the southern part of the 

 United States, in such a manner that they cross the principal boun- 

 daries between vegetational areas. Distributions of this character 

 seem to indicate that the plants in question are probably controlled by 

 temperature conditions rather than moisture conditions. Several 

 plants of this character and of more limited range were selected, be- 

 cause they are commonly found in swamps or marshes and may there- 

 fore be thought of as growing through a wide range of atmospheric 

 conditions, at the same time that they are subjected to a relatively 

 uniform set of soil-moisture conditions. It is to be anticipated that 

 plants of this character differ markedly in their distribution from those 

 whose range is greatly influenced by soil-water conditions. 



The construction of the vegetational maps has involved the examina- 

 tion of a large body of ecological, floristic, and geographical literature. 

 It was originally our plan to publish a complete list of the sources that 

 were used in the compilation of these maps, but the appearance of 

 Harshberger's Phytogeographic Survey of North America has since 

 made it superfluous to do so, inasmuch as this author has given a very 

 thorough bibliography of the literature of American vegetation, in- 

 cluding nearly all of the publications that we have used.^ 



In the construction of the maps of vegetation we have been heavily 

 indebted to the maps pubHshed by the United States Forest Service, 

 to the maps of grazing-lands published by the United States Bureau 

 of Plant Industry, and to the detailed maps which have been published 



'Harshberger, J. W., Phytogeographic survey of North America, Die Vegetation der Erde, 

 13: 863 p., 32 figs., 18 pis., 1 map, Leipzig, 1911. 



