268 Plant Life and Evolution 



PLANT DISTRIBUTION IN THE UNITED STATES 



The North American continent illustrates very 

 clearly the most important factors governing plant 

 distribution, and the United States with its 3,000,000 

 miles of territory, reaching from the Atlantic to the 

 Pacific, offers unusual opportunities for studying 

 the most important factors of Phytogeography. 



The Eastern Forest. In a general way the 

 United States may be divided into three great re- 

 gions, extending east and west. East of the Mis- 

 sissippi the country was originally covered by an 

 almost unbroken forest, of which a very large part 

 has disappeared with the clearing of the land, but 

 enough of which remains even in the more densely 

 settled regions, to make it evident what was its 

 character; and in the remoter districts, especially 

 in the southern mountains, there may still be found 

 tracts of practically virgin forest. This forest is 

 characterized by the predominance of deciduous 

 trees, only occasionally, as in the pine-barrens and 

 cypress-swamps of the South, or in the more north- 

 ern forests, are conifers the predominant trees. 



Owing to the absence of high mountains, or other 

 barriers, the flora of the eastern third of the United 

 States is remarkably uniform, many species occupy- 

 ing the whole area, the differences in the different 

 portions being mainly those caused by variations in 

 soil, heat, or moisture. In the northern portion the 

 prevailing trees are oaks, elms, maples, hickories, 



