492 PLANT GEOGRAPHY 



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of eastern Tennessee and North Carolina, the Adirondacks and 

 the White Mountains, in swampy river valleys, in a few great 

 marshes, or in sterile, sandy pine barrens, that one can find the 

 original flora in its natural condition. 



Comparing our forest region with the parts of Europe which 

 resemble it most in soil and climate, our flora differs notably in 

 possessing such leguminous trees as the locust and the honey 

 locusts, in the abundance of members of the heath family, and 

 in wealth of Composites, especially asters and golden-rods. 



In very many instances our eastern flora when it differs most 

 notably from that of Europe greatly resembles that of China and 

 Japan. This is undoubtedly due to the fact that these American 

 species and kindred Chinese and Japanese ones had in an earlier 

 geological age a common ancestry. 



On account of the great length of the territory along a north 

 and south axis and the diversified nature of its surface, the flora 

 of the forest region varies from a sub-tropical one in southern 

 Florida to one with a plentiful sprinkling of sub-arctic species 

 along portions of the northern border, particularly on the higher 

 mountains. 



469. The plains region. This region rises with a gradual 

 ascent from the prairies (some of which occur from Ohio 

 westward and over great areas border the west bank of the 

 Mississippi), until an elevation of five thousand feet or more is 

 attained, when the plains reach the Rocky Mountain system. 

 There is no sharply defined line of demarcation between the 

 prairies of western Kansas, western Iowa, Minnesota, Nebraska, 

 and South Dakota, with less than 20 per cent of the surface 

 wooded, and the high plains, wholly treeless except along the 

 streams (Plate IX, upper figure), that flank the eastern border 

 of the Eocky Mountains. The lack of trees in the prairie and 

 plains region has been attributed to various causes, but the prin- 

 cipal ones are doubtless forest fires, the scanty rainfall, and the 

 occurrence in winter of severe drying winds, at a time when 



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the roots can draw no moisture from the frozen soil. 



