THE PRAIEIES 213 



sory factors as topography, large bodies of water, character of 

 plant covering, etc.. "which may influence its rate. 



Accessory or Secondary Factors. 

 Topography 



Among the accessory factors none is of greater importance 

 than topography because of its influence upon the conservation 

 of moisture, as has been shown in this paper. Its importance, 

 however, has been variously estimated. Dana (1849 and 1875) 

 considered it together with winds and temperature. Foster 

 (1869) noted its effect upon moisture. "Worthen (1866) found 

 prairie on both high and low ground, and Williams (1904) called 

 attention to the fact that in southern ^lissouri prairies occur at 

 an elevation of 600 to 800 feet, whereas in the northern part 

 they reach an elevation of 1200 feet. "White (1870), Mac^Iillan 

 (1892) and Baker (1908) found forests on the south sides of the 

 valleys of streams, but only where the region south of the 

 stream is rough. Macbride (1894 and 1899) reported and illus- 

 trated the interesting fact that even in the more heavily forested 

 portions of northeastern Iowa the forests are found chiefly on 

 northern and eastern slopes. Cameron (1897) also noted the 

 abundance of forests on rougher areas, and Todd (1878) dis- 

 cussed the distribution of prairie and forest with reference to 

 topography, and noted the effect of topography on the humidity 

 of the air. The writer (1900) also discussed the effect of topog- 

 raphy on winds in their relation to forest growth, and more 

 recently (1911) its influence on evaporation. Local variations 

 in flora are determined in large part by this factor because of 

 its influence upon both evaporation and drainage. 



IXSUFFICIEXT DraIXAGE 



Standing water in marshes and ponds has frequently been re- 

 garded as a cause of the treelessness of prairie. Atwater (1818) 

 and Bourne (1820) early offered this in explanation of the phe- 

 nomenon, and McGuire (?) (1838) also advocated a swamp 

 origin. Lesquereaux (1857. 1860 and 1866) advanced the opin- 

 ion that prairies originated in post-glacial lakes and swamps, and 

 Richard Owen (1862) and Worthen (1882) expressed similar 



