The Vegetation of North America 329 



Scattered throughout the prairies were large and small her- 

 baceous plants, including milkweeds, sunflowers, rosin weeds, cone- 

 flowers, asters, and goldenrods. These plants gave color to the 

 prairies at certain seasons. They never made up a large part of 

 the original prairie covering, however, and they were most 

 numerous on the borders between the prairie and the forest and 

 on eroding slopes. 



Among the explanations sometimes given for the treelessness 

 of the prairies are the fires set by the Indians and by hghtning. 

 That these fires occurred in the autumn there is no doubt, and 

 that they killed young trees on the forest edge and acted as a 

 check to tree invasion there can be no doubt also. The prairies, 

 however, preceded the prairie fires, and the fires could at best 

 only delay forest invasion — not prevent it over such vast areas. 

 Deciduous forests occurred throughout the prairies along the 

 streams, on river bluffs, on valley slopes, and on the flood plains. 

 The highly fertile character of the prairie soil caused them to be 

 occupied by farms as rapidly as they could be broken and properly 

 drained. Today patches of original prairie are far more difficult 

 to find than patches of original forest. 



In Ilhnois and Iowa the prairies occupy for the most part 

 upland areas between the stream valleys. In Kansas and 

 Nebraska, where the region that was dominated by the big 

 bluestem reaches its western Kmit, the prairies were confined 

 to the river valleys and lowlands. 



The plains grassland formation. Between the Rocky Moun- 

 tains and the prairies and from Saskatchewan to Texas is a vast 

 rolHng plain more or less dissected by streams and covered with 

 grasses. West of the Rockies, from Montana to Washington 

 and California, are similar areas of grassland bordering the 

 forests. 



The annual rainfall varies between 10 and 20 inches and is 

 distributed irregularly in showers and occasional heavy down- 

 pours. As the depth of evaporation from a water surface is 



