506 



PRACTICAL BOTANY 



are bare of trees except where they have been planted or occur 

 naturally in belts along the streams. Some of the principal 

 reasons that have been given for the treeless condition of the 

 prairies and the plains are the frequency with which prairie 

 fires were set by the Indians, the scanty rainfall, and the de- 

 structive effect of violent dry winter winds, acting on the trees 



when they can get no water 

 from the frozen soil. Ex- 

 cessive evaporation, due to 

 exposure to high winds at 

 all seasons, seems to be a 

 cause of prairies. Such 

 winds are particularly fa- 

 tal to seedling trees which 

 are not yet deeply rooted. 

 Large areas also have never 

 been reforested since their 

 vegetation was swept away 

 by great geological changes 

 in the Mississippi Basin. 1 



The prairies in their nat- 

 ural condition were rather 

 closely covered by coarse 

 grasses, forming a very 

 tough sod. This, when 

 turned over by the break- 

 ing plow, was firm and durable enough to be used by the 

 early settlers for the walls of sod houses. Among the prairie 

 grasses are found several plants of the Pea family, such as the 

 lead plant or shoe strings, 2 prairie clover, 3 ground plum, 4 and 

 some others. Composites are very abundant and characteristic, 

 among them being goldenrods and asters, the blazing star, 5 the 

 cone flower, 6 and several species of tickseed. 7 Some sunflowers 8 



1 See Pound and Clements, Phytogeography of Nebraska, Vol. I, pp. 67-70. 



2 Amorpfia canescens. 8 Petalostemum. 4 Astragalus. 6 Liatris. 



6 Rudbeckia. 7 Coreopsis. 8 Helianthus. 



FIG. 376. Occurrence of trees above the 



ordinary timber line, in sheltered valleys 



and ravines, Rocky Mountains 



