THE NORTH TEMPERATE ZONE 



107 



role in the destruction, by close cropping of any young trees that 

 may have attempted to get a foot-hold. Once established, how- 

 ever, the dense mat of grass offers very little opportunity for the 

 growth of seedling trees. 



Further west, the extreme weather conditions, dry and cold, 

 are sufficient to account for the absence of tree growth on the 

 western plains, which naturally would assume their present con- 

 dition of arid prairie or steppe. 



A study of the wet prairie near Chicago x indicates that it was 



Fig. 24. — Sand prairie, valley of Illinois River. Photo., Dr. A. G. Vestal. 



originally a swamp. While grasses are the predominant plants 

 of this prairie, they are associated with a number of other char- 

 acteristic forms, some of which have very attractive flowers. 

 Where there are remnants of the original vegetation, as along the 

 railway in places, one may see in spring, even near Chicago, masses 

 of pink phlox, shooting star (Dodecatheon), the beautiful bird's- 

 foot violets (Viola pedata), and the big leaves of the rosin-weed 

 (Silphium), which later sends up its tall stems bearing big sun- 

 flower like blooms. In the late summer and autumn, golden-rods 

 and asters abound, and the pink blazing-star (Liatris) is also 



1 Cowles, H. C., Bot. Gazette, XXXI, p. 145, 1901. 



