558 



RELATION TO ENVIRONMENT. 



from getting a start. Also prairie fires may have destroyed 

 them in the seedling stage. But it is quite clear that climatic 

 factors have been the dominant ones. ist. The greater amount 

 of rainfall is in the spring and early summer, when the grasses of 



Fig. 505- 



Typical prairie scene, a few miles west of Lincoln, Nebraska. (Bot. Dept., Univ. 

 Nebraska.) 



the prairie most need it. 2d. This is the time when seeds of 

 trees would germinate. Those enabled to start would die out 

 during the drought of late summer and autumn. 3d. The firm- 

 ness of the soil and dense mat of grasses would largely prevent 

 seedlings getting a foothold in the ground. 4th. The heavy and 

 drying winds during the dry season and winter cause excessive 

 transpiration, and scatter the litter, thus preventing the accumula- 

 tion of humus which is so necessary for the retention of moisture. 

 1045. Prevailing grasses in the prairie region. Among the 

 prevailing grasses in the prairie region are two types: ist, the 

 "sod-formers," long-stemmed grasses which make close forma- 

 tions, as the drop-seed (Sporobolus asperifolius), Koehleria 



