MESOPHYTE ASSOCIATIONS. 241 



gested perhaps that which refers the western prairies to 

 the prevailing dry winds is the most prominent. 



The extensive plains of the West develop the strong 

 and dry winds which prevail over this prairie region, and 

 this brings about extremes of heat and drouth, in spite of 

 the character of the soil. In such conditions a tree in a 

 germinating condition could not establish itself. If it is 

 protected through this tender period it can maintain itself 

 afterward, but the drying winds forbid any plant with a 

 prolonged and sensitive juvenile period. These prairies, 

 therefore, would represent a sort of broad beach between 

 the western plains and the eastern prairies and forests. 



What seems to be a confirmation of this view may be 

 observed in certain north and south valleys in the Missouri 

 region which lies on the border between plains and prairies. 

 The eastern slopes of such valleys, exposed to the wind 

 from the plains, are without trees ; while on the western 

 slopes, protected from this wind, trees occur. 



Probably the oldest explanation of such prairies is the 

 occurrence of prairie fires, but this would appear to be too 

 local a cause for what seems to be a continental feature. 

 Recently, however, the fire theory has been revived, and 

 evidence has been brought forward to show that in some 

 places, at least, a forest growth would appear if fire and 

 stock were kept out. In fact, the claim is made that Ne- 

 braska is becoming gradually forest-clad. 



170. Pastures. — This term is applied to areas drier than 

 natural meadows, and includes the meadows formed or con- 

 trolled by man (see Fig. 207). They may be natural, or 

 derived from natural meadow areas, or from forest clear- 

 ings; therefore they are often maintained in conditions 

 which, if not interfered with, would not produce a meadow. 

 In general, the pasture differs from the natural meadow in 

 being drier, a fact often due to drainage, and in develop- 

 ing lower and more open vegetation. Naturally the plant 

 forms are prevailingly grasses, and their cultivation is the 



