H> o s t e I s i a 31 



posed. The vegetation on the north 

 slopes of the mesas differs, more or 

 less markedly, from that of the south 

 slopes. The varying amount of mois- 

 ture and sunshine are the important 

 factors in determining this difference. 

 The east and west ends of the mesas 

 are also different in their plants. The 

 west end, being closer to the foothills, 

 is sheltered from the afternoon sun, 

 while the east end is not thus pro- 

 tected. In the spring, small streams from 

 the melting snow on the hills water 

 the west end of the mesa. The 

 snow also lies longer there. Pine trees 

 and quaking asps grow there, while at 

 the east there are no trees at all. 



If one travel westward on one of 

 these mesas a distance of one or two 

 kilometers in the spring of the year 

 he sees a gradual transition from the 

 xerophytic formation of cactuses, Yucca, 



