32 fl> o s t e i s i a 



Astragalus, sage brush and desert 

 grasses to the semi-xerophytic pine 

 groves with their Pulsatillas and lark- 

 spurs. The shrubs Cerco carpus, Rides, 

 Edwinia and the skunk bush [Rhus 

 trilobata) grow among the pines or 

 east of them for a distance. The last 

 named shrub is abundant on the north 

 slopes of mesas, even well out from the 

 hills. If one look westward from the 

 plains into the ravines between two 

 adjacent mesas, the south side of the 

 ravine, i. e. the north slope, will be 

 seen well covered with skunk bush, 

 while there are but few of these plants 

 on the south slopes. 



Sometimes a mesa will have cer- 

 tain shrubs growing near the top on 

 the south slope, but none farther down 

 on the same slope. Since it is mois- 

 ture chiefly which determines the dis- 

 tribution of the shrubs, this may seem 



