5?2 RELATION TO ENVIRONMENT. 



out the dry season, trees are present, especially the mesquite 

 trees. Tree yuccas (Y. arborescens, etc.) occur on higher ele- 

 vations, but not on the lower dry ground. These yuccas on the 



Fig. 510. 



Winter range in northwestern Nevada, showing open formations; white sage 

 (Eurotia lanata) in foreground, salt-bush (Atriplex confertifolia) and bud-sage 

 (Artemisia spinescens) at base of hill, red sage (Kochia americana) on the higher 

 slope. (After Griffiths, Bull. 38, Bureau Plant Ind., U. S. Dept. Agr.) 



mountain slopes are just beneath the coniferous forest line which 

 forms a horizontal zone on the mountain slopes, marking the 

 lower limit of snowfall in a very sharp manner from the lower 

 zone of rainfall. The snow-storage here on the desert mountains 

 provides a source of water which is doled out later in the season 

 and permits the forest growth. The tree yuccas just beneath 

 this forest probably get some of this moisture. Besides the 

 plants mentioned above, cacti, salt-bushes (Atriplex), grease- 

 woods (Sarcobatus), etc., occur. These all show the usual 

 adaptations to desert conditions, reduced leaf and stem surface 

 to check transpiration, and an increase of the root system for 

 absorption. The extent to which the reduction of surface for 

 transpiration is carried in desert plants and the corresponding 

 increase of root system for absorption is shown by Coville in the 

 case of a prickly-pear cactus examined. The aerial part con- 



