SEASONS IN SOUTHERN ARIZONA 



269 



not exhibit any marked xerophilous qualities, except that 

 the dormant seeds or quiescent roots are capable of with- 

 standing great periods of drought without damage. 



The greater proportion of the area, however, is occu- 

 pied by true desert species, which by structure and physiolo- 

 gical capacity are well equipped for the arid conditions 

 prev^alent. In some, structures which prevent loss of water 

 are most noticeable, while in others, the capacity for the 

 absorption an.d retention of surplus water during times of 

 precipitation is developed to an enormous degree. 



Fig. 4. — The Desert Laboratory, on Tumamoc hill, Tucson mountains sur- 

 rounded by a vegetation in which winter perennials are an important 

 feature. 



A number of species are found over its entire breadth 

 from elevations of 3,000 and 4,000 feet in southeastern 

 Arizona to the dunes near the shore of the Gulf, a distance 

 of 200 miles. Among these are to be included Cereiis gigau- 

 teus, Covillea tridentata, EnceVia farinosa, Prosopis vehitina, 

 Fouqiiicria splcudcus and Park'uisouia microphyUa, all of 

 which are essentially desert species, showing marked xero- 

 phytlc structures. Some of these species extend hundreds of 

 miles north and south of the region under discussion, with 



