12 DESERT BOTANICAL LABORATORY 



ocotillo and lechuguilla on rocky places. The soil under the creosote 

 bush is almost always gravelly. 



As the road rises, near the station of Strauss, to the plateau and 

 crosses it westward at an elevation of a little more than 4,000 feet, the 

 same flora appears as that of the Tularosa desert; mesquite, Tucca 

 radiosa^ and grass, or either of these plants alone with the grass, and 

 Ephedra a frequent accompaniment. Creosote bush occurs only oc- 

 casionally and on gravelly slopes near desert hills. 



TUCSON AS A LABORATORY SITE. 



The woody vegetation of the desert in the vicinity of Tucson con- 

 sists chiefly of creosote bush ( Covillea tridentatd) interspersed with 

 several species of Opuntia^ most of them with cylindrical stems (Plate 

 IX), and occasional plants of joint pine {Ephedra triftirca^ and 

 barrel cactus {Echinocactus), w^ith an abundance of mesquite {Pro- 

 sopis^ and cat's claw {Acacia greggii) (Plate X) in the lower drain- 

 age areas. Upon the foothills occur in addition the giant cereus( Cereus 

 giga7iteus^ see frontispiece), two species of palo verde {Parkinsonia 

 microphylla ixnd P. torrey ana) ^ocoiiWo {Fouquieria splendens)^ two 

 species of Lycluni and many other woody plants. This is of course in 

 addition to a great variety of annual vegetation which may spring up 

 between the sparse shrubs after any drenching rain, particularly the 

 repeated slow rains of winter. 



In recommending a site for the Laboratory the Board kept in mind 

 four principal requirements : 



1 . A distinctly desert climate and flora. 



2. A flora as rich and varied as possible, while still of a distinctly 

 desert character. 



3. Ready accessibility. 



4. Habitability. 



Much of the arid region of the western United States is only par- 

 tially or relatively arid and does not therefore contain those pronounced 

 types of drouth resistant vegetation which it is the first object of the 

 Laboratory to investigate. Such semi-desert areas are the western por- 

 tions of Kansas and Nebraska, and the intramontane valleys of south- 

 ern California. Another sort of location, to be avoided for a like 

 reason, was a desert which was likely to be reclaimed by irrigation, 

 such as that about Phoenix, Arizona. The desert character of a small 

 area, even though carefully reserved, might be seriously modified by 

 seepage or other changes following irrigation development in the 

 vicinity. 



