DISTRIBUTION OF SHRUBS. 121 



At Tucson, ,at an elevation of 2200-3000 feet, five 

 characteristic species that inhabit the washes, but, compara- 

 tively speaking, are with one exception almost absent away 

 from them, are Parkuisoiiui. lorrcyuiiui. Wats., Jccuici grcggii 

 A. Gray, Prosopis jiili flora (Sw.) D. C. Zizphiis lycioides A. 

 Gray and Crltis pallida l\)rr. llie first- named is confined, 

 practically without exception, to certain washes and their 

 banks; the second likewise, except that it follows the water 

 courses upward to finer ramifications, and occurs, as a few in- 

 dividuals only, on the nortiiwest slope of Tumamoc Hill; the 

 third is on the upland found only as small and widely separ- 

 ated individuals or groups of such; the fourth, usually spar- 

 ingly represented here as cisewher within the region observed, 

 is the almost invariable companion of Prosopis in' its typical 

 lowland woods; the fifth occurs either on washes where 

 considerable water is present at times, or about cliffs or ex- 

 tremely rocky places, usually facing northward. 



As the foothills of the Tucson Mountains are reached, 

 Parkiusonia torreyana at least in one place can be seen to 

 climb the hillside along with Paikinsonia inicrophyUa, Celt'is 

 pallida hides no longer behind rocks, but comes out more 

 openly, and Acacia greggii becomes a pronounced constituent 

 of the general shrub cover. One mile up Ventano Canyon in 

 the Santa Catalina Mountains, at about 3500 feet altitude, 

 is a north slope that supports Acacia grcggii, as well as the 

 Prosopis and Zizypli us. The latter pair may be found com- 

 monly on gravelly upland soil west of Tres Alamos, at about 

 4000 feet, while in another part of the San Pedro Valley, 

 east of P^airbank, at similar elevation, Zizyphiis was noted 

 on upland soil, this time alone, in company with other shrubs. 



Prosopis jiiliflora, while fringing the saccaton bottom of 

 Babocomeri Creek, and lower down developing into a fine 

 riparian forest, spreads over the bluffs on either side, dotting 

 a vast plains area gently sloping away toward the 

 distant mountains, in the low, matted form char- 

 acteristic of the species in the upland habitats of 



