122 PLANT WORLD. 



its higher range. This is but one example of a 

 dozen that might be given to illustrate the higher distribution 

 of this king of desert species. Acada ^^reg^gu, at this same 

 altitude of about 4500 feet, while developing to tree-like 

 size in the water courses, was seen in places to spread all 

 over the seemingly desiccated bluffs adjoining. Again, be- 

 tween Mescal and Pantano, the former at 4000, the latter 

 at 3400 feet, is a drift area gently sloping westward, the 

 easternmost extension of the Tucson Plains. On this are 

 found Acacia gri'ggU, Zizphiis lycioides and the mesquite 

 as typical constituents of the upland society. 



Descending from the Rincon mountains toward Vail by 

 way of \Iountain Springs Ranch, Acacia greggii is again 

 met growing generally over the open foothill country, here 

 not over 3400 feet above the sea. With it is associated, in 

 even greater numbers than in the 1 ucson mountains, CcJt'is 

 pallida, yet there are no rock outcrops, and no washes. But 

 most interesting of all, because so strictly confined to certain 

 washes lower down, is the appearance of a third species, 

 Parkinsonia torrcyana, as a common and evenly distributed 

 member of this upland society. Another significant fact is 

 that its sister species, Parkinsonia microphylla, at lower eleva- 

 tions a very conspicuous and prevalent element of similar 

 landscapes, is absent. Not until Vail is nearly reached, does 

 it appear. Finally, approaching the Santa Rita Mountains 

 at Helvetia, beginning with the first gentle rise where Lippia 

 was seen, at probably 4000 feet, the only four shrubs that 

 are thoroughly characteristic of the upland are again the 

 four most typical wash species of Tucson, viz.. Parkinsonia 

 torreyana, Acacia gt'eggii, Prosopis and Zizyphiis. Again, 

 as near Vail, Parkinsonia microphylla is conspicuous by its 

 absence. Similarly, Acacia grcggi has left behind its 



congener. Acacia conslricta, although at lower altitudes the 

 latter species is a close and abundant companion and follows 

 the washes and gullies much further toward their head. 



