Plant Associations at the Desert Laboratory. 59 



of the species just named are prominent constituents of the 

 vegetation of these washes, but since their characteristic habitats 

 are elsewhere, other species have been chosen to give their name 

 to this association, viz, the palo verde {Cercidium Torreyanum) 

 and catclaw (Acacia Greggii). 



At present, as the process of base-leveling slowly proceeds, 

 these species are advancing from the flood-plain toward the adja- 

 cent hills. Cercidium Torreyanum is already found far up one of the 

 * gulches of Tumamoc Hill. The catclaw deports itself in the 

 same wav. Ephedra trifurca, another characteristic species, 

 ranges still more widely, approaching the habits of the creosote 

 bush in attaining its best development where there is an abund- 

 ant water supply, but like the latter, capable of maintaining 

 itself successfully where the water content of the soil is lower, 

 especially on light, more or less sandy soils. 



A few other woody species find in this habitat a congenial 

 home. Condalia spathulota attains here its best development. 

 Condalia lycioides, a companion of the mesquite, advances with 

 it up the washes, and Celtis pallida, here a plant of the cliffs, also 

 occurs in the wash, absolutely avoiding the intervening slopes, 

 thus presenting further evidence, if such were needed, that soil- 

 water is a factor of prime importance in determining the distri- 

 bution of this association of plants. 



Five of the species that have been named as members of 

 this association have been observed by Blumer to change their 

 topographic location with altitude. These are Cercidium 

 Torreyanum, Acacia Greggii, Prosopis velutina, Condalia lycioides, 

 and Celtis pallida. At Tucson, at an altitude of 2,200 to 3,000 

 feet, all of these inhabit the washes, and with a single exception 

 (Celtis pallida) are hardly met with at all elsewhere; but at 

 higher elevations, 3,500 to 4,500 feet, in the neighboring moun- 

 tains of the Tucson Range, the Santa Catalinas, and the Rincons, 

 they are found spreading out on gravelly and other upland soils, 

 no longer confined to washes, and deporting themselves as ordi- 

 nary members of the shrubby upland growth there prevalent. 

 Taking these higher elevations as the point of departure, it is 

 found that even 1,000 feet lower all these species exhibit a marked 

 tendency to confine themselves to watercourses or, at all events, 

 to places where there are good conditions as regards soil-moisture 

 and some degree of protection from the more extreme desert con- 



