ENVIRONMENT OF THE ROOTS. 1 3 



matter; and through the digging of two wells, one on the pump lot and 

 one on the experimental garden of the Desert Laboratory, both within 

 70 meters of the western side of the flood plain, a soil section to a depth 

 exceeding 12 meters has been obtained. 



Along the course of the river the soils are adobe clay and apparently 

 homogeneous to the depth examined. At the western side to the same 

 depth, that is, about 5 meters, the soil is of a character similar to that near 

 the river, but below 5 meters the adobe gives place to stratified sands and 

 gravels. Caliche was not found in the soils of the flood-plain to the depth 

 observed and probably none exists there. 



WEST WASH. 



West Wash lies along the western base of Tumamoc Hill and separates 

 it from the benches of the bajada which stretch farther west to the main 

 part of the Tucson mountains. It receives the drainage from the western 

 face of Tumamoc and from a small portion of the Tucson range. The 

 wash, for the present purposes, may be said to be differentiated into a 

 channel from 5 to 10 meters in width and a small plain which separates 

 the channel from the benches to the west. 



The soil of the channel is a coarse sand which reaches to an undetermined 

 depth. The soil of the plain is a sandy loam to a depth exceeding 2 meters. 

 In both channel and plain the water quickly disappears from the surface ; 

 hence, as will appear repeatedly, the conditions of plant life here are mark- 

 edly different from those of the river flood-plain, as well as from those of 

 the other habitats to be described. 



The only plant seen growing in the channel of the wash was a specimen 

 of Curcubita digitata, with a fleshy root. Along the banks of the channel, 

 which are less than a meter below the level of the adjacent plain, there is a 

 fairly heavy growth of Acacia constricta, A. greggii, Covillea tridentata, 

 Ephedra trifurca, Parkinsonia torreyana, Prosopis velutina, and Zizyphus 

 parryi, with occasional specimens of Echinocacius wislizeni. The flood- 

 plain of the wash contains all the species named as occurring along the 

 channel, but the growth is more scattering and the plants may be some- 

 what smaller. 



THE BAJADA. 



The bajada is the drainage slope of the mountains and constitutes the 

 mesa, or Covillea plain. In places in this vicinity it extends in a gentle 

 gradient, said to be about 4 per cent, for distances of 10 miles or more. 

 The slope of the bajada from the base of a mountain range, when viewed 

 in profile and at a distance, constitutes one of the most striking features 

 of desert topography. Where the slope of the bajada is short there are 

 practically no cross-drainage channels, but where it comprises a wide extent 

 of territory, physiography, soils, and other physical characteristics peculiar 

 to it are developed which serve to greatly increase the diversity of this 



