758 
Journal of Agricultural Research voi. xxvm, No. s 
BOTANICAL COMPOSITION 
A typical mesquite thicket contains no other trees or shrubs than the mes- 
quite ( Prosopis glandulosa Torr.)* In a large part of the area where the mesquite 
dominates, however, there are several other shrubs mixed with it. Here Atriplex 
polycarpa (Torr.) S. Wats., Dondia intermedia (S. Wats.) Heller, Atriplex linearis 
S. Wats., and Lycium parviflorum A. Gray are found on low hummocks between 
the mesquite trees. These hummocks are accumulations of light soil, usually 
sand, blown about by the wind and lodged around the bushes. 
APPEARANCE 
The appearance of a mesquite thicket depends on the abundance of the trees. 
Where conditions are favorable the mesquite trees form impenetrable thickets. 
FiO. 13.—A sketch illustrating the distribution of mesquite in desert valleys. This distribution is de¬ 
pendent on soil moisture, either on a high water table, where the soil is heavy or, where the soil is loose 
and rocky as at the base of slopes and along water courses, on the deep penetration of flood waters follow¬ 
ing rains. 
But in the large areas where the mesquite mixes with the desert-sage, there are 
wide open spaces covered with hummocks of desert-sage and sometimes seep- 
weed. 
PHYSICAL CONDITIONS INDICATED 
Soil Moisture 
Like the seepweed, the mesquite grows in two very distinct zones, with 
regard to soil moisture: (a) Areas in the lowlands where the water table is high, 
but not high enough for such shrubs as saltbush, seepweed or arrowweed. Here 
the growth is best, forming dense, often impenetrable, thickets. The soil is 
heavy, a clay loam, and usually saline, but not excessively so; ( b ) areas along 
washes and on hillsides where it is mixed with other plants forming the “hills 
mixed’’ association just above the creosote bush association or the areas in the 
sandhills where it is mixed with chamiso (Fig. 13). The soil of the areas in the 
second group is just the opposite of the bottom lands, being either almost pure 
sand or in the “hills mixed” areas a stony, loose loam, well drained and non- 
