722 
Journal of Agricultural Research 
Vol. XXVIII, No. 8 
Herbarium material was collected whenever necessary. The plant communi¬ 
ties have been studied in local areas and the classification applied over a wider 
area. The factor studies have included soil moisture determinations at the 
period of maximum soil moisture content and at the end of the growth period 
during the drought season. These soil moisture determinations have been 
interpreted by means of the wilting coefficient determinations on the same sam¬ 
ples either made directly or by means of the moisture equivalent. 
The soluble salt content of the soil has been determined by means of the 
electrical resistance method calibrated by means of gravimetrical determinations, 
and the composition of the soil salts has been determined in each region. For 
climatic and weather conditions we have relied on the United States Weather 
Fig. l.—*Sketch map of the southwestern desert region, showing the location of the areas in which detailed 
studies of the vegetation have been made. The heavy black line shows the general area studied. 
Bureau records, since detached determinations continuing throughout a year or 
two, or only a short period, give little reliable summation of the climatic con¬ 
ditions or indication of the normality or abnormality of the weather conditions. 
When cultivated crops were grown, direct correlation was made between vegeta¬ 
tion and crop production. 
LOCATION OF STUDIES 
In this paper a study has been made of the vegetation of Coachella Valley, 
Calif., and of the Gila Valley in Arizona. Less detailed studies have been made 
at Casa Grande, Ariz.; Las Vegas and Searchlight, Nev.; Death Valley Junction, 
Saratoga Springs (Death Valley), Goffs, Needles, Hesperia, and the Mohave 
Desert, Calif.; and St. George, Utah (fig. 1). These studies were made during 
the years 1914, 1915, 1916, and 1917, the observations made during 1917 covering 
a more extended area. Similar studies have also been undertaken in New Mexico, 
the data from which will appear in a later publication. 
