1910] LIVINGSTON—SOIL MOISTURE 245 
river plain, where it is more characteristic. Numerous low forms 
are active throughout the year, notably Encelia farinosa, Sphaeral- 
cea pedata, etc., and a great array of summer and winter forms 
are to be found at the proper season. 
2. The Larrea society of the gravelly slope exhibits, in many 
localities, almost no woody species other than the creosote bush 
(Larrea tridentata). The bushes are uniformly and openly dis- 
tributed over a surface which is otherwise nearly bare of vegetation, 
excepting during early spring and late summer, when numerous 
low forms appear. Of the desert shrubs within its climatic range, 
Larrea seems best fitted to withstand prolonged drought both of 
air and soil. Ephedra occurs upon the slope as well as in the wash, 
but makes its best growth in the latter place. 
3. The Cercidium torreanum society of the wash is characterized 
by this form of palo verde and by mesquite (Prosopis), Condalia 
lyciotdes, and Acacia Greggii. The three latter forms are also 
characteristic of the Prosopis society of the plain. Ephedra 
trifurca and the creosote bush (Larrea tridentata) attain here their 
most luxuriant growth, though the latter, at least, is most char- 
acteristic of the Larrea society. Aside from these and some other 
woody plants, the vegetation of the wash consists mainly of small 
forms, which are active only during the periods of moist soil, 
either in winter or summer or at both seasons. 
4. The Prosopis society of the river plain is dominated by 
mesquite, two forms of Acacia (A. constricta and A. Greggii), and 
Condalia lycioides. Annuals of many forms are numerous during 
the moist periods. The most striking of the low perennials is 
Bigelovia Hartwegii, which scarcely occurs outside this group, 
except for scattered plants in the Parkinsonia society. 
METHODS OF DETERMINING AND OF REPRESENTING SOIL CONDI- 
TIONS.—The method used in determining the water-holding powers 
given above was to place the soil in a sheet-metal cylinder about 
12 cm. high and ro cm. in diameter, with perforated bottom (the 
latter covered externally with thin cloth), then to saturate the soil 
y standing in a vessel of water, at the same time pouring water 
on the soil surface, and finally to allow the soil mass to drain till 
no more liquid water came away below. From the weight of the 
