634 Part IV. Chapter 5. 
A. Yuman District. 
This district includes the Colorado Desert, which occupies a great valley, 
or depression, 180 miles (280 km) long and 30 to 50 (50—80o km) wide, the 
valley of the Gila River and the country south of the plateau coincident with 
the New Mexican-Arizonan deserts previously described (ante p. 574). 
It includes the delta country of the lower Colorado River. The vegetation 
of these deserts as noted before (p. 585) is of the most varied components. 
THORNBER has tabulated the various desert species from the standpoint of habi- 
tat and vegetation forms, and if one views the desert landscape consisting of 
Tumamoc Hill, where the desert laboratory is situated, the mesa-like mountain 
slopes, the floods plains about Tucson, Arizona, he sees the plants species 
numerically as follows: trees 15; shrubs 39; woody twiners 5; dwarf shrubs ı7; 
half shrubs 32; perennial herbs 108; biennial herbs 3; long-lived annual herbs 
57; winter annuals ı22; summer annuals 44; algae 7, a total of 449 distinct 
species’). 
Desert Formation. The woody vegetation of the desert in the vicinity of 
Tucson, Arizona consists of Larrea mexicana (= Covillea tridentata), several 
species of Opuntia, Ephedra trifurca, Prosopis juliflora, Acacia Greggü, Cereus 
(Carnegiea) giganteus (upon the foothills), Parkinsonia microphylla, P. Tor- 
reyana, Fouquiera splendens, and two species of Lycium, such as /. Fremonktii. 
A large number of species with perennial root-stocks and bulbs push up shoots, leaves and 
flowers under the stimulus of the rising temperature and moisture supply. Such vegetation 
awakens in January, blooms early in February and matures fruit in March ia ie Such winter 
perennials are Brodiaea capitata, Anemone sphenophylla, Pentstemon Wri rightii, P. Parryi, Hilaria 
utica, Cassia Covesii, Franseria deltoidea, Encelia farinosa, Covillea ee Baileya multira- 
diata, and Calliandra eriophylla. Among the winter annuals near Tucson which spring from: ger- 
minating seeds in the wet winter season and soon form fruit and seed for the next growth are 
species of Astragalus, Harpagonella Palmeri, Pectocarya linearis, Plantago, Phacelia, Amsinckia, 
Daucus, Bowlesia, Eremiastrum, Microseris, Eschscholtzia mexicana, according to the observations 
of Mac DousAL. The spinose and succulent forms of the early, dry summer are Echinocereus 
Fendleri, Opuntia versicolor, O. spinosior, O. fulgida, O. Bigelowii. The hillsides are yellow with 
the flowers of Parkinsonia mierophylla and P. Torreyanus, while the desert here is characterized 
also % summer perennials, summer annuals and plants of the dry after summer. 
ver Bank Formation. The banks of the Agua Fria River?) and its tributaries in the 
PR region of central Arizona were in many places covered with large celumps of Prunus demissa, 
Rhamnus californica, Platanus racemosa, Fraxinus velutina (= F. pistaciaefolia), ), Juglans californica, 
covered with vines of Vitis arizona. In many places, the river bed was a complete tangle of 
Fallugia paradoxa, Baccharis glutinosa and SE 
1) SPALDING, V. M.: Distribution and Movements of desert Plants. Publ. ıı 3 Carnegie In- 
stitution of Washington. 1909; MENDENHALL, W.C.: Some desert watering Places in S. E. Calif. 
and S. ev. U. S. Geol. Surv. Water Supply Paper No. 224; STANDLEY, PAUL C.: Notes on 
the Flora of the Pecos River National Forest; Muhlenbergia V: 17—30; Mac DoucaL, D. T.: 
The Course of the vegetative Seasons in southern Arizona, The Plant World, Sept., Oct., Nov., 
Dec. 1908. 
2) Toumey, J. W.: A Bit of the Flora of central Arizona, The Botanical Gazette XVII: 
162—164. 
