SHREVE: ACROSS THE SONORAN DESERT 287 
Carnegiea gigantea, Acacia paucispina, Opuntia fulgida and 
Fouquiera splendens. In the vicinity of Tucson Olneya is con- 
fined to the warmest situations, sheltered south exposures which 
are not subject to the influence of cold air drainage. In the 
valleys between Pitiquito and Libertad it is much more ubi- 
quitous, and occurs abundantly in situations where cold air 
drainage operates to produce minimum temperatures that are 
considerably below those of the upper slopes and foothills of 
each valley. The behavior of Carnegiea remains unchanged, 
however. In the Tucson region it is much more abundant on 
the south slopes of hills than on the north, and this is true of it 
down to the point at which it begins to be infrequent and to 
yield its place in the landscape to Pachycereus. The latter 
cactus in turn, and also Lemaireocereus, are like Carnegiea in 
exhibiting greater abundance on south slopes than on north ones. 
arkinsonia microphylla and Cercidium Torreyanum are both 
abundant down to the Gulf, showing no change in their habitat. 
Parkinsonia aculeata, which is widely cultivated throughout the 
southern United States, is first seen within 60 miles of Tucson. 
It is nowhere a common tree but occurs singly or in small groups 
at widely separated localities. 
The Arizona shittimwood, Bumelia rigida, is a tree known 
only from four localities in Arizona, from the Dragoon Moun- 
tains to the Baboquivari Mountains. Its foliage and the char- 
acter of the localities in which it is found, indicate that it isa 
tree of relatively high water requirement. A closely related 
and very similar species, B. occidentalis, was detected at three 
places in Sonora, one of them near a natural water hole, and 
the second near a well. The third locality was in the center 
of a large valley where there are no wells, and it was interesting 
to note that an unsuccessful well had been dug to a considerable 
depth near the Bumelia trees, indicating that the high water 
requirement of the tree had not gone unobserved by the Mexican 
ranchers. 
The valleys lying east of the Sierra Picu have a deep loam 
soil and a rich cover of vegetation. Covillea is infrequent here, 
and large trees of Parkinsonia, Olneya and Prosopis are inter- 
mingled with Carnegiea, Pachycereus, large colonies of Lopho- 
cereus, occasional colonies of Lemaireocereus, and frequent groups 
of a large Yucca, often reaching 25 ft. in height. Smaller cacti 
