Vol. 51 | No. 7 
BULLETIN 
OF THE 
TORREY BOTANICAL CLUB 
JULY, 1924 
Across the Sonoran Desert 
FORREST SHREVE 
The type of desert so well known in southwestern Arizona 
extends southward into the Mexican state of Sonora for about 
300 miles with relatively slight changes in the general physiog- 
nomy of the Vegetation and in the local distribution of the plant 
communities. This natural province, which may well be termed 
the Sonoran Desert, is bounded on the north on the southern 
edge of the Colorado Plateau, in central Arizona, on the east by 
the Sierra Madre Occidental, and on the west by the Colorado 
River and the Gulf of California. To the southward it merges, 
at first gradually and then rapidly, into the xerophytic tropical 
thorn-forest which is characteristic of the west coast of Mexico 
south of Cape Corrientes. Many plants characteristic of the 
Sonoran Desert extend to the west of the Colorado River and 
thence northward, and many of them occur in Lower California 
and in the more elevated desert regions of Chihuahua and 
Coahuila. Considered as a whole, however, the Sonoran Desert 
is a very distinct area with respect to its plant life. In. the vari- 
ety of its plant communities, the relatively large number of 
species of striking growth-form, and in the abundance of suc- 
culents, it is very distinct from the Colorado and Mojave Deserts 
and from the deserts of Chihuahua and Coahuila. 
The distinctive character and relative luxuriance of the 
Sonoran Desert may be attributed to the occurrence of bi- 
seasonal rainfall. Many of its characteristic plants fail to range 
into the Californian region of low rainfall confined to the winter 
and spring. The mountain barriers to the north and east offer 
adjacent territory with higher rainfall, but they also present 
{The BuLLETIN for June (51: 225-282) was issued July 12, 1924.] 
283 
