284 SHREVE: ACROSS THE SONORAN DESERT 
conditions of lower temperature into which the desert forms are 
unable to move. To the south the slightly greater rainfall and 
the much higher temperature conditions are responsible for a 
distinct type of vegetation in which only a few of the Sonoran 
orms are represented. The precipitation is least along the 
western edge of the desert and increases with altitude toward 
the east, ranging from about 4 inches to about 16 inches. The 
temperature conditions in the northern part of the Sonoran 
Desert are such that at Phoenix and Tucson frost occurs several 
times every winter, but a freezing temperature never persists 
throughout the day. At Guaymas freezing temperatures are 
unknown. 
The vegetation of the northern portion of the Sonoran Desert 
has been described and illustrated by several writers,* with 
particular reference to the Tucson region. It is the purpose of 
this paper to describe some of the salient features of the vegeta- 
tion as observed in an expedition from the Desert Laboratory, 
at Tucson, to the shores of the Gulf of California at a point half 
way between Guaymas and the mouth of the Colorado sate 
This trip was made by automobile in November, 1923, i 
mediately following the first of the winter rains. The course 
lay southwest from Tucson to the International Boundary, which 
was crossed at El Sasabe, thence south to Altar, west to Pitiquito, - 
and then south and west to the shores of the Gulf at Libertad, 
thence southward to Punto Kino. The return was made by 
essentially the same route. 
It happens that the International Boundary between El Paso 
and Yuma lies very close to the divide separating the drainages 
which run northward into the Gila River and those that flow 
south and southwest into the Gulf of California. The approach 
to El Sasabe from the north lies across the grassy plain known as 
the Baboquivari Valley. The entry into Mexico is made among 
low hills at an elevation of 3500 ft. Another extensive grassy 
plain is then traversed, falling gradually south and east to the 
Rio Seco. The coarse, granitic loam of this plain supports only 
* MacDougal, D. T. a liao of North American Deserts. 
Pub. 99, Carnegie Inst. Washn., Hornaday, W. T. Camp Fires on 
Desert and Lava. New York, agian 1908. Spalding, V. M.  Dis- 
tribution and Movements of Desert Plants, Pub. 113, Carnegie Inst. Washn., 
