288 SHREVE: ACROSS THE SONORAN DESERT 
and numerous species of shrubs, together with mats of the gray 
grass, Hilaria, all combine to make a highly diversified com- 
munity of plants that constantly gives one the impression of 
being in some gigantic botanical garden. 
In the Sierra Picu and the lower ranges of hills which lie 
between it and the valley of the Magdalena River, the traveler 
from the north finds more plants that are new to him than in 
the valleys. The general groundwork of the vegetation is 
composed of those species that extend north to the edge of the 
Sonoran Desert, but a large number of shrubs, cacti, vines and 
Fic. 2. Pachycereus Pringlei, 8 miles south of Libertad, Sonora. 
herbaceous plants are already added to the vegetation here 
within a linear distance of 100 miles from the Boundary. One 
of the most striking and abundant of these is Elaphrium micro- 
phyllum, the copal tree, of the family Burseraceae. In the 
largest trees the trunk emerges from the rocks with a diameter 
of 12 to 14 inches. It branches freely and dwindles rapidly in 
thickness, so that the limbs 4 ft. from the ground are seldom 
more than I or 2 in. in thickness, while the twigs are of the 
ordinary size. The cortex is very thick, forming nearly half the 
thickness of the trunk, and exudes a milky and resinous sap 
when cut. The leaves are pinnate and dark green, the twigs a 
rich brownish red. The bark is stripped from the twigs by 
