1901] VEGETATION OF WESTERN TEXAS 105 
exhibited in their general aspect, is the most convincing demon- 
stration of the identity of this region with the northern prairies, 
and its small affinity with the southern ones. The cultural pos- 
sibilities still further emphasize the extremes of these two 
divisions of the Lower Sonoran zone. As regards moisture con- 
ditions, both are transitional between Austro-riparian and Lower 
Sonoran, but the Rio Grande plain is in the zone of rice, cotton, 
sugar cane, of fruits like the fig and pomegranate; while the 
northern prairie province is in the belt of corn, wheat, and oats, 
and of such fruits as the apple. In short, the two provinces are 
transitional both as to temperature and moisture zones, the Rio 
Grande to the semi-arid and arid tropical, the central prairies to 
the Upper Sonoran and Carolinian zones. 
The extreme in the direction of Lower Sonoran arid condi- 
tions is reached in the bolson desert provinces of trans-Pecos 
Texas. Certainly westward of the prairie plains of the Stockton 
plateau one is within the arid plateau province of that part of the 
Lower Sonoran zone, which then prevails westward to the Pacific. 
Indices of this arid province are the Yucceae, Agaveae, Cacta- 
ceae, and genera like Fouquiera, Larrea, and Flourensia. Ele- 
ments of this extremely arid portion overlap to some extent 
portions of the Edwards plateau and of the south debris slope 
of the Staked plains, as will be seen subsequently. Finally, the 
entire area covered by the Lower Sonoran provinces is occupied 
by the single and generally dominant Prosopis juliflora (mes- 
quite), which, though dependent upon certain peculiarities of 
soil structure, is absent from no considerable portion of the 
zone, and in its growth and occurrence reflects the measure of 
climatic conditions prevailing in the several provinces. This 
Species marks well the transition from Lower to Upper Sonoran 
on the Staked plains. 
Upper Sonoran.—This zone occupies the Staked plains and 
Panhandle above 3500 feet, the higher slopes and summits of 
the Guadalupe (except the highest points in the range), Davis, 
and other northern mountains above 4000 or 4500 feet, and of 
the Chisos mountains in the Great Bend above 6000 or 7000 
