1901 } VEGETATION OF WESTERN TEXAS II! 
transition flora in the west, all of which gradually disappear 
in the prevalence of intensely xerophytic species. On the other 
hand, increasing aridity has intensified the Mexican semitropical, 
the south plateau Sonoran flora, and the Great plains flora. The 
transition from the eastern mesophytic to the western xerophytic 
zones presents interesting phenomena of geographic range. For 
Fic see tet in the Texas region, in inches per year.—From Hill’s 
Phys. Geog. Texa ‘ 
example, /uglans nigra is succeeded by /. rupestris, and Juniperus 
Virginiana by J. sabinoides and other species. Prominent Austro- 
riparian and semitropical species, like Pinus palustris, Magnolia 
grandifiora, and Persea Borbonia, abruptly stop; and xerophytic 
Lower Sonoran and semitropical species as abruptly begin, as 
Prosopis juliflora, Acacia Farnesiana, and Parkinsonia aculeata. 
WIND. 
The enormous expanse of the plains area in Texas and the 
daily heating of a thinly covered soil promote movements of 
