Igor | VEGETATION OF WESTERN TEXAS 209 
the formations differing somewhat in temperature and moisture 
conditions. But the annuals of the prairies in the west Texas 
region, as well as those in other parts of the Lower Sonoran zone, 
constitute a vegetation phenomenon which periodically over- 
shadows everything else. This phenomenon is the annual wave 
Fic. 11.— Hills covered with xerophytic timber, central mountainous region of 
Edwards plateau, 
of vegetative growth and floral display which sweeps over the 
prairies, temporarily submerging the grass vegetation itself. In 
the mutual adjustment of grasses and herbaceous annuals in this 
climate, the latter, mostly pure mesophytes, must take advan- 
tage not only of the returning rainy season but of the optimum 
period preceding the intense summer heat, and of the absence 
of competing grasses at that time of the year. It is a matter of 
record among cattle men that in the early spring, when grass 
forage is scarce, their herds often feed largely upon the 
