200 BOTANICAL GAZETTE [SEPTEMBER 
with the Upper Sonoran areas to the north. But within the 
province itself there exists this diversity, that the southwestern 
border receives much from the arid Sonoran flora of the south- 
west, at the northeast the incoming flora is a semi-mesophytic 
one from the northern prairies. This relation to the flora north- 
Fic. 8.— Edge of chaparral thicket, Port Lavaca. 
ward is promoted by the continuation of the red beds formation 
as far as southern Kansas. 
The ecological conditions are most favorable for a pure grass 
plains formation. While it lies beyond the zone of chaparral 
formations, the mesquite already covers most of the western 
half of the province, undoubtedly modifying the structure and 
content of the grass formation. Asto the general type of grass 
formation, the semi-arid climate makes of this a characteristic 
dry grass plains flora. Without a more specific study of this 
flora it will be impossible to show satisfactorily wherein lies its 
