196 BOTANICAL GAZETTE [SEPTEMBER 
margin near Austin) northward beyond the Red river, its eastern © 
boundary being the Black prairie or rich black land area of 
Texas. The western boundary of this province is a prominent 
and irregular escarpment overlooking the central denuded 
region, and the upper cross timbers at the north. The prairies, 
which are the dominant physiographic feature, are wide undula- 
tions interrupted by broad erosion valleys and occasional butte 
remnants of the former plain. The elevation of the province is 
from 800 to 1200 feet. The soil covering is but a thin veneer- 
ing overlying alternating beds of chalky limestones and marls, 
which are very poor receivers or retainers of moisture. This 
condition suffices to offset the comparatively high annual rain- 
fall (28 to 32 inches), so that this is a province of pronounced 
xerophytic grass formations, but still a transition from the meso- 
phytic Austro-riparian to the xerophytic Lower Sonoran zone. 
In its temperature relations the province stands almost on the 
transition to the Upper Sonoran and Carolinian zones respect- 
ively, thus differing notably from the Rio Grande plain province 
which passes into semitropical. 
Pure grass formations.— Ecologically the grass formations are 
strongly xerophytic and partake of the characters of the grass 
vegetation of the more westerly plains, though in a moderate 
degree. As already seen, the mean annual rainfall is sufficient, 
other things being equal, to give existence to semi-mesophytic 
formations. But the periods of brilliant sunshine and dry air 
which prevail, especially during the growing season, very quickly 
dry out the superficial covering of soil and drive vegetation into 
the resting stage. Under present conditions of pasturage the 
grass covering has been sadly reduced from its former occurrence. 
Formerly it was very compact and luxuriant. With the break- 
ing up of this, moisture relations have changed, and the forma- 
tion is now more xerophytic. Floristically it is impossible to 
say, with the meager data available, how the association of spe- 
cies compares with that of prairies and plateau plains westward, 
or to what extent the formation has changed under pasturage- 
It is certain from the general aspect of the formation, however, 
