198 BOTANICAL GAZETTE [SEPTEMBER 
out especially on bare chalk slopes. Sometimes a single species 
prevails upon such places, but commonly there occurs a mixture 
embracing nearly all the species. Thus a careful collection 
upon a typical chalk area will yield more than three fourths of 
the species of the typical formation (fig. 74). 
In all species the underground parts are reservoirs of food 
(and not infrequently of water) protected against the extremes 
of heat and drouth to which the arid soil is subjected. When 
the soil is wet by rains these subterranean organs quickly throw 
up vegetative organs, and perhaps flower and mature fruit before 
the brief period ends and they are again driven into the resting 
stage. The underground parts are of several types, as succulent 
fibrous roots with a thick zone of mucilaginous or saponaceous 
tissue (Krameria secundiflora, Yucca rupicola); woody subterra- 
nean tuberous caudex (Liatris pycnostachya); mucilaginous bulbs 
with impervious coats (Allium, Cooperia), and deep irregular 
or fusiform roots with hard flinty (sclerenchymatous) coat 
(Asclepias decumbens). 
THE CENTRAL DENUDED REGION.—The granite and Carbonifer- 
ous areas and the upper cross timbers together constitute a belt 
lying below the western escarpment of the Grand prairie, where 
the prairies are more or less interrupted by a more broken relief, 
which in part of the granite area becomes positively mountainous. 
This belt is marked on the more level areas by a surface cover- 
ing or by deep beds of sand and gravel, where conditions favor 
the occurrence of an open timber formation and even a close 
forest covering in the upper cross timbers. Discussed from the 
standpoint of grass formation, the granite and Carboniferous 
areas (Burnet, Brownwood, and Palo Pinto countries of Hill, 
see fig. 1) are open-timber grass prairies, which with the spread 
of the mesquite (Prosopis juliflora) are losing their last remnants 
of pure grass formation. 
The grass formation of this belt (the upper cross timbers not 
included) appears to have about the same character as that of 
the sand plains of the Rio Grande plain. The ecological condi- 
tions are very similar except as to temperature, and the results 
