216 BOTANICAL GAZETTE [SEPTEMBER 
extreme than in the erosion area of central Texas, as shown in 
the following data. The annual rainfall is from 14 to 26 inches 
(approximate averages for each area respectively ); the evapora- 
tion capacity is from 80 to 60 inches; the extremes of daily 
temperature in the trans-Pecos mountains is much greater than 
in the central region. Consequently the arborescent vegetation 
is one of more absolute adjustment to xerophytic conditions, 
and the semi-mesophytic species found in the central erosion 
area are either wholly excluded from the trans-Pecos area or rele- 
gated to the watered cafions, Of the arborescent species, Pinus 
edulis, Juniperus pachyphloea, J. occidentalis, J. monosperma, Quercus 
grisea, and Q. Emoryi are the principal ones. Not all of these 
are confined to the mountain slopes, for in watered cafions some 
of them form heavy timber of large size and good quality; but 
on the slopes their growth is dwarfed, the wood hard, the stems 
gnarled and misshapen, and they stand in a scant open forma- 
tion. 
The timber formation just described extends to certain 
physiographic features east of the Pecos river, These are the 
Staked plains escarpment, the remnant buttes of the Callahan 
divide (at least west of the 1ooth meridian), the sand hills of 
the southern Staked plains, and the cafion bluffs of the Devils 
and Nueces rivers. In the last instance, it is very dilute, being 
lost in the local formation, but in the other cases it is practically 
the only timber. 
On the escarpment of the Staked plains, even as far as the 
northeastern border, there occur Pinus edulis, Juniperus occidentalis 
(in many straggling groves of small growth, according to 
Havard), and Quercus grisea. On the remnant buttes of the 
western half of central Texas there are close formations of a 
much dwarfed Q. grisea, in which this species attains a growth 
of only two to five feet, covering the ground in true chaparral 
fashion. Here also is found a very scattered formation of 
Juniperus occidentalis, which occupies also the lower mesas, becom- 
ing one of the chaparral species covering them. 
