1901] VEGETATION OF WESTERN TEXAS 267 
some elements of the forest belt spread into regions far beyond 
its normal boundaries. Thus, the Red river, Trinity, and Brazos 
carry purely mesophytic forests to the g7th meridian, and heavy 
timber far into the central province; and the Colorado, aside 
from the timber of its cafion course, carries big growths of pecan 
and sycamore beyond the rorst meridian. 
Floristically the streamway formation does not include the 
16.— Chaparral formation on Staked plains ee Big springs; a north- 
deilovas extension of the trans-Pecos Sonoran chaparral 
Species which give the east Texas forest its distinctive character. 
The pines, gums, magnolia, maple, and water oaks are not pres- 
ent, but on the contrary a selected group of species, which more 
readily adapt themselves to conditions of the open country, 
greater exposure to dry winds, greater sunlight, and less avail- 
able water supply. Such are the hackberry, pecan, sycamore, 
elms, and some oaks. These species are constant associates in 
the streamway timber of the prairies. Farther out on the plains, 
toward the headwaters of the rivers, after the streamway tim- 
ber above described has ended, the plains cottonwood occurs in 
its characteristic ‘ groves”’ about basin springs, as, for example, 
at Big Springs, and in similar places in trans-Pecos Texas. 
