Igor] VEGETATION OF WESTERN TEXAS 121 
waging a successful struggle against grass vegetation. A further 
result of temperature conditions upon the grass formation is to 
determine, in conjuncticn with moisture, the floristic content of 
the subordinate elements in these formations — the mixture of 
annuals and lignescent perennials with the grasses. 
With regard to moisture conditions in general, scant rainfall, 
low humidity, and brilliant sunlight, are such as to give the 
grass vegetation a pronounced xerophytic structure. In this 
respect the grass formations agree with those of the dry plains 
westward, in which, with the approach of the resting period, 
coincident commonly with rainless periods, the aerial parts 
become ‘“‘cured;’’ that is, they die, retain without loss their 
nutritive qualities, and remain in a good state of preservation. 
Since the’ 98th meridian is a mere arbitrary boundary, 
selected with reference to separating approximately the xero- 
phytic vegetation regions from the mesophytic, it is evident that 
at the eastern border of the Rio Grande plain, especially in the 
coast region, the grass formations approach more nearly the 
mesophytic structure; while at the west, along the Rio Grande, 
they approach the pronounced xerophytic aspect of distinctly 
arid regions. 
With respect to the relation of grass formations to woody 
formations in the Rio Grande plain, the encroachment of the lat- 
ter has been so vigorous as practically to destroy continuous 
areas of open grass formation. Much of the province is cov- 
ered by impenetrable thickets of chaparral. There are broad 
stretches of savanna where the grass formation is more open, 
but the areas are studded with isolated individuals or clumps of 
live oak, or by open post oak formation. 
On the basis of geologic structure and soils, three types of 
grass formation may be distinguished: (1) those of the flood 
débris plains; (2) those of the sand plains; and (3) those of 
the coast prairie. The first two are by far the most extensive 
and important. The third is but a slight extension of the coast 
prairie formation which is so characteristic on the Gulf coast 
from central Louisiana westward to our region. 
