118 BOTANICAL GAZETTE [AUGUST 
of moisture, these are a result of the physiographic and geologic 
peculiarities, and may occur independently of rainfall and 
humidity. 
The plant life of the region in general may be included in 
the following formations: (1) grass formations, (2) woody for- 
mations, (3) succulent formations, (4) rock formations, (5) 
halophytic formations. 
I. GRASS FORMATIONS. 
The consideration of p'ant formations in western Texas may 
begin appropriately with the grass formations, for, excepting 
only the highest mountain summits of trans-Pecos Texas, the 
climate is a “‘ grass plains climate,” and the grasses may be said 
to form the matrix of the vegetation of the region. Texas is 
thought of commonly as a land of grasses, and properly so as 
regards the portion considered in this paper. Under what may 
be called natural conditions, to distinguish them from conditions 
which prevail under the present era of exploitation, the grass 
formations held their own in the perpetual struggle against 
woody vegetation. With the advent of the cattle business, 
however, this advantage was lost, and the present is an era of 
the rapid encroachment of timber formations. These phenom- 
ena and their causes will be-specially considered in a subsequent 
paragraph. Mention is made of the matter here to explain that 
in discussing the grass formations as they now exist we are 
dealing with a vegetation which, though still the dominant type, 
has not only a more restricted distribution than formerly, but is 
undergoing perceptible changes, not only in restriction of its 
area as the dominant formation, but in the association of species 
within the formation. 
Along with the grass plains vegetation will be discussed 
those types of formation which, though distinct enough as for- 
mations periodically, never for more than a brief period form 
the dominant vegetation, and are in every case distinctly a 
prairie feature. Such are the prairie annuals (generally meso- 
phytes) and the lignescent perennials (tropophytes). 
