1901 | VEGETATION OF WESTERN TEXAS 123 
THE SAND PLAINS.—These sandy grass plains constitute the 
most considerable feature of the southern half of the Rio Grande 
plain, and arms from them reach well toward the northern 
boundary. In some areas the sands are so deep and shifting as 
to render any stable vegetation impossible. Such areas are not 
yet well enough known to discuss here. The conditions as 
regards earth moisture in the sand plains are such as to leave 
them covered with grass vegetation and accompanying herba- 
ceous plants when the flood débris plains are bare of the corres- 
ponding formation. This is because they are not only better 
receiving areas, but the underground water is more available, for 
as the altitudes are low the distance to water is not great, and 
the open texture of the ground aids in that short distance in 
bringing water to the surface vegetation. 
The grass formations of the sand plains are different from 
those of the flood débris plains in two important respects ; first 
in the more open character of the formation, and second in their 
floristic content, in which the secondary elements —the prairie 
annuals and lignescent or succulent perennials—are especially 
involved. The open character of the grass formation permits an 
uncommonly varied growth of sand plains species, Many of the 
more important of these are of semitropical affinities, and hence 
not found in the more northerly or more elevated provinces. 
THE COAST PRAIRIE.—This is typically a sod prairie with 
grasses and sedges of mesophytic requirements, and annuals 
similarly adapted to wet, Jow, coast lands. Such prairie is con- 
tinuous between Houston and Corpus Christi, except for stream- 
way interruptions; though in passing westward along the line of 
decreasing rainfall the formation gradually becomes of xero- 
phytic stamp, and the semi-marsh land species are succeeded by 
grasses of the plains. Within the province of the Rio Grandc 
this formation passes into that of the sand plains. 
UNIVERSITY OF TEXAS. 
[ Zo be continued. | 
