272 BOTANICAL GAZETTE [ OCTOBER 
have a pure formation of straggling Parkinsonia Texana, and 
basalt ridges are thickly covered by a short bushy formation of 
Acacia Berlandieri. The prevalence of Mimoseae and Caesalpineae 
in the Rio Grande chaparral deserves special mention. No less 
than 30 per cent. of the number of species and a far higher per- 
centage of actual individuals would represent the strength of 
these elements in the formation, This also emphasizes the 
semitropical affinities of the formation. The condition now 
prevalent in the Rio Grande province represents a state of 
unstable equilibrium between chaparral and grass formations (fig. 
These conditions are of a nature to favor the encroachment 
ofthe chaparral. This interesting phenomenon is discussed else- 
where in this paper. | 
Coming next to consider individual features of the chaparral, 
certain conspicuous types are of special prominence. 
Prosopis juliflora or mesquite formation.—1n the Rio Grande 
plain this occupies the level areas of fine compact silty débris. 
Similar ‘‘ mesquite flats,” as they are commonly called, are found 
in similar situations throughout the lower Sonoran area in Texas, 
but the species also occupies black prairie soils in some areas. 
The most massive formation of mesquite occurs on the silty 
débris plains on the northeastern border of the Rio Grande 
plain near San Antonio. From thence southeastward the country 
is a vast mesquite forest some fifty years old and fifteen to 
twenty feet tall. In the lower Rio Grande valley individual trees 
attain (for this species) gigantic size, reaching two feet in 
diameter and thirty-five feet in height. 
Acacia Farnesiana or huisache formation —This prevails espe- 
cially on the low moister coast prairie clays. The species is 
more limited than the mesquite in both moisture and temperature 
requirements. Its spread has been on the coast prairie as far 
east as Galveston. Although cultivated as far inland as at Aus- 
tin, its natural spread is not much beyond the coast country and 
the lower Rio Grande. 
Acacia amentacea formation —This formation is the prevalent 
one in the northern half of the Rio Grande province. It is the 
