644 Part IV. Chapter 5. 
This semiarid region is largely mesquite (Prosopis) plains, varying from 
open grassy land with scattered mesquite to a scrub forest of this tree, in 
places densely filled with other thorny bushes and cactuses, as along the 
southern stream valleys and over much of the plains of the lower Rio Grande. 
Scattered oaks, according to BAILEY‘), and other scrub trees distinguish the 
more elevated, rougher parts of the region and narrow strips of good timber 
are found along some of the streams. The bulk of this chaparral consists of 
Zisyphus obtusifolius, Condalia obovata, Koeberlinia spinosa, Opuntia Engelmannt, 
O. leptocaulis, Parkinsonia aculeata, Acacia farnesiana, with Tillandsıa recur- 
vala etc. 
Fig. 30. LANDSCAPE WITH CACTACEAE IN MEXICAN STATE HIDALGO. 
Columnar form of Cephalocereus senilis (Haw.) K. Sch. — Ball form of Echinocactus ingens Zucc. 
(Engler-Prantl, Nat.-Fam. III 6a No. 2.) 
Larrea Mexicana in a formation of a single species is especially characteristic of high 
gravelly mesas and of the bolson deserts extending even to highly charged alkaline soils; it is a 
shrub and is disposed in such regular open growth as to appear like plantations. | 
Succulents, Yucca and Agave. Approaching the crest of broad undulations or any 
relief featäre visible above the general surface of the plains in trans-Pecos Texas, a unique yucca 
vegetation is seen to occupy such situations. Again on the rim of the dad bolson basins, 
ciation of yucca trees “with caudex ten to twenty feet high and one to two feet in diameter”. — 
») BAILEY, V.: See Bibl. p. 75. 
