280 BOTANICAL GAZETTE | OCTOBER 
is conspicuous on the high grass plains of the Stockton plateau 
and westward. It also occurs east of the Pecos at the foot of 
the Staked plains, and to a less degree on the summit of the 
south half of the plains. At the base of the plains in Borden 
county it is very common and of large size, reaching seven and 
eight feet in height. This Opuntia does not occur as a continu- 
ous formation, but at intervals serving rather to punctuate the 
grassy landscape. This is always noted by travelers who observe 
the landscape of the trans-Pecos grass plains traversed by the 
railways. 
THE BROAD CYLINDRICAL AND SPHERICAL CACTI occur upon arid 
stony slopes of trans-Pecos Texas as the most extreme type. 
These associate with Agaveae and Yucceae to make the type of 
formation to be discussed subsequently. East of the Pecos, on 
stony hillsides, rock bluffs, and even on coarser gravel ridges, 
cespitose species of Cereus occur, often (as in the case of 
C. caespitosus) forming patches from a few yards to many rods 
in diameter, as on broken granite slopes in Llano county. 
THE YUCCA VEGETATION. 
The prominent yucca vegetation of the so-called ‘ yucca 
belts” is a part of a mixed formation to be considered presently. 
Independently of this, certain areas of yucca vegetation are to be 
noted (fig. 24). First, the occurrence of Yucca Treculeana in 
the lower Rio Grande and Gulf coast region, where it is said to 
form ‘‘ open stunted forests.” Again on the cafion sides in the 
central erosion area the same species (apparently ) forms a con- 
spicuous feature of the cajion flora. Second, the constant occur- 
rence of Yucca rupicola upon the talus débris of slopes leading 
down from crumbling limestone formations. The formation, of 
which this is but an element, is one of the most distinct minor 
formations of the cretaceous areas east of the Pecos. Third, 
Yucca angustifolia is particularly characteristic of the Staked 
plains region. Here it follows the gentle slopes about shallow 
basins or the wave-like undulations where the soil is looser and 
more sandy. Fourth, Nolina formations on the Rio Grande 
