278 BOTANICAL GAZETTE [OCTOBER 
formations of this type are also in the main different from those 
-of the other two. With few exceptions, this latter might be 
designated as a kind of rock vegetation, for the most significant 
elements are constantly associated. with high, arid, stony slopes. 
As an ecological type the succulent vegetation is to be regarded 
as a more efficient adaptation to extreme aridity than the grass 
or woody vegetation. Consequently, it is to be found in its most 
typical display, in the arid regions of trans-Pecos Texas. 
In this discussion little account can be taken of the numer- 
ous ephemeral species like those of Sedum, Talinum, Portulaca, 
and others, but the consideration is particularly given to those 
more evident as features in the landscape. Of the elements 
composing this more conspicuous vegetation, there are three 
types, each forming a distinct and close genetic group. These 
are the Cactaceae, the Yucceae, and the Agaveae. A brief con- 
sideration of these may precede the discussion of the three types 
as associated in a single formation. 
THE CACTUS VEGETATION. 
A rather general survey of this group gives three types, 
each associated with a distinct physiographic feature: (1) the 
flat-jointed opuntias of the Rio Grande plain and central Texas ; 
(2) the woody cylindrical-stemmed opuntias of the high plateau 
plains ; and (3) the cylindrical and spherical cacti of arid stony 
slopes. 
Opuntia Lindheimeri formation —The prevalence of this forma- 
tion in the Rio Grande plain has already been noted in discuss- 
ing chaparral vegetation. The species here considered has 
spread beyond the Rio Grande plain to the north and ele 
beyond the Colorado. To the east and southeast of Austin 
there are some extensive and typical prickly pear fields. It 1s 
present with the mesquite in the granite country in great abund- 
ance. In the Rio Grande plain the quantity of vegetation 
formed by this species is of almost incredible bulk. The level 
silty débris plains are its common habitat, but stunted forma- 
tions of it also occur on arid gravelly soils, and on chalk soils 
