PLANTS OF THE SoToL REGION 623 
rocky steppes (Felsensteppen), in this case made up of Yucceae, 
' Agaveae, Cactaceae and desert shrubs like Covi//ea, Flourensia, 
Koeberlinia, Fouquieria and various Mimoseae. That is the typical 
Sotol area. As a matter of fact, this kind of formation occurs on 
most other foothills and mountain slopes westward, and especially 
in the Great Bend region, whether the geological formation be 
limestone or not, so long as there is the unstable covering of 
stony débris. 
It would not be accurate to class all such country as desert, 
although the Great Bend region is as much so as the Sahara it- 
self; but it is all a region in which plant life is beset by extreme 
environmental conditions such as the following : 
A precarious rainfall of fifteen to nine inches annually ; an 
€vaporation capacity in the air equal to from six to ten times the 
annual precipitation; strong and constant dry winds ; a dry, arid, 
stony soil ; intense and prolonged illumination, heightened by the 
glare of the stony earth and by the altitude and the vaporless air ; 
intense heating of the air, the stony earth and the plant bodies 
growing therein, in this respect approximating the maximum inso- 
lation endured by plant life in any region. 
The species, of which only the tissues concerned in transpira- 
tion are described in this paper, were taken from the vicinity of 
Langtry on the Rio Grande near the mouth of the Pecos; but all 
of them abound in the most extreme conditions of the Great Bend 
region and elsewhere. 
1. Agave Lecheguilla. This is one of the less succulent- 
leaved agaves. The long, hard, spiny points of the leaves and 
their hard margins constitute a large proportion of mechancal tis- 
Sue, the leaves being narrow and pike-formed because of the devel- 
opment of this tissue. The plant is, however, possessed of ample 
Storage capacity for erecting a flowering stalk eight to twelve feet 
tall and one to two inches in diameter at base. The epidermis 
Fg; cu) consists of long, columnar cells with a heavy cuticular 
layer and great thickness of water storage cellulose like Frankenta 
bracteata of western Australia and the South African aloes. At 
the junction of the epidermis and the underlying assimilation tis- 
Sue, the walls of each are rather thick, thus binding the two more 
firmly together and preventing the tearing loose of the epidermis 
