1901] VEGETATION OF WESTERN TEXAS rL3 
In the third place, the transporting power of the wind has a 
marked influence on the selection of species. First, by dune 
formation and the assemblage of characteristic dune species, but 
particularly of sand-binding plants. This is true on the Gulf 
and on sandy stretches in the interior (Staked plains sand area, 
Rio Grande sand areas). In the second place, by the erosion 
and blowing away of soil-forming débris which tends to accu- 
mulate upon and modify arid patches, thus rendering the sub- 
stratum almost naked rock or chalk or gravel, and inducing a 
new vegetation. The process of wind denudation is much more 
common, since after over-pasturing and dry weather thousands 
of acres are left unprotected by soil-binding grasses, and the soil 
being often a loose, fine silt, or adobe of sandy nature, is easily 
lifted by even a moderate breeze. It was shown above that such 
soils are also greatly washed by the heavy rainstorms which 
come at irregular periods. 
As to the moisture-carrying capacity of the winds, of course 
the Gulf wind starts inland fairly well saturated, but meeting 
constantly warmer and drier conditions its relative humidity is 
greatly decreased, so that a precipitation of the moisture of a 
direct Gulf wind is not common, 
LIGHT. 
It would be of value if there were definite quantitative data 
as to the effect of light upon the vegetation of western Texas. 
Undoubtedly its effect is felt in the general dwarfing of the vege- 
tation, and is plainly seen in the peculiar types of arboreal 
vegetation, as the ‘“‘orchard tree”’ outline of crown. This is 
observed characteristically in the mesquite, but the live oak, 
water elm, post oak, and hackberry also assume this form. The 
mountain cedar is a compact low tree of oval or spherical out- 
line, with diffuse branching from the ground. Undoubtedly the 
dull grayish-green of the vegetation is largely due to the effect 
of light, as there are probably fewer chloroplasts in any given 
assimilative cell. Also there are cases where the epidermis is 
SO constituted as to diffuse light rays by reflecting facets, as 
appears to be the case in many cactuses and agaves. 
