106 
Transactions Texas Academy op Science. 
THE HABITAT. 
The fact most emphasized in the foregoing east-west section, is 
the radical change in the conditions which determine the physio- 
logical adjustment of the vegetation. The rainfall diminishes to 
a bare 9 inches annually; the air loses its moisture; the dry winds 
bear away every meager supply of moisture from the earth’s sur- 
face and from the surfaces of transpiring plants; the sunlight 
becomes more intense and more constant by reason of cloudless 
skies and vaporless air; the temperature variations of air and soil 
reach the daily extremes of burning heat by day and chilling cold 
by night and the seasonal variations are proportionately extreme; 
the elevation increases from nearly sea level at the Sabine to a 
maximum of five thousand feet (the highest point reached by the 
railway; of course mountain peaks exceed this by over 3000 feet),, 
and a level earth covered by a deep, rich moist soil is succeeded by 
a rolling, hilly, canyon fissured region, the fragmented debris of 
whose rock strata lie loose and uncovered or only imbedded in the 
closely packed adobe soil derived itself from the limestone. 
It is evident that the chief cause of this desert condition lies in 
the reduction of the moisture supply. This would more or less 
directly influence the temperature variations, the light inten-ity 
and even more the decomposition of the native rock and conse- 
quently the soil texture and of course its water supply. 
That the factor of moisture alone would be insufficient to bring 
about the desert aspect of the vegetation is, however, clearly shown 
by the presence of a continuous grass vegetation on areas which, 
like the plateau about Alpine and Marfa, have a rich covering 
of deep, porous soil. The fierceness of the desert is manifest where 
the physiographic conditions present dissected areas of naked rock 
as in the lower Pecos and Devil’s River country and along the Rio 
Clrande canyon. 
The general statement will probably be accepted that the vege- 
tation of the region is, in its physiological relations, the product of 
his particular combination of environmental factors, i. e., that its 
< 3 hief characteristics consist in its methods of adaptation to the en- 
vironment. This paper does not attempt to discuss the question 
as to the origin of the peculiar forms of plants — the so-called veg- 
etation forms or ecological forms — which make up much of the 
sotol formation. That — as others have previously pointed out — ■ 
could be solved only after exact measurement of the ecological 
factors on the one hand and by a detailed study of the physiological 
processes through the life cycle of the ecological forms on the other. 
