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Transactions Texas Academy of Science. 
annual mean temperatures. The air, therefore, 1 ecornes exceed- 
ingly parched and dry in Trans-Pecos Texas, and during the hot 
summer months, the dry superheated air currents bear away every 
meager vestige of moisture from the surface soil and draw with 
withering persistency upon the moisture resources of transpiring 
vegetation. Naturally, much of the vegetation has ceased vital 
activity by that time, and the remainder exhibits effective meas- 
ures for resisting the dessicating power of the air. 
Temperature. The temperature relations of the two extremes — 
Orange and Sanderson — differ but little. Both have an annual 
mean of about 70°. The monthly means coincide closely and 
range (at Sanderson) from about 50° during December and Jan- 
uary to 85° in June, July and August. Minimum temperatures 
in winter are low r er at Sanderson than at Orange, but the summer 
maxima are several degrees higher at Orange. Growing tempera- 
tures uninterruped by frost, may be expected from about March 1 
to November 20. Vegetation in the arid west is less visibly 
altered by winter freezes owing to the early disappearance of her- 
baceous annuals and the non foliaceous character of the other 
forms. Also the warmer weather of early spring is more quickly 
felt and responded to by vegetation in the sotol country than at 
the east, probably because less moisture is present in the soil and 
because the soil itself is susceptible to rapid heating — being un- 
covered by a close vegetation. The daily variations in temperature 
in summer are greater at Sanderson than at Orange. The uncov- 
ered, rocky soil, the absence of vapor from the air, the greater 
altitude are factors which favor intense heating of the earth’s sur- 
face and the stratum of air immediately above during the day and 
proportionately rapid and extreme cooling at night. 
Illumination. This factor must be of special moment in its 
relation to vegetation in the sotol country. In a general way, the 
intensity of illumination has been recorded in the experience of 
various persons who have attempted to photograph objects and 
landscapes in the West Texas region. The almost vertical sun, the 
altitude, the vaporless air and prevailingly cloudless sky on the one 
hand and the white reflecting surface of rock on the other, con- 
spire to make a glare of blinding intensity. 
Physiography. At the beginning of this paper, it was mentioned 
that the sotol formation is characteristic of certain physiographic 
features of the arid southwest. It is most strikingly an edaphic 
formation. One may expect to find this formation in one phase 
or another of its associations, on every foot hill and mountain side 
where fragmented rocky debris covers the surface; on the border- 
