11oO BOTANICAL GAZETTE . | AUGUST 
in April of the dry spring of 1898, all the country covered by 
compact clay silt wash from the Balcones escarpment, being soil 
from which rainfall quickly flows away, was practically bare of 
grass and herbaceous vegetation. Even the streamway timber 
vegetation was shedding its leaves. On the outcrop ridges of 
Eocene sand, where all precipitated moisture had been retained 
and kept available, the landscape was fresh with grasses, com- 
posites, legumes, and many other annuals. This was repeated 
in a more noticeable degree during the same month in passing 
from Eagle pass to Carrizo springs, where again the compact 
clay and silt alternated with wide stretches of Eocene sand. The 
availability of underground water in the sand beds is evidenced 
by innumerable wind pumps, while in the clay silt country the 
surface water is collected in ponds. 
HumIpIty AND EVAPORATION CAPACITY.— The conditions of 
air moisture in the west Texas region have an important bear- 
ing upon the vegetation. As may be noted in fg. 5, illustrating 
evaporation capacity, this increases in proportion to the decrease 
in rainfall, reaching its maximum of 80 inches at E] Paso, with 
this important exception, that in the high mountains of the 
Guadalupe and Davis ranges, where the rainfall is double that 
at El Paso, the evaporation capacity is 90 inches annually. 
This moistureless condition of the air under these circumstances 
reacts in several ways to pile up extremes of aridity. First, 
there is no blanket of vapor or clouds, so that the force of the 
sunlight is intense: and second, temperatures become extremely 
high during the day, and because of rapid radiation sink to 
a low point at night, except in the region of the Gulf winds. 
Under such circumstances the adaptations in the structures of 
plants are most strongly marked. It follows that moisture 
determines, far more than any or all other factors, the structural 
aspects of the flora. 
Moisture determines, as we have already seen, degrees of 
intensity of the Lower Sonoran flora. Lack of moisture has diluted 
the tropical elements, the Gulf zone semitropical, the Austro- 
riparian and Carolinian floras in the east, and the mountain 
