30 
Psyche 
[March 
Cameron counties of Texas and the Mexican state of Tamaulipas, 
beginning on the east at the Gulf of Mexico and ending approx- 
mately at the level of Mission, Texas on the west. Nowhere is this 
Valley much more than 25 or 30 km. wide either north or south of 
the river. It constitutes an island of fertile soil, relatively high 
humidity, and comparatively lush vegetation surrounded landward 
on all sides by desert scrub. 
Because of its latitude (26 degrees N.) and proximity to the Gulf, 
the Valley experiences an extremely mild temperature regimen. The 
average yearly maximum at Brownsville is 28 degrees C. and the 
minimum 18.3 degrees C. Summer highs rarely go above 40 degrees 
C. and the average daily range for July at Brownsville is 33.6 de- 
grees C. to 24.2 degrees C. On the other hand, most winters have 
only two or three frosts during which the temperature normally does 
not fall below -3 to -4 degrees C., although the record low for 
Brownsville is -11 degrees C. (registered in February 1899). The 
average daily range for January at Brownsville is 21.4 degrees C. 
to 11.2 degrees C. and such temperatures occur quite consistently 
throughout the Valley in winter, although from November to March 
warm periods frequently are interrupted by cold fronts that bring 
4-10 day stretches of cloudy weather when the temperature stays 
between about 4 and 10 degrees C. 
Precipitation in the Valley is rather scant, averaging 669 mm. 
per year at Brownsville. It occurs in winter as protracted fine 
drizzle, in spring, summer, and fall as occasional thunderstorms, 
and sometimes in late summer and early fall as torrential inunda- 
tions that accompany inland-moving hurricanes. September, with 
an average of 124.8 mm. is the wettest month while March, with 
26 mm., is the driest. Although long-term figures suggest fairly 
even rainfall distribution, there is actually great variation from 
month to month and from year to year. Protracted droughts are 
common but some years may have more than 1000 mm. of rain. 
Vegetation of the Valley ranges from desert scrub to humid sub- 
tropical woodlands best developed along the Rio Grande and in 
the vicinity of water holes. The south Texas flora resembles that 
which grows in many other semi-arid environments from Mexico 
to Argentina. Some of the more conspicuous angiosperm genera 
are Acacia, Bac charts, Bumelia, Celtis, Cercidium, Condalia, Ery- 
thrina, Opuntia, Parkinsonia, Prosopis, Salix, Tillandsia, and Xan~ 
thoxylum. This same element occurs also in the ecologically sim- 
