108 BOTANICAL GAZETTE [AUGUST 
Of equal importance with the total quantity of rainfall for 
plant life is its seasonal distribution. Even in the eastern part 
of the area this is irregular and spasmodic, although a sufficient 
quantity falls on the average to maintain a fairly large percent- 
age of mesophytic species of the Mississippi valley and Gulf 
regions (Carolinian and Austroriparian zones), and to insure 
reasonable certainty of yield from cotton, corn, wheat, and oat 
crops. But the vegetation west of the g8th meridian bears the 
marked individuality of the Lower Sonoran zone in its physio- 
logical adjustment to arid conditions. 
Texas lies in a peculiar position with regard to atmospheric 
movements, which may account for the periodic and excessive 
variations in seasonal and annual rainfall. The El Paso region 
is clearly within the Pacific-Lower Californian field of climatic 
influence, while the Rio Grande plains province is mostly in the 
Mexican climatic zone. The Great plains type of rainfall pre- 
dominates over the Staked plains, and generally far into the 
center of the state. The Gulf type scarcely extends west of the 
Balcones escarpment and Grand prairie, but occasionally it 
carries a season of high rainfall well toward the central provinces, 
as during the season of 1900 (jig. 4). 
The mean annual rainfall at Austin is 38.88 inches, Its low- 
est record is 18.33 inches in 1879, and its former highest record, 
51.79 inches in 1888, was exceeded during 1900. The rainfall 
from January I to November I, 1900, reached 51.19 inches. This 
is a fluctuation equal to the total mean, The fluctuation between 
Gulf and Mexican type is shown in a remarkable degree at 
Brownsville, where the mean is 31.52 inches, the minimum 8.88 
inches in 1870, and the maximum 60.06 inches in 1886, a varia- 
tion of 167 per cent. of the mean. The amount of rainfall in a 
single month and its departure from the monthly average may 
be excessive. In the last case cited (Brownsville), the rainfall 
for September 1886 was 30.57 inches, or 23.27 inches above 
the September mean. The June rainfall at Fort Clark in 1899 
was 22.32 inches, or 19 inches above the mean, over 75 per cent. 
of this excess falling in twenty-four hours. It thus appears that 
