102 BOTANICAL GAZETTE {AUGUS1 
extremes, which are reflected in the structure of species, stand 
out most significantly in a comparison of two extreme points. 
The greatest daily fluctuation at Galveston is 5 to 7°F. in April; 
at El Paso 25 to 27° in June-July. During the February norther 
of 1899, points on the Staked plains above 4000 feet registered 
more than 20° below zero, and it was 12° above at Brownsville 
on the same date. These figures indicate the extremes which 
may occur to bring about a periodic reduction of the Austral 
flora. The following temperature zones as measured by the 
flora prevail in the western Texas region. 
DILUTE XEROPHYTIC TROPICAL.—This can be recognized only 
in the extreme southern part of the Rio Grande embayment, 
especially in the present Rio Grande valley. The mean annual 
temperature of the valley, however, from Laredo to the mouth, 
is 73°, slightly below that of southern Florida. The monthly 
means correspond to those of the middle part of the Florida 
peninsula from September to March, except the January mean 
which falls some degrees below. The summer means rise much 
higher than in any part of Florida. A record of sixteen years 
at Brownsville showed a minimum temperature of 18° (the 
minimum in February 1899 was 12°) and five years without frost. 
At Indianola a record of fifteen years showed a minimum of 
15° and four years without frost. Probably a freeze severe 
enough to kill tropical woody vegetation occurs in periods of 
ten to twelve years. The fatal temperature for tropical plants 
in this region is that due to the northers, which bring abnormally 
low temperatures suddenly, and not infrequently during the 
growth season. The actual poverty of a tropical flora, however, 
is due equally to the aridity of the region. The floral affinities 
of the tropical element are with the Neotropical Gulf zone, and 
are a part of the Tamulipan division of this zone.3 
The following species, as illustrations, will be recognized as 
belonging to groups of chiefly tropical distribution, but even 
some of these pass into northern and southern subtropical terri- 
tory: Sabal Adansonii, S. Mexicana, Monanthochloa littoralts, 
3 MERRIAM, C. H.: Life zones and crop zones, Pp. 52. 
