272 BOTANICAL GAZETTE [APRIL 
Wats.) Sarg., and Salix spp. fringe the streams. East of the conti- 
nental divide, P. Wislizenii occurs only in the southern part of Colo- 
rado.'3 Opuntia arborescens, the chandelier cactus, extends over 
large areas in the Arkansas Valley; this plant is also restricted in its 
distribution on the plains to the southern part of the state. , 
In their Phytogeography of Nebraska, Pound and CLEMENTS 
have divided that state into four regions, two of which, the sand-hill 
region and the foothill region, extend into Colorado. The sand-hill 
region in Colorado includes a narrow strip along the eastern part of 
the state. Their “foothill region,” so called because of the great 
number of mountain plants which here find their eastward distri- 
butional limit, occupies the rest of the plains area. In describing the 
general appearance of the sand-hill region, they say: “The most 
noticeable character of the sand-hill vegetation, after one has become 
accustomed to the great variety of species which the sparse vegetation 
of each hill affords, is its extreme monotony. This is due to the 
predominance of bunch grasses, which are the controlling element in 
the covering of the hills, hillsides, sandy ridges, and sandy tablelands 
of the water sheds. The principal formation of the sand-hills, then, 
is the bunch grass formation, a grass formation of exposed hills and 
ridges of pure sand.” : 
EASTERN LOWER FOOTHILLS AND MESAS** 
Precipitation: mean annual 15-20 inches; about 75 per cent. during the 
growing season; in the northern part of the state the early spring is the wettest 
season, in the southern part July and August are the wettest months; more 
snow than on the plains. Temperature: mean annual 45°-50°; mean summer 
65°-70°; mean annual and diurnal ranges less than on the plains. Average 
date of last spring frost April 27 to June 16. As a result of temperature inveF- 
13 The writer recently found a few trees of this species at localities in the southern 
part of Rio Blanco County. This locality is west of the continental divide and far 
north. 
+4 The following botanical papers discuss quite fully the ecology of the easter? 
lower foothills and mesas of northern Colorado: RAMALEY, FRANCIS, Botany of 
northeastern Larimer County, Colorado, Univ. lo. Studies §:119-131- 19 
ALEY, FRANCIS, AND Ropgrns, W. W., Ecological notes from north-central Colo- 
do 111-117. 1908. Ropsrns, W. W., Studies of mesa and foothill ve8™ 
tation; 4. Deciduous trees and shrubs of the mesas. Ibid. 6:36-49. 1908. ROBBINS, 
AND Dopps, G. S., Studies in mesa and foothill vegetation; 3. Distribution of 
conifers on the mesas. Ibid. 6:31-36. 1908 
