1910] ROBBINS—VEGETATION IN COLORADO 271 
SUNSHINE AND CLOUDINESS 
Colorado is a state of sunshine. On the average, 50 per cent. of 
the days are clear, 33 per cent. partly cloudy, and 17 per cent. cloudy. 
e Vegetation 
PLAINS"? 
Precipitation: mean annual 10-20 inches; about 75 per cent. during the 
growing season; small amount of snow. Temperature: mean annual 45°-50° 
and above; mean summer 65°-70° and above; mean annual range 40°-50°; 
diurnal range great. Average date of the last spring frost varies in different 
localities from April 28 to May 10; average length of growing season about 
5 months, 6 days. Relative humidity: annual amount 50-60 per cent. 
The plains are generally spoken of as including that portion of the 
State east of the foothills. The climatic and vegetative conditions 
vary, however, in different parts. The “high plains’? north of the 
South Platte River, the Arkansas-Platte Divide, and the high lands 
east of the foothills in the southern part of the state are regions over 
which foothill forms have extended. It will be observed from jigs. 5 
and 6 that these areas have a lower mean annual temperature (45°— 
50°) and a lower mean summer temperature (65°-70°), with a con- 
Sequent less evaporation rate than the plains proper. 
Over the entire plains area grasses are the dominant forms of 
vegetation; in some places not more than 25 per cent. of the ground is 
covered. Some of the common grasses are Andropogon Hallii Hack., 
Calamovilja longifolia (Hook.) Hack., Bouteloua oligostachya (Nutt.) 
Torr., and Bulbilis dactyloides (Nutt.) Raf. Ewurotia lanata (Pursh) 
Mogq., Sarcobatus vermiculatus (Hook.) Torr., and Chrysothamnus 
Sraveolens (Nutt.) Greene are abundant in many localities. On 
ridges and buttes occur various shrubs, such as Yucca glauca Nutt., 
Prunus Besseyi Bailey, and Ceanothus pubescens (T. and G.) Rydb., 
also various mat and rosette-forming herbs. Populus angustifolia 
James, P. Sargentii Dode, P. acuminata Rydb., P. Wislizenii (S. 
*t The classification of the plant zones into plains, foothill, montane, subalpine, 
and alpine are as outlined by. RAMALEY, FRANCIS, Plant zones in the Rocky Moun- 
tains of Colorado, Science N. S. 26:642, 643. (Nov. 8) 1907. 
RaMatry, Francis, Scientific expedition to northeastern Colorado; 8. Botany. 
Account of collections made. Univ. Colo. Studies 4:161-164. 1907. This report 
S'ves a list of plants characteristic of the “high plains.” 
