264 BOTANICAL GAZETTE [APRIL 
TEMPERATURE 
GENERAL ACCOUNT.—Fig. 5 shows the mean annual temperatures 
of Colorado. The general north-south direction of the isotherms 
readily shows the influence of the mountains. The eastern portion 
of the state and a small area along the Grand River in the western 
part have a mean annual temperature above 50°F. The influence 
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Fic. 5—Mean annual temperatures: the general appearance of this figure ug 
very similar to the one giving mean summer temperatures (jig. 6), and to the one 
showing mass elevations ( fiz. 2). 
of the Arkansas-Platte Divide is seen in the eastward deflection of 
the isotherm 50. Isotherm 45 follows closely the eastern foothi 
line, 6000 feet altitude; it indicates the lower limit of the Rocky 
Mountain yellow pine (Pinus scopulorum [Engelm.] Lemmon). 
Isotherm 40 follows closely the 8000-foot contour line, and marks the 
_ lower limit of the lodgepole pine (Pinus Murrayana Oreg- Com.) 
and the usual upper limit of the Rocky Mountain yellow pine. On 
the western slope these isotherms run to higher altitudes. There is . 
decrease in the temperature with an increase in altitude, most localities 
3 Throughout this paper all temperature readings are Fahrenheit. 
