280 BOTANICAL GAZETTE [APRIL 
relations are quite different on the two slopes. On the eastern slope 
the foothill zone extends from an altitude of 8000 feet down to 6000 
feet, and is an area timbered with Rocky Mountain yellow pine. 
On the western slope, localities between 6000 feet and 7500 feet are 
dry, having a precipitation 5 to 10 inches less than similar altitudes 
of the eastern slope; the vegetation is chiefly sage brush, associated 
with Sarcobatus vermiculatus and Chrysothamnus spp. Pinyon pine 
and cedars occur on the ridges. Rocky Mountain yellow pine is 
scattered in northwestern Colorado; it extends up to 8000 feet, but 
forms no distinct zone. Below 6000 feet east of the continental divide 
are grass-covered plains with a precipitation of 10-20 inches. At 
the same altitudes on the western slope one meets with semi-desert 
conditions. The annual precipitation is below 10 inches and the 
vegetation is predominantly sage brush. 
In general it may be said that the state west of the continental 
divide is drier from the vegetation standpoint than the eastern slope. 
Only about 50 per cent. of the rainfall on the western slope occurs 
during the growing season, in contrast with the occurrence on the 
eastern slope of about 75 per cent. during the same period. Hence 
for any two localities east and west of the continental divide with 
equal annual precipitation amounts, the locality west will have the 
more xerophytic vegetation. 
In connection with the preparation of this paper, the writer wishes 
to express his deep appreciation of helpful suggestions and criticisms 
given by Professor FRANCIS RAMALEY. 
UNIVERSITY OF COLORADO 
BouLpeEr, Coto. 
