262 BOTANICAL GAZETTE [APRIL 
the same altitudes on the eastern slope. The precipitation of Colorado 
comes in showers rather than in prolonged rainy spells. 
There is considerable variability as to the wettest part of the 
growing season. In the region north of the Arkansas-Platte Divide, 
the rainy months are April and May, with the exception of localities 
over the extreme eastern part of the state, where the maximum 
rainfall occurs in June. Consequently, the vernal vegetation of this 
region is more luxuriant than one would judge from the annual 
precipitation; the rest of the season is dry and is marked by the 
blooming of xerophytic species, mainly composites. ‘The Arkansas- 
Platte Divide and the entire Arkansas Valley receive their maxi- 
mum rainfall during the summer, July showing the greatest monthly 
amount. In the mountains July and August are the wettest 
months. Over the western slope the spring is wetter than the other 
seasons, but July or August generally shows the greatest monthly 
precipitation. 
On account of the seasonal distribution of precipitation east of the 
continental divide there is very little snow. Furthermore, on account 
of the dry soil and air the snow soon disappears after falling. This is 
especially true of the plains, and as a result the vegetation does not 
have during the winter the protection which snow offers. The 
absence of snow on the plains, with the consequent low soil-moisture 
content, the great diurnal and annual ranges of temperature, together 
with the presence of a low relative humidity and dry winds, bring 
about conditions which account for the absence of trees. 
As a result of the shortness of the rainy season, at most localities 
on the plains and lower foothills there is an acceleration in the growth 
of vegetation. Seasonal rains are usually followed by drought periods, 
and mesophytic plants must hasten to complete their development, 
hence the vegetative appearance changes rapidly. 
Very wet and very dry years, or a consecutive series of the 
not uncommon in Colorado. The mean difference between the 
wettest and driest years is somewhat greater for localities east of the 
continental divide than for localities west. Plants living in 4 place 
where there are occasional very dry and very wet years must be those 
that have adapted themselves more or less successfully, not only to the 
normal precipitation, but to the variability of the same. 
se, are 
