230 
during the dry, hot days of May and June, before the summer rains begin. 
During the wet season it will almost take care of itself, but when the 
rains cease in August the principal danger of the whole year comes, and 
as day follows day of hot, dry, scorching weather, the tree must be lib- 
erally supplied with moisture to take the place of that absorbed by the 
thirsty air. 
This region does not receive the benefit it might from the rain-fall, 
on account of the deep, gravelly, porous soil, through which the rain 
sinks quickly, leaving but a small portion behind, a large part of which 
is soon taken up by the air, and but little remains for the tree. 
EVAPORATION. 
The evaporative power, or thirstiness, of the air is increased by high 
temperature, low humidity, low pressure, and high rate of motion. It 
is most powerful in summer in the sun and wind, and only slight in 
winter in shade and when the air is caim. In any place like Colorado, 
where are found many upturned faces of broken rocks and large uncov- 
ered areas of sand and gravel, evaporation and radiation will be much 
greater than in places where the-surface is protected from sun and wind 
by vegetation. This is shown by the entire absence of dew on the 
plains and the rocky slopes of the mountains, and its formation when 
trees and lawns are planted. 
Trees are injured by rapid evaporation, because it produces an unnat- 
ural activity in the roots and stem to supply the drain from the cells of 
the leaves and bark, and unless the tree has a large root-system and 
great vigor in all its functions, it will be unable to keep up the proper 
circulation. This is a reason for the fact already cited, that the trees 
which succeed best here are those of rapid, vigorous root growth, with 
a solid leaf structure. Evaporation is more rapid in winter, and 
therefore fall planting is objectionable. Imported trees always thrive 
best if they are cared for in a bursery for a year or two, until their 
roots, leaves, and bark become accustomed to the new conditions of 
climate. 
WINDS. 
The winds of this region, which, from the level character of most of 
its surface, are very constant and often of great velocity, have a strong 
influence upon the production of increased evaporation. In the day- 
time throughout the year the prevailing winds are from the south and 
southeast, while at night they are from thenorth. This is an advantage 
to trees, for the south and east winds, always more or less laden with 
moisture, come ata time when evaporation is most active, and tend to 
relieve its evil effects. The north wind, direct from the region of per- 
petual ice and snow, is the wind of blizzards and storms, and is the 
most injurious of all, from the sudden changes of temperature, already 
described, which it produces. The warm west or northwest winds, | 





