228 
The small amount of moisture in the atmosphere of Colorado is indi- 
cated by the almost total absence of dew, fog, mist, and hoar.frost, by 
the seareity of clouds, and by the rapid evaporation of rain and snow. 
Tt is also shown by the power in this climate of the solar rays to heat 
solid bodies exposed to them, while the neighboring air remains com- 
paratively cool. “Humidity is least on the open, unsettled plains, where 
there is nothing to prevent rapid evaporation, and it is greatest in the 
canyons and parks among the mountains, where the mosses, grasses, and 
trees retain the moisture in the Boul; giving it out slowly and continu- 
ously to the air. 
Trees depend upon the air for a portion of the moisture they receive, 
which, absorbed through the leaves, keeps up the free circulation neces- 
sary for their growth. Without moisture leaves can not take up from 
the air the carbonic acid and other gaseous products upon which - 
they exist. Where this moisture is limited, more is demanded of 
the roots, and the relative proportion between them and the leaves 
inust be increased. This therefore is one reason for the slew growth 
of trees here as compared with other places, and only those succeed 
well that have large and rapid root-growth, and especially those with a 
tap-root, or roots which naturally reach far down in the ground, where 
they find a greater supply of moisture. Atreein Colorado may haveall 
the irrigation if can possibly need, but will never have the rich, luxu- 
riant foliage seen along the Atlantic and Pacific coasts. During the 
spring and autumn rain-storms, already noticed, there is much humidity, 
as shown by mists and fogs, and under its influence the foliage of the 
trees always swells and expands, and starts into new growth. Moisture 
in the air will also benefit a tree through its roots, if enabled to reach 
them through a spongy, porous soil, or by cultivation with a spade or 
plow, which loosens the soil around the tree and allows the air to pen- 
etrate toa sufficient depth. This accounts for the fact that trees in cul- 
tivated ground will endure drought better than those not so favored. 
The porous nature of the soil in this region is therefore not so great a 
disadvantage as it might be, for it helps to carry to the roots whatever 
moisture theair may provide. tLrrigation would be unnecessary if we had 
the moist atmosphere which enables farmers on the southern plains of 
Texas, for instance, to raise the richest crops without irrigation. 
RAIN-FALL. 
The rain-fall, or moisture precipitation, is the most important factor 
of any climate in relation to trees. It is also the most irregular of me-— 
teorological elements. But the records of thirteen years will show what 
may be accepted as the rule, though subject to many exceptions. The 
average annual rain-fall at Denver is 14.99 inches, and at Colorado | 
Springs 15.87 inches. In comparison with this, at Los Angeles it is 

