228 



The small amount of moisture in tlie atmospliere of Colorado is indi- 

 cated by the almost total absence of dew, fog", mist, and hoar frost, by • 

 the scarcity of clouds, and by the rapid evaporation of rain and snow. 

 It 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 tlie moisture in the soil, 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- 

 must be increased. This therefore is one reason for the slow 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. A tree in Colorado may have all 

 the irrigation it 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 notised, 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 ii spongy, porous soil, or by cultivation with a spade or 

 l)low, which loosens the soil around the tree and allows the air to pen- 

 etrate to a 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 the air may provide. Irrigation wouldbeunnecessary if wehad 

 the moist atmosphere which enables farmers on the southern plains of 

 Texas, for instance, to raise the richest crops without irrigation. 



EAIN-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 



