18 The Colorado Experiment Station 
rounding air condenses on the plant, and, if further lowering of 
the temperature takes place, the condensed vapor freezes, and 
“frost” is formed. 
The rate of radiation of heat from a plant surface is increased 
and hence the formation of frost favored by the following condi¬ 
tions : (1) Clear sky; (2) dry air; (3) still nights; (4) rare atmos¬ 
phere (as at high altitudes). Clear nights are the rule in Colorado, 
and hence the radiation at night is pronounced. A blanket of 
fog or clouds tends to retard the loss of heat from bodies on the 
earth’s surface. The actual amount (not relative amount) of water 
vapor in the air decreases with altitude, and since water vapor ab¬ 
sorbs and retains heat radiated from the earth’s surface, the atmos¬ 
phere at high altitudes is less able to hold this heat than atmos¬ 
phere at lower elevations. It is a common observation that frosts 
are more prevalent on still nights than when the air is in motion. 
Radiation of heat proceeds with greater rapidity in a rare atmos¬ 
phere, such as is found at high altitudes, than it does in a dense 
atmosphere, such as is found at low elevations. 
Freezing of Plants .—In the freezing of plant tissues, injury 
may result in two ways: (1) Water may be withdrawn from the 
plant cells and ice crystals from this water form in the spaces be¬ 
tween cells; subsequently the ice in the intercellular spaces may 
evaporate, and cause death of the plant tissue by drying out. It is 
well-known that many plants may survive extremely cold weather 
provided it is not accompanied by a dry atmosphere; (2) again, 
the injury from low temperatures may result from the actual freez¬ 
ing of the cell sap, which may cause a mechanical rupture of the 
living material in the cell itself. 
It is a familiar observation that some of the more tender 
plants are injured by temperatures above the freezing point; and 
that, on the other hand, there are many plants that may withstand 
temperatures considerably below the freezing point. This state¬ 
ment may apply not only to dormant plant parts, but to swelling 
buds, open flowers, and forming fruit as well. The plants at tim¬ 
ber-line and above are subject to freezing temperatures almost 
every night in the year. The exact nature of this immunity to low 
temperatures is not known. 
There are remarkable differences between varieties of plants, 
between individuals of the same variety, and even between differ¬ 
ent buds on the same tree, in their resistance of low temperatures. 
This resistance may depend much upon the age of the tissue, and 
the treatment and care the plant has had during its life. Old plant 
tissue is more resistant than newly-formed tissue. A pruning prac- 
