Frost and the Prevention of Damage by It. 37 



watery and its freezing point high. For this reason a frost which 

 follows a period of weather favorable for rapid growth will cause 

 more damage than the same frost following a period of cold cloudy 

 weather and consequent slow growth. 



When a plant freezes a portion of the cell sap is withdrawn from 

 the plant cells, gathering in the intercellular spaces in the form of 

 ice. If thawing takes place gradually and the cell walls have not 

 been ruptured, this moisture is again taken up by the cells as it is 

 melted, without serious damage. If thawing takes place rapidly, 

 however, the intercellular ice is liquefied faster than it can be ab- 

 sorbed by the cells; a part of it is lost by evaporation and the cells 

 are broken down. When fruit crops are damaged by frost the 

 greatest damage is often found on the portion of the tree where 

 the morning sun strikes first. When clouds gather on the eastern 

 horizon before sunrise and obscure the sun for a few hours in the 

 morning after a cold night, damage to vegetation is likely to be 

 slight, provided the temperature does not fall much below the critical 

 temperature. Heavy smoke from orchard^ heaters or smudge fires 

 may also lessen damage from frost through causing a slow thawing. 

 Of course, if the temperature falls sufficiently low a great deal of 

 damage may be done even when the rate of thawing is slow ; in other 

 words, the prevention of a rapid rise in temperature in the morning 

 may often not be sufficient in itself to prevent injury. 



It is possible that some of the protection from frost damage ob- 

 tained by irrigating is due to making available to the damaged 

 plant cells a larger supply of water through increased flow of sap, 

 to replace that lost through freezing. 



INFLUENCE OF HUMIDITY ON RATE OF FREEZING 



Fruit growers in nearly all sections are convinced that with the 

 same temperature the amount of damage by frost will be greater 

 when the humidity is low than when it is high. Recent studies by 

 I. G. McBeth, of the Leffingwell Rancho at Whittier, Calif., indicate 

 that under certain conditions citrus fruits will be damaged in a 

 shorter time when the humidity is high than when it is low, the 

 temperature being the same in both instances. It was thought this 

 was due to greater conductivity of moist air, the heat being con- 

 ducted away from the fruit more rapidly, causing the temperature 

 of the fruit to fall more nearly at the rate at which the temperature 

 of the air was falling. 



However, in making these investigations allowance was not made 

 for the influences of radiation and the liberation of heat by condensa- 

 tion. Under orchard conditions, blossoms and leaves exposed to the 

 sky lose their heat rapidly by radiation and their temperature may 



