338 FUNDAMENTALS OF FRUIT PRODUCTION 



SO widely as is warranted. It should be understood, however, 

 that cold weather aside from frosts may damage fruit crops and it is 

 not always necessary that the temperatures go below the freezing point. 

 Dorsey shows that cold weather, though the temperature remains above 

 freezing, immediately following the pollination of certain plum varieties, 

 results in such a slow growth of the pollen tube that abscission of the 

 style often takes place before fertilization, the result being as complete 

 failure to set fruit as though frost had actually occurred during the 

 blossoming period. Low temperatures also prevent the bees from 

 effecting pollination. 



Frosts and Freezes Distinguished. — Furthermore, not all freezing 

 temperatures are due to frosts. English writers use the term "frost" 

 to designate freezing temperature of any kind but usage in the United 

 States restricts ''frost" to a kind of cooling well recognized and limited 

 in its scope. A "freeze," as distinguished from a frost, is due to the 

 importation of cold air from other regions and may be accompanied by 

 a high wind; a frost is due to a local cooling of air and occurs during 

 calm weather. A frost may take the form called a "hoar" frost, with 

 a visible deposit of frozen moisture, or it may be a "dry" or "black frost" 

 with freezing temperatures but with no deposit. Freezing temperatures 

 may accompany snow squalls. All of these may injure orchard fruits. 

 Against freezes the fruit grower is generally unable to contend by 

 palliative methods; against frost much effort has been expended and it 

 is upon frost that much horticultural thought has been centered. 



Relation of Radiation to Frost. — Some knowledge of the nature of radiation 

 is necessary to a proper understanding of the nature of frost. It is generally 

 considered by physicists that all substances are constantly receiving and emanat- 

 ing heat. This radiation heat travels in straight lines through ether and through 

 air, being absorbed by them little or none. Striking a solid substance it is in 

 part reflected and in part absorbed, the amounts of reflection and of absorption 

 varying with the substance. During a clear day the heat received by any sub- 

 stance through radiation from the sun and from other substances is in excess of 

 the amount emitted through radiation; during a clear night the heat lost by 

 radiation exceeds that gained. On a cloudy day the sunlight is cut off to a 

 great extent and the substance is warmed less than on a clear day; during a 

 cloudy night much of the heat lost by radiation is reflected by the clouds and the 

 substance is cooled less than on a clear night. There is some absorption of 

 radiant heat by the atmosphere. Radiant heat from the earth is absorbed by 

 water vapor, carbon dioxide and ozone. 



Radiation is proportional to the exposed surface, and the amount of heat 

 stored and available for radiation is to a large extent proportional to the volume 

 of the radiating substance. Therefore vegetation, which has a large surface in 

 proportion to its volume, cools by radiation with relative rapidity. 



Though air freely permits the passage of radiation heat, it radiates little 

 itself in comparison with other substances. There is, it is true, an appreciable 



