40 



Farmers' Bulletin 1096. 



the effect on the size of the final crop. After a few seasons, he will 

 have collected enough data to enable him to know with considerable 

 accuracy how low it is safe to allow the temperature to fall before 

 lighting the heaters. 



The following table of critical temperatures as recorded by a well 

 exposed thermometer in the orchard is meant to give a general idea 

 of what the blossoms will endure for a half hour or less without 

 injury. 



Temperatures endured by blossoms for 30 minutes or less. 



i Bosc 27. 

 CITRUS FRUITS. 



Owing to the lateness of the blossoming period of most varieties 

 of oranges, the fruit has practically reached maturity before the 

 frost season arrives in the subtropical sections where they are grown 

 commercially. In this case, therefore, we have to deal with the 

 chilling of a relatively large bulk in comparison with deciduous 

 fruit or blossoms. 



The thick pithy rind of the orange is a poor conductor of heat 

 and the protection it affords causes the temperature of the interior 

 to fall much more slowly than the temperature of the outside air. 

 When the air temperature is falling rapidly the interior of the 

 fruit may be as much as 7 warmer than the air surrounding it, 

 and the temperature inside the fruit lags from an hour to an hour 

 and a half behind the temperature of the air. As the freezing point 

 of orange juice is about 28 F., the temperature inside the fruit will 

 not fall much below this point until the fruit is frozen. (See fig. 16.) 

 It is characteristic of any liquid that in the freezing process sufficient 

 latent heat is given off to maintain the temperature at the freezing 

 point of the liquid until the whole quantity of liquid has been frozen. 



Eadiation is of great importance in damaging citrus fruit and the 

 fruit exposed to the sky is always the first to be frozen. It is prob- 

 able that any beneficial effects that may be obtained by covering an 

 orchard or tree are due in a large degree to diminishing the rate of 



