The Temperature Relation 407 



244. The date-palm. - - One of the most important ap- 

 plications of a study of the relation of plants to the heat 

 units of the region in which they are grown is that made by 

 Swingle respecting the date-palm. He has shown that as 

 the heat units increase, the general adaptability of arid 

 regions to date culture is advanced, and a certain minimum 

 may not be exceeded for any type of date. From such a 

 study it was considered possible to foretell with approxi- 

 mate accuracy what section of the Southwest might be 

 utilized in date culture. 



245. Control of temperature. - - It is obvious that limi- 

 tations of expense impose pronounced restrictions upon 

 the exercise of control over the temperature factor in the 

 open. With a few intensive crops, such as asparagus, 

 waste steam has been utilized to some extent in forcing in 

 open culture, but proper control of temperature for forcing 

 or for producing crops out of season is usually confined to 

 greenhouse and hot-bed culture. 



In some sections of the United States the loss of the 

 entire peach, apple, or other fruit crop may occur in conse- 

 quence of one or two late frosts, when, as experience has 

 shown, the temperature may fall from 4 to 14 below 

 freezing. 1 Recently a control or prevention of this loss 

 has been successfully accomplished by means of coal or 

 oil heaters. The general plan is to place from 60 to 100 

 small ovens or heaters per acre at appropriate distances 

 apart. Then, if by midnight the indications are that a 

 freezing temperature will be reached in the early hours of 



1 Paddock, W., and Whipple, O. B., " Fruit-Growing in Arid Regions." 

 (Frost Injuries, Secondary Bloom, and Frost Protection.) Chapter 19: 

 324-354, 1910. 



