5IO 



Bulletin 316 



cellular spaces, where injury from freezing occurs. Moreover, plants 

 differ greatly in their ability to withstand cold, and a temperature that 

 will destroy at one stage of the plant's development may cause little or 

 no injury at another stage. Again, injury may dependon other conditions. 

 If the water that is abstracted from the cells by the process of freezing 

 and that congeals in the intercellular spaces is again returned to the cells — 

 that is, reabsorbed — as is sometimes the case, there 

 will be little or no injury to the plant. This occurs 

 when the process of thawing takes place very slowly. 

 A cloudy morning or a cold rain after a frost is often 

 the means of saving a crop by retarding the thaw- 

 ing process. From the foregoing, it will be seen that 

 it is very difficult, if not impossible, to select a critical 

 or injurious temperature that will apply in all cases; 

 and it is even more difficult to choose a place where 

 such a temperature may be obtained. 



It is a common experience that vegetation at the 

 surface is sometimes killed when the temperature of the 

 air four feet above the surface remains above freezing ; 

 but rarely is there an absence of frost or of injurious 

 temperature when the air temperature four feet above 

 the siu-face falls to 32 . This is due to the fact that 

 on clear, quiet nights when frost is likely to occur, the 

 air at the surface is nearly always colder than it is a 

 few feet above the surface. This difference may 



amount to as much as 10 , or even 15 , in as many 

 ^%l?nm~xfm2 ^^et. but Usually it is much less. The difference is 

 atid minimum greater on clear nights than on cloudy nights, for clouds 



tisi7im!ent'' 'giv^s ^^^ ^^ ^ blanket and retard the escape of heat from the 



the highest and the earth, thus tending to maintain the whole stratum of 



%TeaTdi7al7o ^ir between the earth and the clouds at a higher 



the current temper- temperature than if no clouds were present. The 



ature^ at^ anj^time.^ difference is greater also on quiet nights, for winds mix 



cellent thermome- the air, preventing the colder, heavier air from settling 



^Price' $2to^$^^^' ^^ ^^^ ground and thus maintaining the temperattire 



more uniform. Hence, since the temperature at night 



usually increases from the surface upward to a certain limit, which is 



above the height attained by ordinary vegetation, it is obvious that 



freezing temperatures may occur at the surface when the air temperature 



four feet above is considerably above freezing, but rarely, if ever, will 



vegetation at the surface escape injury when the temperature at a height 



of four feet falls to 32°. 



