6 BULLETIIT 1099, U. S. DEPAKTMENT OF AGRICULTURE. 



tance in frost resistance, for it allows the tissue beneath it to be 

 undercooled below the freezing point. 



As shown in a previous publication (2), the freezing-point depres- 

 sion of tomato-plant juice can be increased about 0.2° F. by ex- 

 posing the plants for five days at 37.4° F. This may be consid- 

 ered an adaptation to low temperature; but it can not be carried 

 far, because certain physiological disturbances occur which will result 

 in the death of the plants although they are not frozen. It is there- 

 fore seen to be impossible to harden tomatoes to make them immune 

 to frost, like cabbages and such other biennials and perennials, which 

 can be frozen stiff without injury if they are allowed to adapt them- 

 selves to cold during a preliminary hardening period. We have to 

 deal with the factors of undercooling and freezing-point depression 

 in the case of tomatoes rather than with frost immunity. 



STUDIES IN THE FIELD. 



In experiments on the freezing of tomatoes in the field, a series of 

 temperature measurements was made by means of mercury ther- 

 mometers of temperature at Bell, Md., on a clear, still night on which 

 the first frost of the autumn of 1919 occurred. The temperature 

 measurements from which the data shown in Figure 1 were obtained 

 were taken in a field in which 26 varieties of tomatoes were under 

 test to determine their relative frost resistance. The field was lo- 

 cated on a gentle slope, and down the incline an almost imperceptible 

 stream of cold air flowed.'* The current of cold air was rather shal- 

 low, flowing down hill in a layer only a few feet deep. The 

 coldest part of the current was about 10 inches above the ground. 

 At the ground the lower part of this current seems to have been 

 warmed by radiation from the earth. At 18 inches above the ground 

 the air current was somewhat warmer, owing to the tendency of the 

 denser and colder air to move downward. One can frequently see 

 smoke or mist floating on these denser strata near the ground. 



The curves in Figure 1 give the air temperatures at ground level 

 and 6 inches and 18 inches above the surface of the ground as well as 

 the surface and internal temperatures of three tomatoes situated from. 

 10 to 12 inches above ground. These temperatures were determined 

 by thermometers placed at the surface of each tomato or inserted 

 into holes in the fruit made by removing plugs of the same size as 

 the thermometers with a cork borer. From the graphs it is seen that 

 the temperature within the tomato lagged considerably behind that 

 of the air immediately outside. This lag is due to the heat capacity 

 of the tomato and increases with the size of the fruit. With the out- 



* For a more detailed discussion of temperature relations, see " Frost and the Preven- 

 tion of Damage by It." (7). 



