6 BULLETIN 1099, U. S. DEPARTMENT 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- 
5 For a more detailed discussion of temperature relations, see ‘‘ Frost and the Preven- 
tion of Damage by It.” (7). 
