♦ 
THE FREEZING POINT OF POTATOES. 5 
in the period beginning on April 1 . Unfortunately, specimens of these 
varieties could not be obtained for this work earlier in the season, 
since some of them were being held for another purpose. The data 
presented here bring out the fact that the relations of the freezing 
points of the members of the varietal groups are fairly consistent; 
also that freezing points can possibly be used in helping to classify 
varieties. The average freezing points of the early and midseason 
groups represented here by the Triumph, Cobbler, Michigan, and 
Rose families are distinctly higher than those of the late variety 
groups represented by the Green Mountain, Rural, Up-to-Date, and 
Peachblow families. It is somewhat uncertain where to class the 
American Giant variety. Prof. Stuart, on being asked his opinion, 
expressed uncertainty, but stated that judging from its parentage it 
should possibly be classed with the Green Mountain group, section 2. 
However, if one were classifying potatoes according to freezing 
points alone, this might belong with the early varieties. 
UNDERCOOLING. 
It has already been brought out that generally undercooling is neces- 
sary before potatoes begin to freeze and that according to Muller- 
Thurgau the range of undercooling is from 26.25° to 20.10° F. (-3.2° 
to— 6.6° C). According to the writers' experience, the degree of 
undercooling reached before freezing begins depends for one thing 
upon the rate of undercooling. This is in accord with the results of 
Jones, Miller, and Bailey, 10 who say that "the supercooling range 
seems to depend upon the air temperature and the rate at which 
this temperature is dropped. Thus at —3.5° C. (25.7° F.) the 
supercooling point approaches the air temperature. If the air tem- 
perature is dropped slowly to —5° C. (23° F.) or below, it will 
approach —5° C, while if dropped rapidly to the same point it will 
be much higher, i. e., nearer —3° C." This also depends to some 
extent upon the variety. The most important factors limiting the 
extent of undercooling are accidental external disturbing influences, 
such as j arring, or the freezing of moisture that accidentally may be 
on the outside of the material, or even touching with a crystal of ice. 
Any of these factors in greater or less degree will instantly terminate 
the undercooling process at any point below the true freezing point. 
As undercooling advances, the stability of the liquid state of the 
sap within the tissue becomes more and more strained until a point 
is reached when the slightest perceptible jar will terminate the under- 
cooling, and freezing will begin instantly. The temperature of the 
tissue then rises with extreme rapidity to the true freezing point. 
No attempt has been made to record accurately the time occupied 
while the temperature rises from the maximum degree of Under- 
go Jones, L. R., Miller, M., and Bailey, E. Op. cit. 
