2 BULLETIN 895, U. S. DEPARTMENT OF AGRICULTURE. 
and Bailey 5 the freezing point is near 28.4° F. (-2.0°C.). Potatoes, 
since their sap is a solution of salts, sugar, and various soluble mate- 
rials, freeze at a lower temperature than pure water. Muller-Thurgau 
found that the expressed sap froze at 30.8° F. (-0.65° C), while the 
living potato tissue froze at a lower temperature. He also brought 
out the fact that undercooling usually takes place before freezing. 
According to his results a temperature ranging from 26.25° to 20.3° F. 
was necessary before freezing commenced; subsequently the tem- 
perature rose to the true freezing point. Muller-Thurgau further 
showed that the living tissue required more undercooling than the 
expressed sap. Jones, Miller, and Bailey 5 brought out further data 
on the subject of undercooling, which will be discussed later. 
PLAN OF THIS INVESTIGATION. 
For greater accuracy, the thermoelectric method was used to 
measure temperatures. Practically the same arrangement for tem- 
perature measurements was used as is described and figured by 
Harvey. 6 A 5-junction copper-const ant an couple of the needle type de- 
scribed by Taylor 7 was used in all determinations, in connection with 
a Wolff type potentiometer and a Wolff suspension galvanometer. 
With this arrangement it was not possible to use entire potatoes. 
However, in order to determine whether the freezing points of such 
sections were comparable with those of whole potatoes, a different 
type of freezing apparatus was temporarily used, in which whole pota- 
toes as well as sections could be frozen. It was shown that the results 
were identical. The advantage of using sections of potatoes as 
described is apparent, since these can be frozen much more quickly 
than whole specimens. 
Standard sections from at least two places near the center of each 
potato were cut with a cork borer. These sections were about 
three-eighths of an inch in diameter by 1| inches in length. A hole 
was bored lengthwise nearly the full length of the section, into which 
the thermocouple was thrust. After all surface moisture was blotted 
off with a clean towel, the thermocouple with the section of potato 
was suspended through a stopper in the inner or freezing tube of the 
freezing apparatus. Ordinarily, the surrounding ether was kept 
between 17° and 16° F. Repeated trials showed no consistent dif- 
ference in the freezing points of portions taken from different parts 
of the potato. It was found that considerable time could be saved 
in each determination if the potato was inoculated (see the explana- 
& Jones, L.R., Miller, M., and Bailey, E. Frost necrosis of potato tubers. Wis. Agr. Exp. Sta. Research 
Bul.46, 46p.,illus. 1919. 
6 Harvey, R. B. The importance of epidermal coverings. In Bot. Gaz., v. G7, no. 5, p. 441-444, 2 fig. 
1919. 
7 Taylor, G. F. Some improvements on the needle-type thermocouple for low temperature work. Jour. 
Indus, and Engin. Chem., v. 12, no. 8, p. 797-799. 1920. 
