2 BULLETIN' 916, U. S. DEPAETZvIEXT OF AGRICULTURE. 
PHYSIOLOGY OF THE FREEZING PROCESS. 
The frost necrosis of potato tubers is described by Jones, Miller. 
and Bailey (l) 2 as being characterized by three types, viz. ring, net. 
and blotch. In the ring type only the vascular ring, an area near the 
surface, is discolored. In the net type threadlike areas radiating 
from the center are discolored. The third type is marked by irregular 
blotched areas. Ordinarily no further change takes place after the 
injured potato thaws, although in the most severe cases the tubers 
soon begin to break down if kept warm. 
Frost injury seems to be the result of actual ice formation within 
the potato tissue. Abbe (1) , in his investigations of the effect of freez- 
ing upon plant tissue, considers that as the tissue cools water exudes 
from the cells into the intercellular spaces. After sufficient under- 
cooling this water freezes. The concentrated sap remaining in the 
cells will not freeze until cooled below the freezing point of water. 
On thawing, this intercellular water escapes by transpiration and the 
plant wilts. Goppert (3) and Sachs (6) both observed the presence 
of ice within the cells and the intercellular spaces of plant tissue. 
However, Sachs found crystals of ice usually present in the inter- 
cellular spaces. Muller-Thurgau (5) was one of the first to publish 
(1880) upon frost injury of potatoes in his investigations upon the 
freezing point of plant tissue. The phenomena of undercooling were 
investigated by him to a considerable extent. He showed that plant 
tissue required undercooling below the true freezing point before 
actual freezing commences. He also showed that the expressed sap of 
the potato tuber freezes at a higher temperature than the living tis- 
sue. The ultimate freezing point of potatoes is placed at 30.21° F. by 
Muller-Thurgau and between 28° and 26° F. by Appleman (2). 
Vaughan and Miller (7) state that "actual freezing does not begin 
in potatoes until the temperature drops below 28° F." Jones, Miller, 
and Bailey (1) place the freezing point at 28.-1° F. 
The author of an unsigned article in the Potato Magazine (8) states 
that exposure at 30° F. for 9 or 10 hours or at 16° F. for 1 hour 
will develop signs of frost necrosis. In results published by Wright 
and Harvey (9) the freezing point varied from 29.67° to 28.13° F, 
according to variety and season. In this investigation the freezing 
points of 18 different varieties were determined. The freezing 
points varied in different varieties and according to family groups. 
The early and medium-early potatoes froze at a higher point than 
the purely late varieties. The subject of undercooling was discussed. 
It was stated that potatoes, since their sap consists of salts, sugar, and 
other soluble material, freeze at a lower temperature than pure water. 
2 The serial numbers in parentheses refer to " Literature cited " at the end of this 
bulletin. 
