OF TREE BRANCHES AT FREEZING TEMPERATURES 53 
(32° F.), and that it may be considerably lower; that as the sugar 
season approaches it frequently rises above this point, and during 
its course is usually above, but occasionally falls below it; and 
that a part of the sap in the vessels of the outer rings freezes during 
the winter and may freeze during the sugar season.”’ 
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Fic. 19. Curves showing the change in width of frost crack and change of tem- 
perature of tree trunk and atmosphere. At A the tree trunk was at a uniform 
temperature to a depth of five inches. From this point on, the frost crack decreased 
in width consistently with the rise of temperature of the trunk of the tree. At B, 
where the thermometer readings showed the greatest difference of temperature 
between the successive layers of the tree trunk, the frost crack was nearly closed. 
In experiments on certain plants evidence has been presented 
showing that freezing apparently is arrested by what appears to 
be a sub-cooling process; i. e., the confines of the liquid content 
