This section is from the book "The Gardener's Monthly And Horticulturist V22", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
Geo. W. D., Kent, O., writes: "I was much interested in what was said in your valuable Monthly in regard to foliation and heat, but was, I confess, startled by your statement that the sap is never frozen in the living cells without killing, those cells dying afterwards. I grant that if the cells and ducts were full of sap, this would be the result, but I believe the truth of the matter to be that when the tree is in its dormant state they are not full, and in this state what moisture remains can be, and is frozen without bursting the cells and without injury to the tree. I have noticed that in very cold weather the young shoots of trees are smaller, and that the thin outer bark is wrinkled in consequence of the shrinkage".
[The observation of our correspondent is quite correct. Not only do twigs shrivel under severe frost, but the actual girth of a tree is less after a few hours of severe frost than it was before. In experiments made by the writer of this, there has been as much as a quarter of an inch shrinkage in a luxuriant silver maple tree about three feet round before the thermometer fell to near zero. Now if the sap froze the trunk would expand and not shrink. It should not be forgotten that when moisture is in a finely divided condition it does not "freeze." The atmosphere in winter is often full of moisture, though the thermometer may be at zero, and moisture in trees is so constituted that it does not freeze under low temperatures, but evaporates through the tissues, and the branches shrink just as our correspondent has noticed. Some trees or plants have not this power. A geranium has not. Its sap does freeze, and the plant is killed; but when trees die in winter, that do not have their sap freeze, they die because the branches dry up.
The sap does not freeze, it evaporates. - Ed. G. M].