This section is from the book "The Gardener's Monthly And Horticulturist V26", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
A correspondent inquires whether the sap freezes in winter in trees or not. We have been so often over this topic before, that it seems superfluous to go over again. But there are so many new readers of the magazine, and so much interest evidenced in the subject that it may do no harm to allude to it again. Experiments have shown that instead of the branches of trees expanding in winter time, as they would do if the sap froze, they actually contract. If we break a twig in the sharp freezing weather we find it crack "short off," just as it would when half dry in summer time. If examined closely the bark will actually have the appearance of being dried, showing wrinkles. Now if this same twig be taken into a warm room it soon changes its appearance, the bark becomes smooth, and the twig will bend short without breaking, and thus we conclude that the sap instead of having froze and expanded, had actually contracted, and we have the lesson that the sap does not freeze. The whole subject is curious, and it is singular that so much misapprehension exists, in view of the fact that a continual flow of liquid through the plant all winter long is a necessary condition of its existence.
That there is a great amount of evaporation going on, we know, and that this evaporation increases with the lowness of the temperature. That liquid is turned into ice does not alter the fact. There is evaporation from ice as well as from water. This evaporation must be supplied, and is supplied during the winter by what is known as "root pressure." In short, the tree would die from sheer evaporation if the circulation was suspended by its liquids freezing.
"A. J. M." writes: " One could ask a thousand people who had cut frozen timber if the sap freezes; all would probably say ' Yes.' When the peach is exposed to 200 or 300 and the wood and bark are ruptured, what does it unless it is the sudden expansion of the sap caused by freezing? Cold expands and contracts ice. Ice or frozen sap will flow under pressure, and pressure enough would supply a flow of ice for all required evaporation, while if the wood fiber was not hardy enough to sustain the pressure it would be ruptured by the freezing".
[We have never known any one to contend that sap will not freeze. If such there be, he may see icicles of frozen sap, often a foot long, pendent from a wounded branch of maple tree, any time in Philadelphia during the spring of the year.
What is contended is, that sap cannot freeze without rupturing some tissue and that this ruptured tissue is either already dead matter, or will speedily become so by the rupture; and this is just what our correspondent shows to be the case. - Ed. G. M].