Seasoning of Wood

by J. B. Wagner

It is generally supposed that trees contain less water in winter than in summer. This is evidenced by the popular saying that "the sap is down in the winter." This is probably not always the case; some trees contain as much water in winter as in summer, if not more. Trees normally contain the greatest amount of water during that period when the roots are active and the leaves are not yet out. This activity commonly begins in January, February, and March, the exact time varying with the kind of timber and the local atmospheric conditions. And it has been found that green wood becomes lighter or contains less water in late spring or early summer, when transpiration through the foliage is most rapid. The amount of water at any one season, however, is doubtless much influenced by the amount of moisture in the soil. The fact that the bark peels easily in the spring depends on the presence of incomplete, soft tissue found between wood and bark during this season, and has little to do with the total amount of water contained in the wood of the stem.

Even in the living tree a flow of sap from a cut occurs only in certain kinds of trees and under special circumstances. From boards, felled timber, etc., the water does not flow out, as is sometimes believed, but must be evaporated. The seeming exceptions to this rule are mostly referable to two causes; clefts or "shakes" will allow water contained in them to flow out, and water is forced out of sound wood, if very sappy, whenever the wood is warmed, just as water flows from green wood when put in a stove.

Composition of Sap

The term "sap" is an ambiguous expression. The sap in the tree descends through the bark, and except in early spring is not present in the wood of the tree except in the medullary rays and living tissues in the "sapwood."

What flows through the "sapwood" is chiefly water brought from the soil. It is not pure water, but contains many substances in solution, such as mineral salts, and in certain species—maple, birch, etc., it also contains at certain times a small percentage of sugar and other organic matter.

The water rises from the roots through the sapwood to the leaves, where it is converted into true "sap" which descends through the bark and feeds the living tissues between the bark and the wood, which tissues make the annual growth of the trunk. The wood itself contains very little true sap and the heartwood none.

The wood contains, however, mineral substances, organic acids, volatile oils and gums, as resin, cedar oil, etc.

All the conifers—pines, cedars, junipers, cypresses, sequoias, yews, and spruces—contain resin. The sap of deciduous trees—those which shed their leaves at stated seasons—is lacking in this element, and its constituents vary greatly in the different species. But there is one element common to all trees, and for that matter to almost all plant growth, and that is albumen.

Both resin and albumen, as they exist in the sap of woods, are soluble in water; and both harden with heat, much the same as the white of an egg, which is almost pure albumen.

These organic substances are the dissolved reserve food, stored during the winter in the pith rays, etc., of the wood and bark; generally but a mere trace of them is to be found. From this it appears that the solids contained in the sap, such as albumen, gum, sugar, etc., cannot exercise the influence on the strength of the wood which is so commonly claimed for them.