It is obvious that such views could apply only to sapwood, since it alone is in living condition at the time of cutting.  Heartwood is dead wood and has almost no function in the existence of the tree other than the purely mechanical one of support.  Heartwood does undergo changes, but they are gradual and almost entirely independent of the seasons.

Sapwood might reasonably be expected to respond to seasonal changes, and to some extent it does.  Just beneath the bark there is a thin layer of cells which during the growing season have not attained their greatest density.  With the exception of this one annual ring, or portion of one, the density of the wood substance of the sapwood is nearly the same the year round.  Slight variations may occur due to impregnation with sugar and starch in the winter and its dissolution in the growing season.  The time of cutting can have no material effect on the inherent strength and other mechanical properties of wood except in the outermost annual ring of growth.

The popular belief that sap is up in the spring and summer and is down in the winter has not been substantiated by experiment.  There are seasonal differences in the composition of sap, but so far as the amount of sap in a tree is concerned there is fully as much, if not more, during the winter than in summer.  Winter-cut wood is not drier, to begin with, than summer-felled—­in reality, it is likely to be wetter.[47]

[Footnote 47:  See Record, S.J.:  Sap in relation to the properties of wood.  Proc.  Am.  Wood Preservers’ Assn., Baltimore, Md., 1913, pp. 160-166.

Kempfer, Wm. H.:  The air-seasoning of timber.  In Bul. 161, Am.  Ry.  Eng.  Assn., 1913, p. 214.]

The important consideration in regard to this question is the series of circumstances attending the handling of the timber after it is felled.  Wood dries more rapidly in summer than in winter, not because there is less moisture at one time than another, but because of the higher temperature in summer.  This greater heat is often accompanied by low humidity, and conditions are favorable for the rapid removal of moisture from the exposed portions of wood.  Wood dries by evaporation, and other things being equal, this will proceed much faster in hot weather than in cold.

It is a matter of common observation that when wood dries it shrinks, and if shrinkage is not uniform in all directions the material pulls apart, causing season checks. (See Fig. 27.) If evaporation proceeds more rapidly on the outside than inside, the greater shrinkage of the outer portions is bound to result in many checks, the number and size increasing with the degree of inequality of drying.

In cold weather, drying proceeds slowly but uniformly, thus allowing the wood elements to adjust themselves with the least amount of rupturing.  In summer, drying proceeds rapidly and irregularly, so that material seasoned at that time is more likely to split and check.