This is produced by deposits in the heartwood of various materials resulting from the process of growth, increased possibly by oxidation and other chemical changes, which usually have little or no appreciable effect on the mechanical properties of the wood.

(See HEARTWOOD AND SAPWOOD, above.) Some experiments[28] on very resinous longleaf pine specimens, however, indicate an increase in strength.

This is due to the resin which increases the strength when dry.

Spruce impregnated with crude resin and dried is greatly increased in strength thereby.
[Footnote 28: Bul.

70, U.S.Forest Service, p.