Certain rot-producing fungi impart to wood characteristic colors which thus become criterions of weakness.

Ordinary sap-staining is due to fungous growth, but does not necessarily produce a weakening effect.[31] [Footnote 29: See Burke, H.E.: Black check in western hemlock.
Cir.No.61, U.S.Bu.

Entomology, 1905.] [Footnote 30: See McAtee, W.L.: Woodpeckers in relation to trees and wood products.Bul.No.39, U.S.Biol.

Survey, 1911.] [Footnote 31: See Von Schrenck, Hermann: The "bluing" and the "red rot" of the western yellow pine, with special reference to the Black Hills forest reserve.Bul.No.36, U.S.Bu.

Plant Industry, Washington, 1903, pp.