the process of destruction is the same.  The mycelial threads penetrate the walls of the cells in search of food, which they find either in the cell contents (starches, sugars, etc.), or in the cell wall itself.  The breaking down of the cell walls through the chemical action of so-called “enzymes” secreted by the fungi follows, and the eventual product is a rotten, moist substance crumbling readily under the slightest pressure.  Some species remove the ligneous matter and leave almost pure cellulose, which is white, like cotton; others dissolve the cellulose, leaving a brittle, dark brown mass of ligno-cellulose.  Fungi (such as the bluing fungus) which merely stain wood usually do not affect its mechanical properties unless the attacks are excessive.

It is evident, then, that the action of rot-causing fungi is to decrease the strength of wood, rendering it unsound, brittle, and dangerous to use.  The most dangerous kinds are the so-called “dry-rot” fungi which work in many kinds of lumber after it is placed in the buildings.  They are particularly to be dreaded because unseen, working as they do within the walls or inside of casings.  Several serious wrecks of large buildings have been attributed to this cause.  It is stated[40] that in the three years (1911-1913) more than $100,000 was required to repair damage due to dry rot.

[Footnote 40:  Dry rot in factory timbers, by Inspection Dept.  Associated Factory Mutual Fire Insurance Cos., 31 Milk Street, Boston, 1913.]

Dry rot develops best at 75 deg.F. and is said to be killed by a temperature of 110 deg.F.[41] Fully 70 per cent humidity is necessary in the air in which a timber is surrounded for the growth of this fungus, and probably the wood must be quite near its fibre saturation condition.  Nevertheless Merulius lacrymans (one of the most important species) has been found to live four years and eight months in a dry condition.[42] Thorough kiln-drying will kill this fungus, but will not prevent its redevelopment.  Antiseptic treatment, such as creosoting, is the best prevention.

[Footnote 41:  Falck, Richard:  Die Meruliusfauele des Bauholzes, Hausschwammforschungen, 6.  Heft., Jena, 1912.]

[Footnote 42:  Mez, Carl:  Der Hausschwamm.  Dresden, 1908, p. 63.]

All fungi require moisture and air[43] for their growth.  Deprived of either of these the fungus dies or ceases to develop.  Just what degree of moisture in wood is necessary for the “dry-rot” fungus has not been determined, but it is evidently considerably above that of thoroughly air-dry timber, probably more than 15 per cent moisture.  Hence the importance of free circulation of air about all timbers in a building.

[Footnote 43:  A culture of fungus placed in a glass jar and the air pumped out ceases to grow, but will start again as soon as oxygen is admitted.]