Page:Popular Science Monthly Volume 29.djvu/453

Rh —illustrated here—can be detected is by their mycelia, consisting of filaments of, usually, white cells, branching repeatedly by lateral ramifications, growing at their apices, lacing and interlacing, forming in many places dense, felted masses. When they grow on the under side of a plank, closely packed boards, and railroad-ties, they are often similar in form to that shown in Fig. 5. So far as the decay of the wood is concerned, the mycelia of the fungi is the most important part. Though these filaments are small, ranging from 0·0004 to 0·002 of an inch in diameter, they are able to pierce the walls of the wood cells when softened by moisture, which many of them seem to generate to aid in their destructive work.

The fungi, instead of propagating by visible seeds, only have microscopic spores, which are freely disseminated by the air to resting places. When proper conditions for germination occur, the spore sends out a mycelium, which, by spreading over the under side of a plank, as seen in Fig. 5, induces, sooner or later, the decomposition of the structure of its host, to partly build up its own.

Where it has once run over the wood in a dense growth, it destroys its strength from one eighth to three fourths of an inch in depth, and if the wood dries, cracks and crumbles to pieces (see Fig. 5)—it forms the so-called "dry rot" in timber, which is said to take place when the wood is perfectly dry. This is a misconception, as it is impossible for decay to commence without moisture, sufficient heat and access of air to supply the amount of oxygen needed in the reduction of the tissue to lower compounds.

If the wood does not dry, the mycelia continues to grow until all of the wood-cells are disorganized and fall to pieces, or, in other words, completely rotted. In many cases, the mycelia works in the inner portions of the timber, as explained later, and does not show exteriorly until decay is well advanced; this is especially true of larger timber.

"Dry rot" was named from the effect produced, and not the cause, to distinguish it from the so-called "wet rot." It has been an unfortunate designation, misleading many people, causing them to believe that timber will rot when dry, and proper precautions have not been taken to prevent decay, on the supposition that it would occur in any event.

The illustration in Fig. 5 is that of the mycelium of the Polyporous radula (? Fr.) spreading on the under side of the plank of station platforms, which were destroyed in a year and a half to two years. It is typical of a large number of the mycelia of the fungi growing in similar conditions. One sees the same general appearance on lumber, plank, and sawed railroad-ties, which are piled together without being separated from each other by a small air-space. Hemlock inch boards can be completely rotted through in six to eight weeks of July and August weather, by the mycelia attacking both sides of the boards when damp, and piled up without an air-space between each. Cargoes