Page:Popular Science Monthly Volume 29.djvu/459

Rh white-oak ties. It is usually dimidiate, as shown in Fig. 9, though, when growing upon the under side of timber above-ground, it is often resupinate; the pores all point downward, the substance of the cap is hard, and, if undisturbed, the pores in the next year's growth form over that of the preceding years, but, enlarging the area, many of them are found twelve to eighteen inches across, and by cutting through them so as to show the section, six to eight years' growth is often seen. In this figure but one year's growth has taken place; frequently two caps form instead of only one, as here shown.

In white-oak timber and ties the earlier growth of the mycelium is not as continuous and uninterrupted as in the yellow pine, but grows more in little white patches, with considerable wood intervening between each. In Fig. 9 they are shown as dark spots. The massive bundles of medullary rays of this wood slowly decay, and preserve their form long after the other tissue has decayed. The large ducts seen in Fig. 3 are not open with a free communication, but filled with a delicate tissue, remains of which are visible in the cut; this tissue will be found quite perfect in ties well advanced in decay.

The fungi so far illustrated in this paper apply mostly to the decay of timber under conditions similar to those of railroad service. In the next paper I shall give two or three illustrations of fungi of more general character. To deal with the great practical question of preventing wood from decay, the subject requires a more special treatment than it has received. Each species of tree, to a great extent, has special fungi, as it has insects which are not common upon other kinds of wood. Red cedar, cypress, locust, and catalpa are very durable in contact with the ground, where some others would quickly decay. The chemical composition of woods is not practically the same, as recently stated, but differs even in the sap-and heart-wood of the same species. Some of the woods have compounds in their cells easily induced to decompose and start the wood-tissue, while others have different compounds requiring inducing agents of greater intensities to begin decay; and it is not true that a fungus which will destroy one wood will destroy all of the other species, and this one fact is of great practical importance, for, in a road-bed filled with the mycelium of one kind of decayed wood, another wood may be used which is not affected by that fungus, and its mycelium would be inert.

In treating wood it is found that the chemical which will prevent the germination of the spore of the fungus may not protect it from the attacks of its mycelium, contained in the ground or upon other decayed timber.

Experience has long since established the fact that wood kept