Page:Popular Science Monthly Volume 29.djvu/623

Rh or in painted poles, the decay does not appear above the ground-line on the exterior until after the breaking away of the interior wood-cells. In chestnut poles, the sap-wood being so much thinner, the cells break away sooner than in the spruce.

From the place of growth at the ground-line the mycelia pierce the upright longitudinal cells more readily than they do those of the medullary system, growing up and down, but faster in the latter direction, from the fact that the moisture is retained below the ground line in greater abundance than above. Accompanying the mycelia is a growth of ferments, either of the Schizomycetes (bacteria) or the Saccharomycetes depending somewhat upon the particular fungi and the wood. These aid in carrying on the destruction by producing fermentations, which extend down the wood-cells preceding the growth of the mycelia. The illustration was prepared from a pole of which, five feet in length of the base was in the ground, and decay had followed down the cells four feet, while above-ground the decay only followed up the cells a few inches; the pole being unpainted allowed the moisture to escape sufficiently to retard the upward growth. It will be seen from Fig. 10 that a few inches below-ground the exterior of the wood is not so quickly affected, and, comparatively speaking remains sound until destroyed from the inside, though retaining the moisture and facilitating the growth of the fungi. This illustrates one of the important principles to be observed in the care and preservation of our timber in structures, for it will be seen that exterior protection to unseasoned timber, or to that which is to be in a damp situation, retains the moisture and hastens internal decay.

In the case of painted posts above the ground the paint prevents the escape of the moisture, the mycelia and fermentations grow farther up the cells and the posts often break off on the inside above the earth, while appearing sound outside, with a cone-shaped fracture.

In Fig. 10 the decay was extending toward the center of the pole very slowly, the fermentation not being communicated with as great rapidity by the medullary cells, which are only one fifth to one third as large as the upright cells.

Fig. 11 shows the tangential section of the tamarack, which is quite similar to that of the black spruce. The three largest bundles of rays contain resin-ducts, while the cell-cavities of the rays can just be seen; also the sections of the lenticular markings on the walls; in Fig. 12, they show in position on the walls parallel to the medullary rays; the latter are the lines partly crossing the cut, and composed of short contiguous cells, which are thick-walled and not easily penetrated by the mycelia or destroyed by fermentations. An example of the slow lateral extension of the decay is found in the white cedar, a transverse section of which was shown in Fig. 2 (see August number). In growing trees of this wood some of the lower limbs often die, and, not breaking off close to the body, the fungi grow before the wound heals