Page:Popular Science Monthly Volume 29.djvu/624

606 and start decay in some of the upright wood-cells. Ties from such trees show decayed spots from one half-inch to an inch in diameter which extend through their entire length of eight feet. When the wounds close up, the decay is checked, and the wood is so durable that the ties are mechanically destroyed in the track before the decay spreads from the spots so as to render them unserviceable.



The manner of decay shown in Fig. 10 is quite similar in principle to that occurring in double bridge-planks; where the under side of the lower plank is exposed to the air, a thin shell from one eighth to one fourth of an inch thick remaining dry, checks the evaporation from below of the absorbed moisture from above, and decay takes place from the fact that the necessary conditions for the fungi to grow are supplied.

A large majority of fence-posts, especially on railways, are from small timbers, with the sap-wood remaining except on the face side; on those of chestnut and oak, Polyporus versicolor, Fr., Fig. 8 (see August number), will often be found fruiting near the ground-line, as seen in the figure, while its mycelium has already partially rotted the post. Many other species of fungi will also be found fruiting, though in a majority of cases the mycelia only will be seen. The bark should always be removed from posts or timber to be used in the ground, otherwise it will furnish means for a growth of mycelia, and the posts or wood will decay much quicker than otherwise would be the case. This is readily seen in the forest; dead trees with the bark on will be found more or less covered with fruiting fungi, the wood decaying with great rapidity, while those with the bark off remain sound for a longer time. A striking object in a forest abounding in birch-trees is to see on their dead trunks, of twenty to thirty feet high, ten to twenty specimens of Polyporus betulinus, Fr. (Fig. 13), from one to four inches in diameter, projecting from small openings in the bark, which clasps around their