6o6 



THE POPULAR SCIENCE MONTHLY. 



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. 



Fig. 11. Tangential Section op Tama- 

 rack, 2 ^. 



Fig. 12. Radial Section or Tamarack, ^. 



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 sup- 

 plied. 



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, Polyioorus 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 my- 

 celia, 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, pro- 

 jecting from small openings in the bark, w T hich clasps around their 



