98 College of Forestry 
their power of disorganizing their own substratum by fer- 
mentation and putrifaction, not only cause the substratum 
to become an unfavorable nutrient medium for the entrance 
and growth of other fungi but may in time render it unfit 
for their continued growth. Under normal conditions the 
woody tissue that has been reduced partially by cellulose 
and lignin ferments will of course be attacked by molds, 
bacteria, and other micro-organisms and split up into simpler 
compounds. It is to these agencies that we must attribute 
the final reduction to “ humus” of the refuse material left 
from woody tissue after its partial destruction by the more 
potent agencies — the wood-destroying fungi. 
The outer, paper-like bark of the yellow birch is excep- 
tionally resistant to decay. Yellow birch trees frequently 
are found with this outer bark sound and entire while the 
inner portion of the bark and all of the underlying wood 
may be completely rotted, either by this or by other wood- 
destroying fungi. This great resistance to decay, according 
to Stevens (1910, p. 336), is to be attributed to the presence 
of betulin —a glucoside which occurs in the form of fine 
granules in thinner-walled cork cells of birch bark. This is 
strongly antiseptic and protects birch bark against the attacks 
of lower organisms.** The inner, brown fibrous bark, how- 
ever, is speedily decayed by the action of the fungus and is 
converted into a whitish crumbling mass, which can be pul- 
verized readily between the fingers to a gritty powder 
containing a large percentage of a crystalline substance, 
doubtlessly calcium oxalate. 
Microscopic Characters of Decayed Wood.— In sections of 
wood in an advanced stage of decay, all stages of decomposi- 
tion, from incipient delignification to the complete disap- 
pearance of the elements, may be seen. Separating the 
pockets or decayed areas are irregular zones of less decayed 
wood (Plate XXI, Fig. 2) varying from a few to several 
cells wide. In the median cells of such zones, all three layers 
1 Outer bark of any kind, however, would be quite resistant to decay, 
owing to its corky nature. . 
