The Biology of Polyporus Pargamenus Fries 135 
strain imposed upon them much longer than would those 
woods which lack these reinforcing structures. 
The outer, corky bark of all the trees studied (both micro- 
scopically and macroscopically) has proved to be exceedingly 
resistant to decay. This outer, corky bark frequently re- 
mains entirely sound and free from decay in many trees 
even after the inner bark and the underlying sapwood may 
be completely rotted. ‘The great resistance of the outer bark 
of most trees to decay is to be attributed to the presence of 
tannins which are noted for their preservative qualities. The 
efficacy of intact and uninjured mature bark as a barrier to 
the entrance of parasitic fungi into the trunk cannot be too 
strongly emphasized. When the outer, corky bark has 
become injured, however, the inner fibrous bark is of no 
avail in keeping out the spores of wood-destroying fungi. 
As a matter of fact the exposed cambial layer, which is exceed- 
ingly rich in food materials, immediately affords a highly 
favorable environment for the germination and subsequent 
development of any fungous spores that may chance to lodge 
thereon. In attacked trees, especially felled ones, the rapid- 
ity with which the fungous mycelium spreads between the 
bark and the sapwood is nothing short of surprising. In the 
course of the decay of the inner bark pockets or cavities are 
produced in it similar to the ones found in the decaying 
sapwood. 
Microscopic Characters.— Prepared slides of the decayed 
woods, when examined under the microscope, show up the 
pocket formations very clearly. The pockets containing wood 
elements in various stages of decomposition are separated 
from one another by irregular zones of less decayed wood, 
varying from a few to several cells wide. 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. In the central cells 
of such zones all three layers of the cell-wall of the elements 
respond to lignin tests as they do in normal wood. The cells 
bordering on these elements are in various stages of decom- 
