The Biology of Polyporus Pargamenus Fries 193 
qualities. The rate at which the sapwood of various dicoty- 
ledonous species decays presents but little variation; the rate 
at which the heartwood decays, however, is slower by reason 
of being proportional to the superior durability of the heart- 
wood over the sapwood exhibited by each species. 
For its entrance to the living tree, Polyporus pargamenus 
is dependent upon human, organic, or inorganic agencies, 
usually gaining entrance through mechanical injuries to the 
host. Ordinarily this organism exists as a saprophyte but 
may, when favorable conditions present themselves, become 
a wound parasite. In the latter case it exhibits in its life- 
cycle two rather distinct phases — pathogenesis and sapro- 
genesis. 
The practice of allowing fires to run through our hardwood 
forests can not be too strongly condemned, since even trees 
but slightly scorched on one side furnish admirable infection 
courts for the entrance of Polyporus pargamenus and other 
sap-rotting fungi. The subsequent deterioration of the stand- 
ing timber of any region incident to fire is a direct source of 
loss to the entire community of that region. 
In a study of fire-killed timber made one year and three 
months after the burning, out of 364 standing hardwood trees 
occurring on an area 100 by 500 feet in central Pennsylvania, 
57 per cent already bore sporophores of Polyporus parga- 
menus. Of the 237 white oak trees recorded, this being the 
principal tree on the tract, 69 per cent bore sporophores, 
the largest diameter classes containing the largest proportion 
of sporophore-bearing trees. This indicates the importance 
of the fungus in causing timber deterioration. 
Under the conditions on this area the incubation period of 
Polyporus pargamenus was less than fifteen months, and, in 
all probability more nearly twelve months. In the case of 
artificial cultures the length of the entire life-cycle varied 
from four months in agar plate cultures to eighteen months 
in cultures on blocks of wood. 
In the protection of our forests the control of Polyporus 
pargamenus, as well as other sap-rotting fungi, must be a _ 
“matter of prevention rather than one of cure. Prevention 
7 
