The Biology of Polyporus Pargamenus Fries 95 
take on merely a yellowish tint, excepting in the terminal 
cells of the erowth ring where they become somewhat more 
strongly colored. 
From the tests described above in full detail, and from 
still other tests employed for the differentiation of both cel- 
lulose and lignin, it is evident that the entire vessel walls and 
the middle lamellx of the other elements are lignified to a 
much greater degree than the secondary and tertiary layers 
of these elements and that the tertiary and secondary layers 
of the wood prosenchyma elements are lignified to an approxi- 
mately equal extent. As a result of the variable structure 
and microchemical reactions of the various elements, we 
naturally would expect to find that the vessel walls would 
resist delignification longer than any one part of the other 
cells. Of the latter the middle lamelle would be far more 
resistant to the solvent action of the fungus than the second- 
ary and tertiary layers of these respective elements. In the 
description of the course of the decay, which is given later, 
we shall see to just what extent this inference is substan- 
tiated. 
Macroscopic Appearance of Decayed Wood.— The first evi- 
dence of incipient decay in yellow birch wood is the appear- 
ance in the wood of small, irregular areas in which the tissues 
have lost their natural brownish color and appear as though 
bleached. The wood between the lighter areas remains mod- 
erately sound for a time, at least it does not begin to decay 
until they are considerably disintegrated; even then their 
decay proceeds very slowly. All parts of the annual ring 
‘are equally susceptible to the attacks of the fungus. 
Cross sections of trunks in the early stages of decay usually 
exhibit a number of conspicuous, irregular zones of blackish 
wood which sharply delimit decayed from sound wood or 
portions of the wood in different stages of decay. In cross 
sections of partially decayed trunks these black zones in the 
wood have the appearance of irregular hnes. Longitudinal 
sections show that they extend for varying distances up and 
down the stem, their general course being parallel to the 
