Brown : Timber Rot 
15 
ing the method of attack and chemical changes in the wood for 
chestnut applies equally well for oak. The first visible evidence 
of attack is the formation of white areas in the tissue due to the 
delignification of the elements. The centers of infection lie be- 
tween the broad pith rays. In the white spots the hyphae run 
vertically and become closely applied to the walls of the cells. The 
fungus works from the lumen outwards, and first removes the 
lignin from the cell walls, leaving pure cellulose behind. As 
soon as the middle lamella is attacked, the cells separate completely 
or cling together loosely at the corners where the thicker walls 
offer more resistance to the fungus. Finally the thin-walled par- 
enchyma cells and the tracheids are entirely dissolved. The 
thick-walled strongly lignified wood fibers persist the longest. 
Only a part of them are dissolved, the remainder forming a white 
cellulose lining and partly filling the cavities (Fig. 15). In the 
final stage the cellulose lining is entirely lacking.^® 
The wood between the infected areas, as in chestnut, remains 
apparently as sound as ever. Here and there a hypha may be 
seen extending horizontally from cell to cell, but connection be- 
tween the pockets is secured mainly, as in chestnut, by strands of 
mycelium which run radially and tangentially. As previously 
noted, the centers of infection have their origin between the large 
medullary rays, while in chestnut they may occur anywhere within 
the ring. Once started, however, the white areas spread and may 
include a portion of a compound ray within their boundaries. The 
decay in oak is comparable to that in chestnut except in minor 
respects. 
The remarkable similarity which has been shown to exist super- 
ficially between the decay of oak caused by S. .frustulosum and 
that of H. rubiginosa is even more striking when a microscopical 
investigation is made. The walls of the infected elements in 
both cases are first delignified, beginning with the tertiary layer, 
and pure cellulose left behind. As soon as the primary layer is 
reached, it is dissolved and the elements separate. Subsequently 
cellulose digestion goes on and the thinner-walled elements are 
entirely dissolved, the thicker-walled fibers disappearing last and 
appearing for a time as a white layer lining the cavities. 
16 What has been said above concerning the restricted peripheral decay in 
chestnut applies equally well to oak. 
