10 
Mycologia 
hyphae run vertically in the cell lumina and are closely applied 
to the tertiary wall. In cross section (Figs. 22 and 26) they ap- 
pear as small black dots within the cell cavity. Viewed in long- 
itudinal section each element is seen to contain from several to 
many hyphae which extend vertically from cell to cell and pene- 
trate the walls at will. Preference is shown for the pits on the 
walls as in the case of some other forms (Figs. 27 and 29), but 
the avenues of penetration are by no means restricted to the pits. 
The hyphae pass directly through the wall with little or no con- 
striction (Fig. 23). Subsequently the perforations thus arising 
are further enlarged through enzyme action so that the hyphae 
appear to pass through openings much too large for them. In an 
advanced stage of decay only the openings are left as the hyphae 
disappear (Figs. 25 and 30). Nests of hyphae also accumulate 
in vessel cavities, but the centers of infection are usually in areas 
where only parenchyma and fibrous tracheids occur. Where pith 
rays cross the diseased areas, they are attacked and eventually 
destroyed. 
Chemical Changes in Chestnut Wood . — The first chemical 
change which is brought about by the fungus is that of deligni- 
fication. It is due to this action that the elements lose their normal 
brown color and become white. The fungus probably secretes 
dissolving enzymes which attack the tertiary layers first and work 
outward. The compounds which are known collectively as lignin 
are entirely dissolved and walls of pure cellulose left behind. Near 
the center of infected areas all stages in the process of deligni- 
fication are to be found (Fig. 26). In the sound cells just with- 
out the diseased tissue, all three cell layers respond to tests for 
lignin (Fig. 22 c), although the tertiary layer appears to be less 
strongly lignified than the others. The elements nearer the center 
of infection are already partially delignified (b). In some the tert- 
iary layer no longer gives the lignin reaction, or but feebly, while 
in others the secondary and primary layers have become involved 
and delignified. The lignin reaction persists longest at the cell 
corners where the wall is thickest, or about the small intercellular 
spaces which often occur there, but even here it disappears even- 
tually, and the tissue that remains consists of almost pure cellulose. 
As soon as the middle lamella is delignified, the cells separate 
