96 College of Forestry 
elements of growth. They are not limited by the annual 
rings, however, for they may cross and recross these repeat- 
edly. Examination shows that these black zones consist of 
wood of unusual hardness. As shown in a previous paper by 
the writer (1917°), these black zones are caused by the infil- 
tration into the bs) of decomposition products formed from 
the woody substance as it decays. They are of common 
occurrence in the decay of all dicotyledonous woods by Poly- 
porus pargumenus and other wood-destroying fungi, but 
appear to be of comparatively rare occurrence in the decay 
of coniferous woods. Further discussion of black zones will 
he presented later. 
Yellow bireh wood in an advanced stage of decay has a 
mottled appearance. The decay is localized mainly in min- 
ute areas, each of which is bounded by a thin zone of firmer 
wood. The areas of advanced decay appear as white spots 
while the less decayed wood bounding these is more or less 
yellowish and resembles superficially a delicate network 
(Plate XX) visible on all three sections. This network is 
rather inconspicuous in dry wood but stands out quite promi- 
nently after the surface of the wood has been moistened. 
As the solvent action of the fungus progresses, the white, 
decayed areas, or pockets, continue to enlarge. Within the 
individual centers of decay the disintegration progresses in 
a direction parallel to that of the woody elements and in 
time the woody elements in these respective areas are reduced 
rapidly both in color and hardness until they become pith- 
hke. 
This method of decomposition results in the formation of 
a series of pronounced pockets or cavities within the wood 
(Plate X XI, Fig 2). In the latter stages of the decomposi- 
tion two or more decayed areas frequently coalesce into one 
so that the wood has a decided ‘ pocketed” appearance. 
Although highly irregular in size and outline, the largest of 
these decayed areas, or pockets, as bounded by the meshes of 
firmer yellowish wood, average two mm. or less in diameter 
and one em. or less in length. In general their length coin- 
cides with the direction of the woody elements, although 
