38 BULLETIN 1128, U. S. DEPARTMENT OF AGRICULTURE. 
from a white-ash longeron with this incipient decay is illustrated in 
Plate I, right side. 
Apparently mycelium does not occur in the brown discolored wood 
in advance of the white spots. It would seem that the wood is not 
weakened until the white spots are found, and the wood with the 
brown discoloration alone need not be rejected. It is an excellent 
hint for close scrutiny of an affected piece, however. The incipient 
decay is somewhat obscured in rough lumber, but is usually readily 
apparent on smooth surfaces. This stage does not extend many feet 
beyond the typical decay, and on long boards the latter will most 
. likely also occur. Once the presence of the typical decay is ascer- 
tained it is a relatively simple matter to determine the limits of the 
incipient stage. 
Areas in which the wood failed to change color upon transition 
from heartwood to sapwood (see p. 16) can be differentiated from 
the initial stages of white-rot by nee larger size, by the straw-yel- 
low color as opposed to the whitish of the decay, by the sharp line 
between the two colors, and by the fact that the spots are much 
larger, without becoming soft and spongy, than would be the case 
with the decay. 
Sweet birch and yellow birch are subject to a white heart-rot (32) 
which, although very similar to the foregoing, is caused by a dif- 
ferent fungus, the false tinder fungus (Pomes igniarius (u.) Gill.). 
The first indication of the incipient decay is a brown discoloration, 
not very apparent against the reddish brown heartwood. Next, 
faintly paler streaks or spots appear, which finally become a yel- 
lowish white, strikingly apparent against the dark background. 
This stage is illustrated by Plate VII. In the center of these streaks 
small spots are found in which the yellowish white wood appears 
to have collapsed. Usually the long axis of these spots is parallel to 
the grain, but in some it may be at right angles to it. The wood up 
to this time appears firm and hard. Next the white streaks merge, 
the wood becomes soft, and finally the entire affected portion of the 
heartwood is reduced to a yellowish white fibrous mass composed 
principally of cellulose, the result of the delignification by the fun- 
gous hyphe. As in the white-rot of ash, hy phe are not found in the — 
brown discoloration. Hence, no reduction in the strength of the 
wood may be expected until the very first indications of the whitish 
streaks or spot, which may be found as much as 8 feet in advance 
of the typical decay. 
One of the most common decays (24) on oaks and es on cer- | 
tain poplars (Populus) is the heart-rot caused by the oak fungus 
(Polyporus dryophilus Berk.). The incipient decay of this whitish | 
piped rot in white oak has a water-soaked appearance in the unsea- — 
soned wood, but when dry the discoloration becomes hazel to tawny — 
in color. The discoloration may extend from 1 to 10 feet in advance 
of any other indication of the decay. The next stage of the decay, — 
- which is best seen on a radial face, is characterized by whitish spots — 
or streaks, usually following the medullary rays, which produce a | 
delignification process; that is, the lignin is removed from the wood, © 
leaving only the cellulose. In the final stages the decayed wood is © 
firm, with a white, stringy appearance, and the delignification is — 
mottled appearance of the wood. This mottling is the result of a 
practically complete. 
ae Ae 
ee 
