46 BULLETIN 871, U. S. DEPARTMENT OF AGRICULTURE. 
An examination of knots on the outside of a log or tree usually 
does not give any reliable indication of the condition of the heart- 
wood with respect to its degree of soundness. In this respect dry- 
rot differs markedly from the stringy brown-rot (Echinodontium 
tinctorium) in white fir and the ring scale or red-rot (Trametes pini) 
in Douglas fir. 
Fire and knots are responsible for over 90 per cent of the infections 
and severe cull cases: other factors are of minor importance. 
About 3 per cent of the infections on the combined areas entered 
through wounds the causes of which it was impossible to exactly 
determine. Some of these may have been fire wounds, others 
lightning. Of all such wounds 23 per cent subsequently became 
infected. 
Lightning is of little importance as a means of entrance for dry-rot. 
On the intermediate area only 2.5 per cent cf the infections are 
traced to this source, on the optimum area 1.5 per cent, and for the 
combined areas 2 per cent. As a rule, lightning causes small super- 
ficial scars offering little opportunity for inoculation, but at times 
large areas of the cambium and bark are killed. This dead bark 
then drops off, exposing the sapwood. which dries out. Cracks 
opening up into the heartwood are formed, and such large areas 
offer a good chance for the lodgment of fungous spores. This con- 
dition is reflected in the percentage of risk of inoculation when it is 
found that 19.2 per cent of the lightning-struck trees on the optimum 
area. 31.8 per cent on the intermediate area, and 25 per cent for the 
combined areas were infected by dry-rot through lightning wounds. 
This figure is higher than that for all other factors except fire. The 
chief reason, then, that lightning wounds are of so little importance 
in relation to decay is not that the character of wounding on the 
whole is such as to offer little opportunity for inoculation, but rather 
that this type of wounding is rare. 
As an actual means of entrance of decay, frost cracks are even less 
important than lightning wounds. These cracks, while often of 
considerable length, even then present only a very narrow opening 
exposed to the air. the chances of fungous spores lodging in such a 
small opening being exceedingly small. Xot quite 1 per cent of the 
infections for the combined areas entered through frost cracks, 
while the risk of infection is only 10.4 per cent lower than for all 
others except broken and dead tops. But though a rare source of 
infection, frost cracks sometimes carry an infection, entering through 
some other type of wound, over a greater linear extent in the heart- 
wood than might normally be expected, thus resulting in a large 
proportion of cull. The pockets of dry-rot do not occur in the wood 
immediately adjacent to the cleft or crack, but are usually found 
some distance removed, leaving the wood around the crack sound. 
