44 BULLETIN 871, U. S. DEPARTMENT OF AGRICULTURE. 



Table X shows that on the intermediate area nearly 76 per cent 

 of all the infections entered through fire wounds; this means of 

 entrance for the optimum area is approximately 59 per cent, while 

 for all the areas combined it is almost 70 per cent. Since fire scars 

 are almost invariably found in the base of the tree, commencing at 

 ground level, these figures are at variance with Von Schrenk's (26, 

 p. 69) idea that "the decay begins somewhere in the upper part of a 

 tree. " 



Besides fire wounds being responsible for such a high percentage 

 of the infections, inoculations through wounds of this character 

 quite commonly lead to very serious and damaging dry-rot, even 

 in some of the younger trees. In many cases, even in old trees, a 

 long continuous pocket of dry-rot, sometimes having a linear extent 

 of 10 feet, will follow a healed fire scar, running out at the end of 

 the wound, with no further decay extending up the tree. Such 

 infections do not appear especially serious, but it must be remembered 

 that the most valuable portion of the trees, the heartwood in the 

 butt log, is damaged. On the other hand, the fungus evidently 

 finds conditions highly unsuitable in the wood back of a large open 

 fire scar. Almost every tree with this type of wound appeared 

 sound on the stump when felled, but serious dry-rot appeared at 

 the first cut above the open fire scar. When such logs were split, 

 it was found that the pockets of dry-rot commenced just at or a 

 little above the top of the open fire scar, but rarely lower down. 

 The avoidance of the dried-out wood around an open fire scar by 

 the mycelium of this fungus is not at all in keeping with the results 

 of experiments of Munch (19, 20, 21), which emphasized the highly 

 favorable influence of an increase in oxygen coupled with a decrease 

 in moisture in the host tissues on the development of various wood- 

 inhabiting fungi. There should certainly be a big increase in the 

 oxygen content of heartwood directly exposed to the air over that 

 protected by a heavy layer of bark and sapwood, thus, according to 

 Miinch's theory, causing very serious dry-rot in the wood around 

 open fire scars. The exact reverse of this is the condition actually 

 existing. However, every wood-inhabiting fungus must have cer- 

 tain minimum physical requirements for its growth and development. 

 Possibly the dried-out wood in this case falls below the minimum 

 water requirement of Polyjiorus amarus, or it may be that certain 

 chemical changes in the wood brought about by more or less ex- 

 posure to the air inhibit the growth of the fungus mycelium. 



Not every fire scar is inoculated, but the chances for inoculation 

 with subsequent infection are rather high, owing to the relatively 

 large area of heartwood exposed offering a broad surface for the 

 lodgment of spores of the dry-rot fungus. On the optimum area, 

 70 per cent of the trees wounded by fire subsequently became infected, 



