20 BULLETIN 871, U. S. DEPAETMENT OF AGEICTTETTJEE. 



words in a letter to the writer, "The sporophores are of the small 

 depauperate type which I find occasionally on trees at high eleva- 

 tions or on old punk knots from which the original sporophores have 

 fallen and are reviving." 



However, for the purposes of this paper these decays may all be 

 treated as one and the same, since the dry-rot in small pockets and 

 the Trametes pir& decay are of negligible importance both in the 

 number of infections and amount of cull resulting. Hence, except 

 in the data on the rate of spread of the dry-rot, they are included in 

 all subsequent pages with the typical decay of Polyporus amarus. 



Xo relation was found between the width of the sapwood and the 

 extent of decay; trees with wide and narrow sapwood seem to be 

 equally affected with the dry-rot. 



RAPIDITY OF SPREAD OF THE DRY-ROT. 



Although the rapidity of the spread of decay caused by heartwood- 

 hihabiting fungi in standing trees has always been of interest, very 

 little work has been done on this line. Hartig (9, p. 115-116), 

 mentions this briefly in relation to the rot caused by Polyporus 

 (Fomes) igniarius in oak. More recently Munch (23) has published 

 some interesting results from studies of the same fungus and host, 

 showing a wide variation of 3.8 to 37.5 cm. (0.12 to 1.23 feet) in 

 the yearly vertical progress of the decay, with an average of 5 to 9 cm. 

 (0.16 to 0.30 of a foot). No tangible difference was found between 

 the upward and downward rate of spread from the point of infection. 

 Munch's results are based on an analysis of only 15 cases, and 

 their value is further reduced by the fact that in determining the 

 age of the infection which entered a tree through an open wound, he 

 assumed that infection must have occurred the yeav the wound was 

 made, or at least a very few years subsequently, even though the 

 wound was still open at the time of analysis. True enough, as 

 shown by Munch (23), Fomes igniarius attacks not only the heart- 

 wood but the sapwood of many trees and Mils the cambium, causing 

 cankers with subsequent callusing, and by counting the number of 

 annual rings in the callus at the point of infection the age of the 

 decay can be determined, provided a canker was formed the year of 

 infection; but this is not uniformly the case, to judge from Munch's 

 (23, p. 519) own statement that " Fomes igniarius produces exceed- 

 ingly variable cankers. Sometimes small points of infection which 

 are scarcely noticeable and are soon healed perfectly ..." 



In securing the figures on the yearly rate of spread of the dry-rot, 

 only those infections were considered the entrance of which could 

 be absolutely traced, without any other possibilities, to a healed 

 scar for which it was possible to determine the exact dates of 

 occurrence and closure. For example, an infection is found in a 





