54 DISEASES OF DECIDUOUS FOREST TREES. 



MANNER OF INFECTION. 



One of the most practical questions in connection with the decay 

 of sapwood of broadleaf species, as well as of conifers, deals with 

 the manner in which the sapwood becomes infected with the fungus. 

 With the possible exception of some of the fungi referred to in a pre- 

 vious part of the bulletin, such as Fomes igniarius and Polyporus 

 sulphureus, most of the fungi which bring about the decay of struc- 

 tural timber do not grow in living trees. The heart wood of the liv- 

 ing tree is accordingly free from the supposed germs of decay, and 

 wherever any decay does take place the spores of the fungi responsi- 

 ble for the same must get into the wood from the outside. 



Although this has been very definitely shown to be the case, there 

 is still a very widespread belief among timbermen that decay starts 

 in the interior of the stick. This has arisen from the fact that pieces 



Fig. 11.— Cross section of an oak railroad tie rotted by one of the sap-rot fungi. Note the season crack 

 through which the fungus obtained entrance to the interior. 



of wood may appear perfectly sound on the outside and still may be 

 wholly decayed in the inner part. In figure 11a striking example of 

 this is shown. This figure shows a section taken from a piece of 

 "bottom" red oak cut in Ma}', 1904. The timber was piled in the 

 open, and the section here shown was taken in 1906. It will be 

 noted that the outer quarter of an inch of wood appears perfectly 

 sound and that immediately inside this sound layer the wood has 

 been completely decayed. There was absolutely no external sign on 

 this timber which would indicate that it was not sound throughout. 

 Instances of this kind are numerous and they naturally give rise to 

 much apprehension on the part of the purchaser of timber, because 

 it is not practicable to cut into timbers for the purpose of deter- 

 mining whether they are sound on the inside. 



149 



