TliG Black Zones Formed by Wood-Destroying Fungi 83 



tion. Lindrotli (190-1) in discussing such a zone of " protec- 

 tion heart/' as he called it, caused by Polyporus nigricans 

 Fries in birch wood, states that it is especially resistant to 

 decomposition and offers a certain protection against further 

 attack by the fungus. If it should happen, as these blackish 

 zones continue to become thicker, that the supply of water 

 within the diseased portions of the wood falls below the de- 

 mand made upon it by the fungus, the mycelium — providing 

 it has not been smothered in the meantime — maj break 

 through and transform another zone into pathologic heart- 

 w^ood. This gives rise to the well-known concentric blackish 

 zones which at first sharply separate the various stages of the 

 decay in wood (Plate 1). 



It is plausible that the occlusion of the vessels and other 

 elements of the wood renders the pathologic heartwood, as 

 well as the healthy wood lying directly without it, relatively 

 free from air; furthermore it inhibits the circulation of air 

 and hence the inroads of the fungal hyphae. It is also pos- 

 sible that the brown decomposition products with which the 

 cell walls frequently are infiltrated may be, to a certain 

 degree, harmful to fungi. At least such infiltrated substances 

 are more difficult for them to assimilate than those constitu- 

 ents which have not vet been oxidized. Owing to this the 

 contents of the cells which otherwise would be an important 

 source of nitrogen for the growth of fungi have assumed 

 through chemical decomposition a form more difiicnlt to 

 assimilate. On 'these groimds greater resistance to decom- 

 position by fungi is to be expected, especially if the fungi 

 which have caused this transfoiination to pathologic heart- 

 wood are already smothered. For fungi coming in later the 

 culture medium is made poor. 



Under optimum conditions for the growth of fungi, heart- 

 wood (normal as well as pathologic) in its typical condition 

 is more highly resistant to decay than the sapwood. In a 

 living tree, however, the sapwood frequently is less attacked 



