92 HEART-KOT 



rounds the I'otted region in advanced stages of disease and 

 definitely limits the scope of the fungus. In cases of annular 

 attack in A\hich a central ]:)illar of wood is left unrotted, it 

 is found that this is surrounded by a layer containing the 

 gummy substance, so that here also it secures the central 

 portion from attack. And when, as so often happens, the 

 rot is confined to certain patches (as seen in a transverse 

 cut), each patch is surrounded by a layer of this gum. 

 How this layer is formed is obscure, but its protecting 

 efficacy is clearly shown by the fact that on one side of it 

 the wood may be entirely rotted, though on the other it is 

 quite sound. This layer may thus heljD to account for the 

 fact that larch trees are so seldom Idlled by Fomes amwsus, 

 for the young wood, being protected by this gummy layer, 

 continues to function. In completely rotted, dead trunks, 

 portions may often still be found which have been saved 

 from rot by the preservative gummy layer, though in such 

 cases they are usually not entirely free from fungi, and one 

 must suppose that the gum is eventually decomposed. 

 Woodmen have told me that they consider the part immedi- 

 ately surrounding the rot as stronger and more resistant to 

 damp than ordinary larch wood. This is readily intelhgible 

 in the light of the above observations. 



In spruce the heart-rot is sometimes surrounded by a dark- 

 brown or grey layer. In this layer some of the tracheides 

 are partially filled with a gum, but it is duller in colour and 

 has a different apj)earance from the insoluble gum of larch. 



Decomposition of the wood. The mycelium in the tracheides 

 consists, in the first place, of septate hyphae which branch 

 freely and have hyaline contents. These hyj)hae vary 

 greatly in thickness, and although all intermediate sizes 

 may be found, they are mostly either large (3"5-5/x in 

 diameter) or small (1-2//). The large ones grow somewhat 

 irregularly, are often wavy in outline, and have thin walls. 

 The fine ones usually grow very straight and arise from 

 thick ones without themselves giving off many branches. 

 In early stages of attack they usually grow, from one 

 tracheide to another through the bordered pits, but later 



