92 HEART-ROT 
rounds the rotted region in advanced stages of disease and 
definitely limits the scope of the fungus. In cases of annular 
attack in which a central pillar 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 help to account for the 
fact that larch trees are so seldom killed by Fomes annosus, 
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 intelligible 
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 appearance from the insoluble gum of larch. 
Decomposition of thewood. Themycelium in the tracheides 
consists, in the first place, of septate hyphae which branch 
freely and have hyaline contents. These hyphae vary 
greatly in thickness, and although all intermediate sizes 
may be found, they are mostly either large (3°5-5p 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 
