84 HEART-ROT 
when the rot reaches the trunk it spreads both upwards 
and downwards. There is some doubt as to the method of 
infection adopted by Poria vaporaria, but probably, like 
Polyporus sulphureus, it can only reach the heart-wood 
through wounds. 
Fomes annosus, (Fr.) Cooke: General. In Britain Fomes 
annosus attacks nearly all kinds of conifers. It has, how- 
ever, two different modes of attack, and as the symptoms 
are very distinct in the two cases, they might easily be 
considered the work of two different fungi. In one case the 
fungus attacks the trees when young, usually about four 
to ten years, and kills them in a year or two. Frequently 
in young plantations individuals or groups may turn a brown 
or reddish-brown colour, lose their leaves and die, and on 
pulling these trees up it is generally found that the roots 
are attacked either by Armillaria mellea or Fomes annosus. 
The identity of Armillaria mellea can generally be detected 
by the presence of rhizomorphs and the large flaky masses 
‘ of mycelium between the wood and the phloem and between 
the scales of the bark of the rootstock, as well as by the 
resin flow at the base of the trunk. Fomes annosus is not 
so easy to identify unless some of the perennial fructifications 
are present, but small pustules of mycelium on the roots 
and the very thin layers of mycelium in the bark, no thicker 
than the thinnest tissue paper. are usually sufficient evidence 
of its presence. The species that are most commonly 
attacked in this way are Douglas fir, Lawson’s cypress, 
Weymouth pine, and sometimes the monkey-puzzle tree, 
but probably many other conifers are liable to this kind of 
attack. In such instances the fungus is a fairly rapid 
parasite, and death follows quickly on attack. Other 
species, such as the iarch, spruce, and Scots pine, are not 
generally attacked by the Fomes until they are comparatively 
old and have made heart-wood. On these trees the fungus 
is a rather feeble parasite and kills living tissues very slowly. 
It flourishes, however, in the heart-wood which is dead, and 
1 The death of some young larch trees in the manner described above 
is reported by Somerville (1898). 
