CHAPTER V 
HEART-ROT. FOMES ANNOSUS 
Various fungi which cause heart-rot. Y'omes annosus: general; his- 
torical. Secretions induced by Fomes annosus: turpentine and resin ; 
soluble gum ; insoluble gum. Decomposition of the wood. 
HEART-ROT of trees is caused by fungi which grow sapro- 
phytically on the dead wood, but are cither incapable of 
growing parasitically, or attack living tissues very feebly. 
The heart-wood of trees is dead, has lost all its protoplasm 
and with it much of its power of resisting fungi. Though 
it is more resistant than dead sap-wood, it is by far the 
most susceptible part of the trunk of a living tree, and 
is liable to become entirely decaycd, leaving only a ring of 
healthy sap-wood which maintains the life of the tree. 
With the destruction of the heart-wood the trec may lose 
none of its vital activity, but it is nevertheless weakened 
in two respects. The central column of tough wood is lost 
and the tree is much more liable to be wind-blown, especially 
as the roots are often weakencd in the same manner as the 
trunk, and if the central hollow has any communication 
with the outside air, a ready means of infection is provided 
for truly parasitic fungi. 
One of the most familiar instances of heart-rot caused 
by a purely saprophytic fungus is afforded by the elm. 
This tree is usually grown in avenucs or hedgerows where 
large lateral branches are allowed to develop which would 
be suppressed in closcly-timbered woods. These branches 
are brittle, and frequent breakages result in numerous large 
scars which expose both sap-wood and heart-wood to fungal 
attack. In summer spores are nearly everywhere prescnt 
to take advantage of these scars, and the trecs become 
infected with such fungi as Fomes ulmarius, which is the 
chicf, if not the only, cause of heart-rot and hollowness in 
