230 THE ESSEX NATURALIST
although, on the whole, beech is relatively resistant to this
parasite. Its mode of attack is interesting. The spores
cannot directly infect living tissue. Spores, however,
germinate readily on old stumps, and the mycelium grows
in the dead wood. Often dense clumps of fruit-bodies are
to be seen on such a stump. Prom the dead roots of this
saprophytic base the mycelium of the fungus grows out as
highly organised fungal cords or rhizomorphs. These pass
through the soil a few inches below the surface. They look
rather like black boot-laces, and may travel many yards
through the soil. When the tip of a rhizomorph comes in
contact with a living root, penetration occurs and the fungal
mycelium becomes established in the cambium and spreads
back to the collar of the tree where the roots meet the stem.
In this region the living tissue around the wood is
destroyed. The tree is thus effectively girdled, and dies
rather suddenly for a tree. The fungus lives on in the dead
tree and infects the wood. In a dead beech killed by the
fungus, the bark becomes loose, and when this is pulled
away, the black sub-cortical rhizomorphs of the fungus,
looking like anastomozing black boot-laces, are readily to
be seen. If the trunk of the dead tree is cut across, very
conspicuous black lines or black zones are to be seen. The
mycelium of the fungus ramifies through the fibres and
vessels of the woody tissue, occupying a definite volume of
the trunk. The black lines represent the outer limit of a
volume of wood so occupied. If part of a transverse section
passing through a black line is examined, it will be seen
that the line consists of wood elements entirely blocked by
dark, thick-walled fungal hyphae. The black line or black
zone probably protects the mycelium in the volume of wood
it surrounds from excessive water loss. Armillaria mellea
is by no means the only fungus to form black lines in wood.
Around the base of the dead tree, in due course, clumps of
fruit-bodies appear, and new subterranean rhizomorphs may
grow through the soil to claim fresh victims. The only
really effective control of the disease is to grub up and
destroy dead stumps—an expensive business.
There are a number of large perennial polypores that
attack living beech trees. One of the commonest of these
is Ganoderma applanatum, which also occurs on a number