SOME FUNGI IN WOOD.
35
a consequence, deficient in air. This difference would doubtless
be more marked but for the presence in the heartwood of sub-
stances tending to inhibit fungal activity. Hiley [18] is careful
to point out that although heartwood is more resistant than dead
sapwood, it is by far the most susceptible part of the living tree.
An interesting illustration of the need for oxygen is seen in
comparing the rotting of wooden posts in sandy and clay soils.
In the former the rot will frequently extend from the base up to
a little above ground level: in clay the post usually snaps off
clean at ground level, and I have on occasion found such posts
to be reasonably sound below ground, where the oxygen supply
was insufficient to permit of fungus development, a case strictly
comparable with immunity from fungal attack of submerged
timber.
Stagnant air favours the rotting of wood; this may appear
difficult to correlate with the need of oxygen, but moving air
will, of course, tend to dry out the wood, and thus to make it a
less favourable substratum for the fungus; in addition, air
currents will tend to prevent fungal spores from settling on.
the wood. Here two factors, moisture and oxygen, have to be
considered, and the former, not the latter, is limiting.
Favourable conditions apart, a fungus must be equipped with
a means of breaking down the resistant cell walls of the wood.
Even if it be a sap stain fungus it must still be able to attack
and destroy the middle lamella at least, before it can pass from
one cell to another. Fungi are not lacking in suitable enzymes.
Schmitz [28, 29] found thirteen enzymes in Fomes igniarius, and
only one less in two other fungi. Passage of the hyphae from
cell to cell may be effected directly through the cell wall or by
way of the pits, where, of course, there is only the middle lamella
to be penetrated: in the wood-rotting types the hyphae pass
through the cell wall as well as through the pits, but the sap
stain fungi generally make use of the pits. Penetration is
probably due to the secretion of enzymes at the tip of the hypha,
which break down the wall material. The manner of penetration
may sometimes be of assistance in the identification of the
attacking fungus. The bore hole may be of the same diameter
as the hypha, or it may be smaller, so that the hypha is con-
stricted where it passes through the hole, suggesting that the
hypha has increased in diameter after penetration, but has been