6 Wood Discoloration

176 8 Habitat of Wood Fungi
Fig.8.8. Changes in the xylem and phloem of hardwoods after wounding (after Liese and
Dujesiefken 1989)
since the defense reactions in the wood begin temporally retarded. In this
bright zone of about 1 cm, the vessels remain open, and the lumina do not
contain inclusions. With increasing distance to the wound, reserve material
is mobilized, and the vessels are closed. In beech, the degeneration of the
parenchyma is limited, as parenchyma cells in the wounded area are divided
by transverse walls and limit the damage by suberization of the wound-near
compartments (Schmitt and Liese 1993).
A closure by tyloses (Schmitt and Liese 1994) only takes place in tree species,
which possess pit sizes of at least 8µm. Trees without tyloses, like lime and
maple, can prevent air embolism by blocking the vessels with plugs. In birch,
the ladder-shaped vessel openings are closed on one side by membranes, and
parenchyma cells excrete fibrillar material in neighboring vessels and fibers
(Schmitt and Liese 1992a).
The tissue behind the wound area, which is discolored by means of accessory
compounds and which contains died parenchyma cells and vessels
out of function, had been termed protection wood. As it is colonized however
frequently by fungi, it obviously does not possess increased durability.
The healthy wood outside this area shows microscopically in an area of a few
millimeters mobilization of reserve material and vessel closure, but no fungi,
so that the actual protective layer obviously lies outside of the visible discoloration.
Also in the phloem the parenchyma dies at the wound surface and the
tissue beneath is set out of function. A wavy-shaped wound periderm, which
attaches the periderm of the young callus bark to the outer bark, develops in
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