Introduction to Fungi, Third Edition

POLYPOROID CLADE
565
distinct components, an outermost primary
wall, inter-wall pillars surrounded by electrontransparent
regions, and an innermost secondary
wall. Wall pillars, which initially develop
immediately adjacent to the spore plasma
membrane, eventually appear to fuse with both
the primary and secondary walls.
Ganoderma applanatum and G. adspersum are
two species with similar basidiocarps which
are confused with each other but can be
distinguished by their basidiospores which are
larger in G. adspersum. Both are wound parasites
of deciduous trees, producing large (up to 1 m),
perennial, brown, woody brackets (conks) which
form a fresh layer of hymenial tubes annually
(Figs. 19.23b,c). Recently formed hymenial tubes
of G. applanatum are chocolate-brown in colour
in contrast to the paler appearance of older
layers formed earlier. The older layers are
less dense and have a lower nitrogen content
than the current year’s growth, suggesting
that nitrogen is translocated from older,
spore-depleted strata to the newly formed
layers (Setliff, 1988). Nitrogen is often in
limited supply in woody substrates, and its
conservation is therefore important for wooddecaying
fungi.
Ganoderma applanatum is less common than
G. adspersum and is usually found on old trunks
of beech (Fagus sylvatica), where it causes a white
heart rot. The hyphal structure of the fruit body
is trimitic. A characteristic feature is that the
skeletal hyphae are of two types, namely arboriform,
i.e. showing an unbranched basal part
with a branched tapering end, and aciculiform,
i.e. unbranched and usually with a sharp tip
(Hansen, 1958; Furtado, 1965). The hymenial
tubes of G. applanatum may be up to 2 cm in
length and about 0.1 mm in diameter, i.e. 200
times as long as broad. The fall of the spores
down this tube raises problems. The hard, rigid
construction of the sporophore minimizes
lateral disturbance to the vertical alignment of
the tubes. Gregory (1957) has shown that the
majority of the spores carry a positive electrostatic
charge, but whether this charge has any
relevance to the positioning of the spores during
their fall by causing them to be repelled from the
tube wall seems doubtful. It has been calculated
that a large specimen may release as many as
20 million spores min 1 during the 5 or 6 months
from May to September (Buller, 1922). Spore
discharge can continue even during periods of
drought, doubtless associated with uptake of
water from the tree host (Ingold, 1954b, 1957).
In addition to wind dispersal, basidiospores of
G. applanatum have been shown to be dispersed
by specialized mycophagous flies visiting the
basidiocarps (Tuno, 1999). Basidiocarps of
G. applanatum are parasitized by larvae of the
mycophagous fly Agathomyia wankowiczii. The
trama is stimulated to proliferate into conical
or cylindrical gall-like outgrowths on the underside
of the basidiocarp. When the larva has
completed its development, it bores an exit hole
through the tip of the gall and drops to the forest
floor for pupation (Eisfelder & Herschel, 1966).
Gall-forming insects rarely attack fungi, and
even other Ganoderma spp. do not seem to be
attacked by A. wankowiczii.
Spores placed on media suitable for germination
may take 6 12 months to develop germ
tubes. Pairings between monosporous mycelia
show that the fungus is tetrapolar, with multiple
alleles. The large output of spores may be related
to the low probability of compatible spores
infecting the same tree trunk.
As discussed above for Trametes, Ganoderma
spp. have also been credited with having
medicinal value against a wide range of
ailments. Ganoderma lucidum (Fig. 19.23a), distinguished
by its stalked, shiny (lacquered) basidiocarps,
grows on the roots of deciduous trees.
It is cultivated in China on sawdust in
plastic bags for its basidiocarps from which
polysaccharides and other pharmacologically
active substances are extracted (Chang & Miles,
2004).
Phanerochaete
There are about 100 species to this genus of
saprotrophic fungi forming resupinate, crust-like
basidiocarps with smooth, wrinkled or spiny
but non-poroid hymenial surfaces. By far the
best-known species is P. chrysosporium, which
has become a ‘model’ organism for the examination
of lignin-degrading enzymes (see Fig. 19.13)