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96 CHAPTER 5

parent hyphae in the nitrogen-rich zone. Regulatory
control by nitrogen seems logical for wood-decay
fungi, because wood has a very low nitrogen content
and these fungi could have evolved special mechanisms
for conserving and remobilizing their organic nitrogen
(Chapter 11). This could apply also to the cords of
ectomycorrhizal fungi, because these fungi have a
significant role in degrading organic nitrogen in other-
wise nitrogen-limiting soils (Chapter 13). In terms of
function, mycelial cords have been shown to trans-
locate carbohydrates, organic nitrogen, and water over
considerable distances between sources and sinks of
these materials. The vessel hyphae seem to act like xylem
vessels of plants, transporting water by osmotically
driven mass flow (Chapter 7). The combination of their
thick walls, the extensive extrahyphal matrix and rein-
forcement by fiber hyphae could enable vessel hyphae
to withstand considerable hydrostatic pressure.

Rhizomorphs

Rhizomorphs serve similar functions to mycelial
cords but have a more clearly defined organization.
A notable example is the rhizomorph of Armillaria
mellea, a major root-rot pathogen of broad-leaved
trees. It spreads from tree to tree by growing as rhizo-
morphs through the soil, and it also spreads extensively
up the trunks of dead trees by forming thick, black
rhizomorphs beneath the bark. These rhizomorphs

resemble boot laces, hence the common name for this
fungus – the boot-lace fungus (Fig. 5.14).
As shown in Fig. 5.15, the rhizomorph has a specially
organized apex or growing point similar to a root tip,
with a tightly packed sheath of hyphae over the apex,
like a root cap. Behind the apex is a fringe of short
hyphal branches. The main part of the rhizomorph
has a fairly uniform thickness and is differentiated
into zones: an outer cortex of thick-walled melanized
cells in an extracellular matrix, a medulla of thinner-
walled, parallel hyphae, and a central channel where
the medulla has broken down, serving a role in
gaseous diffusion. Rhizomorphs branch by producing
new multicellular apices, either behind the tip or by
bifurcation of the tip.
Rhizomorphs extend much more rapidly than the
undifferentiated hyphae of A. mellea, and they can grow
for large distances through soil. However, they need
to be attached to a food base because their growth
depends on translocated nutrients, so one of the
traditional ways of preventing spread from tree to tree
is by trenching of the soil to sever the rhizomorphs.
Almost nothing is known about the developmental
triggers of rhizomorphs, except that ethanol and other
small alcohols can induce them; similarly, almost
nothing is known about their mode of development
because they originate deep within an established
colony in laboratory culture. However, the behavior of
rhizomorphs is of considerable interest, as shown
by the work of Smith & Griffin (1971) on Armillariella

Fig. 5.14Armillaria mellea, “boot-lace fungus”, growing on a decaying tree trunk. (a) Typical fruiting bodies of Armillaria.
(b) Thick, black rhizomorphs that grow beneath the bark of dead trees.


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