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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.

(a) (b)