DIFFERENTIATION AND DEVELOPMENT 95
Fig. 5.11Fruitbodies and mycelial cords of a puffball
(Lycoperdonsp.) growing on decayed wood.
Mycelial cords
Mycelial cords have been studied most intensively in
Serpula lacrymans(Basidiomycota) which causes dry-rot
of timbers in buildings (Chapter 7). Once this fungus
is established in the timbers, it can spread several
meters beneath plaster or brickwork to initiate new sites
of decay. It spreads across non-nutritive surfaces as
fans of hyphae, which draw nutrients forwards from
an established site of decay. The hyphae differentiate
into mycelial cords behind the colony margin.
The early stage of differentiation of mycelial cords
occurs when branches emerge from the main hyphae
and, instead of radiating, they branch immediately
to form a T-shape and these branches grow backwards
and forwards close to the parent hypha. The branches
produce further branches that repeat this process, so
the cord becomes progressively thicker, with many
parallel hyphae. Consolidation occurs by intertwining
and anastomosis of the branch hyphae and by secre-
tion of an extracellular matrix which cements them
together. Some of the main hyphae then develop into
wide, thick-walled vessel hyphaewith no living cyto-
plasm, while some of the narrower hyphae develop into
fiber hyphaewith thick walls and almost no lumen.
Interspersed with these types of hyphae are normal,
living hyphae rich in cytoplasmic contents. The cords
of other fungi, such as the mycorrhizal species Leccinum
scabrum(Fig. 5.12), do not have fibre hyphae but
otherwise show a similar pattern of development. In
mature hyphal cords there is evidence of a large
degree of degeneration of hyphal contents and of the
deposition of large amounts of cementing material
between the hyphae (Fig. 5.13).
The factors that control the development of mycelial
cords are poorly understood, but studies on S. lacrymans
Fig. 5.12Scanning electron micrograph of a mycelial
cord of the mycorrhizal fungus Leccinum scabrum, broken
to show the internal distribution of hyphae. The wide,
central vessel hyphae (vh) are surrounded by narrower
sheathing hyphae (sh). The surface of the mycelial cord
is covered with extracellular matrix materials, and hyphae
(e.g. arrow) radiate into the soil to explore for nutrients.
(Courtesy of F.M. Fox; from Fox 1987.)Fig. 5.13Transmission electron micrograph of a section
of a mycelial cord of Leccinum scabrum, showing wide,
thick-walled, empty vessel hyphae (vh), thin-walled
sheathing hyphae (sh), and abundant intercellular matrix
material (m). Some hyphae (labeled d and also near the
top right) have been sectioned through dolipore septa.
(Courtesy of F.M. Fox; from Fox 1987.)suggest that the availability of nitrogen is a key factor.
Cords were found to develop on media containing
inorganic nitrogen (e.g. nitrate) but not on media con-
taining amino acids. Also, cords growing from a mineral
nutrient medium onto an organic nitrogen medium
gave rise to normal, diverging hyphal branches. So it
was suggested that cords develop when the parent
hyphae leak organic nitrogen in nitrogen-poor con-
ditions, causing branch hyphae to grow close to the