Introduction to Fungi, Third Edition

500 BASIDIOMYCOTA
Fig18.11 Diagrammatic representation of the
sequence of events associated with the formation
of a clamp connection in the dikaryotic hypha of a
basidiomycete. (a) Terminal segment of a hypha
with two genetically dissimilar nuclei. (b) A lateral
bulge in the hypha appears near the paired nuclei.
The leading nucleus moves into the bulge. (c) The
two nuclei undergo conjugate (i.e. simultaneous)
nuclear division. Mitosis of the leading nucleus
occurs within the bulge. (d) The lateral bulge has
developed into a backwardly directed branch or
hook with one daughter of the leading nucleus at
its tip. One of the daughter subterminal nuclei
moves forward.Transverse septa have developed
simultaneously in the main hypha and at the base of
the hook. (e) The tip of the hook has fused with the
wall of the main hypha. Its nucleus has moved into
the main hypha and taken up position behind the
daughter of the subterminal nucleus. (f) The pairs
of nuclei move away from the transverse septum in
the main hypha, the terminal pair moving nearer
the hyphal tip and the subterminal pair distally.
Note that both segments contain dissimilar nuclei
but that their arrangement has been reversed.
nuclei, there would be a tendency for dikaryotic
mycelia to break down into homokaryotic
segments. Whilst it is reasonable to infer that
mycelia with clamps at the septa are dikaryotic,
the converse is not true because there are
numerous basidiomycetes in which the dikaryotic
mycelium does not bear clamps, and this is
especially true of hyphae making up basidiocarps.
For a fuller discussion of dikaryon
formation see Casselton and Economou (1985).
18.7.5 Aggregates of vegetative hyphae
Basidiomycete hyphae can aggregate to form
complex vegetative structures such as hyphal
strands, mycelial cords, mycelial sheets and
rhizomorphs (Butler, 1966; Watkinson, 1979;
Rayner et al., 1985). These often grow more
rapidly than individual hyphae, connect food
bases such as litter fragments in soil, fallen logs
and tree stumps, and are capable of rapid twoway
conduction of water and nutrients.
Mycelial cords
Mycelial cords have been defined by Boddy (1993)
as aggregations of predominantly parallel,
longitudinally aligned hyphae (see Fig. 1.13).
They are especially common in wood-decaying
basidiomycetes but are also involved in the
underground spread of basidiomycetes forming
ectomycorrhiza. Wood-decaying mycelial cord
formers are a very successful ecological group,
using their cords in a variety of strategies to
secure resources, e.g. by displacing other fungi
from food bases or exploring new resources in
the soil. The cords show relatively little differentiation
in structure and pigmentation.
Examples of fungi with mycelial cords are the
stinkhorn (Phallus impudicus) and the agaric
Megacollybia platyphylla. Their white cords can
extend for many metres in the soil and can be
traced back from the fruit bodies to tree stumps
or other food bases if highly motivated students
and digging tools are available (Fig. 18.13a).
Rhizomorphs
The term rhizomorph refers to the root-like form
of these mycelial aggregates which are more
highly differentiated than mycelial cords and are
often pigmented brown or black due to the
presence of melanin in the walls of small,