Introduction to Fungi, Third Edition

538 HOMOBASIDIOMYCETES
In culture, the sclerotia of C. cinereus form on
monokaryons as well as dikaryons growing
aerially or submerged (Waters et al., 1975a,b).
They are quite small (100 1000 mm in diameter),
dark brown to black and more or less spherical.
These sclerotia do not develop basidiocarps.
Three tissue layers are distinguishable in aerial
sclerotia: an outer diffuse layer of apparently
dead cells, a multi-layered rind of heavily
pigmented, closely packed, thick-walled cells,
and a medulla with predominantly thick-walled
cells. These thick-walled cells at first accumulate
rosettes of a glycogen-like polysaccharide in the
cytoplasm, but this disappears as the cells
mature, while a secondary cell wall layer
increases in thickness. It is believed that the
glycogen serves a temporary storage function but
that the secondary wall represents the long-term
storage compartment.
Stipe elongation in Coprinus
Shortly before spore discharge begins in Coprinus,
the stipe undergoes rapid elongation, most of it
taking place in its upper half. Although in
C. cinereus the cells of the vegetative mycelium
are dikaryotic, the cortical cells of the stipe
undergo repeated nuclear division prior to rapid
elongation so that they may contain between
32 and 156 nuclei. Stipe elongation is largely
brought about by turgor-driven cell expansion,
not cell division. The cells increase in length, but
not in width, by about eightfold in the final 15 h
of fruit body expansion. During that period, the
osmotic pressure is actively maintained as water
enters the cells. The solutes contributing to the
maintenance of osmotic pressure have not been
identified. The increase in stipe length is
mirrored by an increase in the chitin content.
Helically coiled chitin microfibrils are present in
the walls of cortical cells, and it is believed that
newly synthesized microfibrils are inserted
between pre-existing ones, a process termed
diffuse extension growth to distinguish it from
the usual apical extension growth of hyphal tips
(Kamada, 1994).
The force of the elongating stipe can be
considerable. Buller (1931) placed weights on an
expanding fruit body of C. sterquilinus which had
a pileus height of 1.4 cm and diameter of 0.8 cm.
This fruit body could lift a weight of 204 g.
The cross-sectional area of the ring of tissue
surrounding the hollow stipe was 29 mm 2
and calculations of the upward pressure of the
stipe gave a value of two-thirds of an atmosphere
(0.7 10 4 Nm 2 ). This may explain why the
expanding fruit bodies of C. atramentarius can
crack asphalt and why other fungi can lift paving
slabs.
Mating behaviour in Coprinus
Some Coprinus spp. are primarily homothallic,
e.g. C. heptemerus. Species with two-spored basidia
are often secondarily homothallic, e.g. C. bisporus
(¼ C. ephemerus var. bisporus). Yet other species
show bipolar (i.e. unifactorial) heterothallism
(e.g. C. comatus and C. ephemerus), or tetrapolar
(i.e. bifactorial) heterothallism (e.g. C. cinereus;
see Section 18.9). M. Lange (1952) has proposed
the term amphithallism (see p. 532) to describe
the behaviour of species of Coprinus in which
both homothallic and heterothallic mycelia can
be raised from the same fruit body, giving as
examples C. sassii, a bisporic species which is
amphithallic-bipolar, and the amphithallictetrapolar
C. plagioporus.
In C. cinereus, as in many other species,
monokaryotic (rarely dikaryotic) mycelia form
globose mucilaginous heads containing numerous
oidia dispersed by insects. Plasmogamy
preceding dikaryotization may be achieved
by fusion between compatible monokaryotic
hyphae or by fusion between a monokaryotic
hypha and a compatible oidium. Monokaryons
may also be dikaryotized following hyphal fusion
with dikaryons (the Buller phenomenon; see
pp. 508 and 566).
Edibility of Coprinus basidiocarps
Some species of Coprinus have basidiocarps which
are edible when young. Coprinus comatus is an
example. The fruit bodies of C. atramentarius are
edible and harmless except if consumed with
alcohol, which results in unpleasant symptoms
of nausea and palpitations. The substance associated
with this effect has been identified as
coprine (Fig. 19.15a), and this is similar in its
effects to the drug disulfuram (antabuse) which