Introduction to Fungi, Third Edition

MYXOMYCETES: TRUE (PLASMODIAL) SLIME MOULDS
47
Fig 2.6 Sporogenesis in a
protostelid (after Olive,1967).
(a) Early pre-spore stage.
(b) Hat-shaped stage.
(c) Appearance of the steliogen.
(d) Beginning of stalk formation.
(e) Later stage in stalk development,
with steliogen extending into
upper part of stalk tube. (f) Mature
sporocarp showing terminal spore,
with subtending apophysis, outer
sheath, and inner stalk tube.
The swarmers eventually fuse to form a diploid
zygote which initiates the plasmodial stage
(Figs. 2.7i,j), from which the sporocarp develops
(Spiegel, 1990). Ceratiomyxa fruticulosa thus
shows features of both the Protosteliomycetes
in producing its spores externally, and the
Myxomycetes (see below) in having a flagellated
stage in its life cycle. Its precise phylogenetic
position remains to be established. This species
is probably homothallic (Clark et al., 2004).
Its whitish semitransparent sporocarps are
rather common on the surface of rotting wood
(Plate 1a).
2.5 Myxomycetes: true (plasmodial)
slime moulds
Fig 2.7 Ceratiomyxa fruticulosa. (a) Fruiting sporocarp bearing
stalked spores. (b) Portion of the surface of the sporocarp
showing spores and their attachment. (c) Spore. (d) Naked
protoplast emerging from the spore at germination. (e) Naked
protoplast before cleavage. (f) Cleavage of protoplast to form
a tetrad of protoplasts. (g) Octette stage: a clump of eight
protoplasts. (h) Uniflagellate and biflagellate swarmer released
from the octette protoplasts. (i) Copulation of swarmers by
their posterior ends. (j) Young plasmodium: c, contractile
vacuole; s, ingested spore within food vacuole. (c i) to same
scale.
The Myxomycetes (zool.: Myxogastrea) are by
far the largest group of slime moulds, comprising
some 800 species in 62 genera which
are currently divided into five orders (Kirk
et al., 2001). General accounts have been given
by Frederick (1990), Stephenson and Stempen
(1994) and Alexopoulos et al. (1996). A monograph
of British species has been compiled by Ing
(1999). These are the familiar slime moulds
so common on moist, decaying wood and
other organic substrata. They are also abundant
in soil and may fulfil ecological functions
which are as yet poorly understood (Madelin,
1984).
The vegetative phase is a free-living plasmodium,
i.e. a multinucleate wall-less mass of
protoplasm. This may or may not be covered