Introduction to Fungi, Third Edition

182 ZYGOMYCOTA
Fig 7.14 Mucor racemosus (a,b) and M. plumbeus (c,d). (a) Tip of a sporangiophore which has formed a sporangium.The columella
(arrow) is visible. (b) Lower region of sporangiophores showing intercalary thick-walled chlamydospores which are typical of the
species. (c) Sporangium with a spiny surface of calcium oxalate crystals. (d) Exposed columellae with finger-like projections. (a,b) to
same scale; (c,d) to same scale.
Zygorhynchus
There are about six species, mostly reported from
soil, often from considerable depth (Hesseltine
et al., 1959). All species are homothallic and,
unusually, form heterogametangic zygospores
(Fig. 7.15). The sporangiophores are commonly
branched and the columella is often broader than
high. The most frequently encountered species is
Z. moelleri, which has been isolated worldwide
from a range of soils and from the rhizosphere of
numerous plants (Domsch et al., 1980). Most
species are mesophilic, but Z. psychrophilus forms
zygospores readily at 5°C. Sporangium development
in Z. heterogamus has been studied by
Edelmann and Klomparens (1994) (see p. 171).
Zygospore development and structure in several
species of Zygorhynchus have been described by
O’Donnell et al. (1978a). The warts on the outside
of the zygosporangia often appear as interlocking,
starfish-like pointed thickenings. The inner
wall of the zygosporangium is ornamented by a
network of ridges and grooves radiating from
centres corresponding to the points of the warts.
The outer wall of the zygospore proper, lying
within the zygosporangium, is similarly
ornamented by a pattern of radiating grooves
and ridges which are a template of the lining
of the zygosporangial wall.
Rhizopus
There are about 10 species which grow in soil
(Domsch et al., 1980) and on fruits, other foods
and all kinds of decaying materials. Rhizopus spp.
also occur frequently as laboratory contaminants.
Rhizopus stolonifer (syn. R. nigricans) grows
rapidly. It is often found on ripe fruits, especially
if these are incubated in a moist atmosphere (see
Plate 3d). Characteristic features of Rhizopus are
the presence of rhizoids at the base of the
sporangiophores (which may grow in clusters),
and the stoloniferous habit (Fig. 7.16). An aerial
hypha grows out, and where it touches on the
substratum it bears rhizoids and sporangiophores.
Growth in this manner is repeated. The
sporangium wall is brittle and the sporangiospores
are dry and wind-dispersed. Some species
of Rhizopus, e.g. R. oryzae, R. microsporus and its
allies, are used as starters in ragi fermentations
of rice (Hesseltine, 1991). Several species
(R. arrhizus, R. microsporus, R. rhizopodiformis) are