Sexual reproduction takes place by zygospores, which function as
resting-spores, and arise in consequence of _conjugation_ (Fig. 81); in
the majority of species these are rarely found, and only under special
conditions. The most common method of reproduction is by endospores, by
acrogenous conidia, by chlamydospores, or by oidia. _Swarmspores are
wanting._ Parasites and saprophytes (order 6 and 7). The zygospores are
generally produced when the formation of sporangia has ceased; _e.g._
by the suppression of the sporangial-hyphæ (_Mucor mucedo_), or by
the diminution of oxygen; _Pilobolus crystallinus_ forms zygospores,
when the sporangia are infected with saprophytic _Piptocephalis_ or
_Pleotrachelus_.
=A.= Asexual reproduction only by sporangia.
Order 1. =Mucoraceæ.= The spherical sporangia contain many spores.
The zygospore is formed between two unicellular branches (gametes).
The unicellular mycelium (Fig. 78) of the Mucoraceæ branches
abundantly, and lives, generally, as a saprophyte on all sorts of
dead organic remains. Some of these Fungi are known to be capable of
producing _alcoholic fermentation_, in common with the Saccharomyces.
This applies especially to _Chlamydomucor racemosus_ (_Mucor
racemosus_), when grown in a saccharine solution, and deprived of
oxygen; the mycelium, under such conditions, becomes divided by
transverse walls into a large number of small cells. Many of these
swell out into spherical or club-shaped cells, and when detached from
one another become chlamydospores, which abstrict new cells of similar
nature (Fig. 79). These chlamydospores were formerly erroneously
termed “mucor-yeast,” but they must not be confounded with the
yeast-conidia (page 94). They are shortened hyphæ, and are not conidia
of definite size, shape, and point of budding. Oidia are also found in
_Chlamydomucor_.
[Illustration: FIG. 78.--_Mucor mucedo._ A mycelium which has
sprung from one spore, whose position is marked by the *: _a_,
_b_, _c_ are three sporangia in different stages of development;
_a_ is the youngest one, as yet only a short, thick, erect
branch; _b_ is commencing to form a sporangium which is larger in
_c_, but not yet separated from its stalk.]
The Mucoraceæ, in addition to the chlamydospores and oidia, have a more
normal and ordinary method of reproduction; viz., by _spores_ which
are formed without any sexual act. _Mucor_ has round sporangia; from
the mycelium one or more long branches, sometimes several centimetres
in length, grow vertically into the air; the apex swells (Figs. 78,
80) into a sphere which soon becomes separated from its stalk by a
transverse wall; in the interior of this sphere (sporangium) a number
of spores are formed which eventually are set free by the rupture of
the wall. The transverse wall protrudes into the sporangium and forms
the well-known columella (Fig. 80 _d_, _e_). The formation of spores
takes place in various ways among the different genera.
[Illustration: FIG. 79.--Chlamydospores of _Chlamydomucor
racemosus_ (× 375 times.)]
[Illustration: FIG. 80.--_Mucor mucedo_: _a_ a spore commencing
to germinate (× 300 times); _b_ a germinating spore which has
formed a germ-tube from each end (× 300 times); _c_ the apex of
a young sporangium before the formation of spores has commenced;
the stalk is protruded in the sporangium in the form of a column:
on the wall of the sporangium is found a very fine incrustation
of lime in the form of thorn-like projections; _d_ a sporangium
in which the formation of spores has commenced; _e_ a sporangium,
the wall of which is ruptured, leaving a remnant attached to the
base of the columella as a small collar. A few spores are seen
still adhering to the columella.]
SEXUAL REPRODUCTION by conjugation takes place in the following
manner. The ends of two hyphæ meet (Fig. 81) and become more or less
club-shaped; the ends of each of these are cut off by a cell-wall, and
two new small cells (Fig. 81 _A_) are thus formed, these coalesce and
give rise to a new cell which becomes the very thick-walled zygote
(zygospore), and germinates after period of rest, producing a new
hypha, which bears a sporangium (Fig. 81 _E_).
_Mucor mucedo_, Pin-mould, resembles somewhat in appearance
_Penicillium crustaceum_ and is found growing upon various organic
materials (bread, jam, dung, etc.).
_Pilobolus_ (Figs. 83, 84) grows on manure. Its sporangium (Fig. 84
_a″_) is formed during the night and by a peculiar mechanism (page 92)
is shot away from the plant in the course of the day. This generally
takes place in the summer, between eight and ten a.m. The sporangium is
shot away to a height which may be 300 times greater than that of the
plant itself, and by its stickiness it becomes attached to portions of
plants, etc., which are in the vicinity. If these are eaten by animals,
the spores pass into the alimentary canal and are later on, sometimes
even in a germinating condition, passed out with the excrement, in
which they form new mycelia.
_Phycomyces nitens_ (“Oil-mould”) is the largest of the Mould Fungi;
its sporangiophores may attain the height of 10–30 c.m.
Order 2. =Rhizopaceæ.= _Rhizopus nigricans_ (_Mucor stolonifer_) which
lives on decaying fruits containing sugar, on bread, etc., has, at the
base of the sporangiophores, tufts of rhizoids, _i.e._ hyphæ, which
function as organs of attachment. From these, “runners” are produced
which in a similar manner develope sporangiophores and rhizoids.
[Illustration: FIGS. 81, 82.--_Mucor mucedo_: _A-C_ stages in the
formation of the zygote; D zygote; E germination of zygote: the
exospore has burst, and the endospore grown into a hypha bearing
a sporangium.]
Order 3. =Thamnidiaceæ.= On the same sporangiophore, in addition to
a large, terminal, many-spored sporangium, many smaller, lateral
sporangia are formed with a few spores. Thamnidium.
=B.= Asexual reproduction by sporangia and conidia.
Order 4. =Choanephoraceæ.= _Choanephora_ with creeping endophytic
mycelium, and perpendicular sporangiophores.
Order 5. =Mortierellaceæ.= _Mortierella polycephala_ produces on the
same mycelium conidia and sporangiophores. _M. rostafinskii_ has a long
stalked sporangiophore, which is surrounded at its base by a covering
of numerous felted hyphæ.
[Illustration: FIG. 83.--_Pilobolus._ Mycelium (_a_, _a_), with a
sporangiophore (_A_) and the fundament of another (_B_).]
[Illustration: FIG. 84.--_Pilobolus._ Sporangium (_a″_) with
stalk (_a-c_), which is covered by many small drops of water
pressed out by turgescence.]
=C.= Asexual reproduction only by conidia.
Order 6. =Chætocladiaceæ.= The conidia are abstricted singly and
acrogenously. _Chætocladium_ is a parasite on the larger Mucoraceæ.
Order 7. =Piptocephalidaceæ.= The conidia are formed acrogenously and
in a series, by transverse divisions. The zygospore arises at the
summit of the conjugating hyphæ, which are curved so as to resemble a
pair of tongs. _Piptocephalis_ and _Syncephalis_ live parasitically on
the larger Mucoraceæ.