Part 21

This group resembles Vaucheria and the other Siphoneæ among the Algæ.

ORGANS OF NUTRITION. The mycelium is formed of a single cell, often thread-like and abundantly branched (Fig. 78). Vegetative propagation by chlamydospores and oidia. Asexual reproduction by endospores (sometimes swarmspores) and conidia. Sexual reproduction by conjugation of two hyphæ as in the Conjugatæ, or by fertilisation of an egg-cell in an oogonium. On this account the class of the Phycomycetes is divided into two sub-classes: ZYGOMYCETES and OOMYCETES.

Sub-Class I. =Zygomycetes.=

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.

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.

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.

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.

=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.

Sub-Class 2. =Oomycetes.=

Sexual reproduction is oogamous with the formation of brown, thick-walled oospores which germinate after a period of rest. Asexual reproduction by conidia and swarmspores. Parasites, seldom saprophytes.

The oospores are large spores which are formed from the egg-cell (oosphere) of the oogonium (oosporangium, Fig. 89, 95). A branch of the mycelium attaches itself to the oogonium and forms at its apex the so-called “antheridium” (pollinodium): this sends one or more slender prolongations (fertilising tubes) through the wall of the oogonium to the egg-cell.

A fertilisation, a passage of the contents of the antheridium to the egg-cell, has as yet only been observed in Pythium; in Phytophthora only one small mass of protoplasm passes through the fertilising tube to the egg-cell; in Peronospora and the Saprolegniaceæ no protoplasm can be observed to pass through the fertilising tube, so that in these instances parthenogenesis takes place; Saprolegnia thuretii, etc., have generally even no antheridia, but nevertheless form normal oospores. Fertilisation of the egg-cell by means of self-motile spermatozoids is only found in Monoblepharis sphærica.

=A.= Asexual reproduction by conidia only.

Family 1. =Entomophthorales.=

The mycelium is richly branched. The family is a transitional step to the conidia-bearing Zygomycetes, since the oospores of many members of this family arise, and are formed, like zygospores.

Order 1. =Entomophthoraceæ.= Mycelium abundantly developed. This most frequently lives parasitically in living insects, causing their death. The conidiophores forming the conidial-layer project from the skin, and abstrict a proportionately large conidium which is ejected with considerable force, and by this means transferred to other insects. These become infected by the entrance of the germ-tube into their bodies. The spherical, brown resting-spores develope inside the bodies of insects and germinate by emitting a germ-tube.