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.
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.
