SECTION XI.

FERMENTATION BY ZYGOMYCETES.

CHAPTER XLIII.

MORPHOLOGY AND SYSTEMATOLOGY OF THE MUCORS. $234. Subdivision of the Order of Zygomycetes. THE Order of Zygomycetes (§ 221) is characterised by the capacity of forming zygospores. According as these resting cells are able to envelop themselves in a more or less completely developed capsule, or remain naked, the corresponding Zygomycetes are divided into two sub-groups. The higher of these, viz. the one forming the spore capsules, has received the name, carposporangial," from Brefeld, who united the whole of the members into a family, a few particulars of which will be given in § 237. Antithetical to these are the remaining, or exosporangial," Zygomycetes, as they were termed by Brefeld. It is with this (larger) sub-group, the sporogenic organs of which remain naked, that we have first to deal.

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The occurrence of several methods of fructification in one and the same species of fungus has already been mentioned in § 223, and is specially prevalent among the Zygomycetes. Apart from the characteristic zygospores, they are able to ensure the reproduction of the individual in other ways, special aptitude in this direction being exhibited by the genus Choanephora, which, in addition to zygospores and gemmæ, also puts forth conidia and endospores. In all other Zygomycetes only one or other of the two last-named methods of fructification is exhibited by any one individual. Sporangial fructification is characteristic of the Mucor family, a number of examples of which will be more closely considered later on.

The other Zygomycetes, on the contrary, exhibit conidial fructification, and are divided into three families, which do not come directly within the scope of the present work, and will therefore be dismissed with a few brief explanatory remarks. The family of the Entomophthoreæ forms a connecting link between the orders of Zygomycetes and Oomycetes. The species of this family are almost exclusively parasitic on living organisms (insects, fungi, ferns), and one of them will, from its effects at any rate, be, superficially, known to the reader, namely, Empusa

musca, the cause of a disease attacking the house-fly in late summer and autumn. From this cause numbers of these insects are found to adhere, straddle-legged, to walls and windows, and become surrounded by a distinct white aureole, consisting of unicellular conidia dispersed by the conidiophores protruding from the body of the insect. These conidia germinate into a structure similar to a budding mycelium (§ 219); and it is owing to this faculty that the wraith of the house-fly has for so long made its appearance in the controversy on the origin of the Saccharomycetes (§ 344).

The second of the three families of conidiophorous Zygomycetes, namely, the Piptocephalideæ, on the other hand, lacks the faculty of producing budding cells (like the third family), and is distinguishable by the feature that the zygospore apparatus consists of five cells instead of three, as also by the fact that the conidia are multicellular. The members of this family are parasitic on various species of Mucor, to the hyphae of which they attach themselves by means of special organs (haustoria), and then penetrate the interior for the purpose of abstracting nourishment. This behaviour they exhibit in common with the species of the third family of conidiophorous Zygomycetes, namely, the Chatocladiaceæ, whose conidia are unicellular and may be regarded as sporangia, whilst the contents are united to form a single endospore, instead of being divided into several. By this feature they reveal the connection, as regards progress of development, between conidial and sporangial fructification.

We can now collect the above particulars into the form of a
SCHEMATIC SUBDIVISION OF THE ORDER OF ZYGOMYCETES.

Of these six families, only the first and last will be dealt with in the following paragraphs, chiefly from the standpoint of Physiology and the technology of fermentation, leaving out of consideration their morphology and development except in so far as mention of these is absolutely essential to our purpose. Readers desirous of obtaining fuller information on the two latter points are referred to ALFRED FISCHER'S (III.) monograph on the Phycomycetes and Zygomycetes.

§ 235.-Subdivision of the Mucor family.

From the explanations in the preceding paragraphs, the Mucoraceae may be defined as Zygomycetes which exhibit sporangial, but not conidial, fructification, and produce naked zygospores. This family may be subdivided into three subfamilies, of which, however, only one falls within the scope of the present work, whilst the others will merely be referred to in order to facilitate comprehension of the connection existing between them.

The sub-family of Thamnidies is characterised by the possession of two kinds of sporangia: on the one hand, a large, normal, polysporous sporangium, on the crown of the sporangiophore (terminal sporangium); and, on the other hand, certain far smaller sporangia, which are situated lower down and put forth by whorled lateral branches of the sporangiophore. This second kind of sporangia are destitute of columella, contain only a pair of endospores-sometimes only a single one and are known as sporangioles. The most closely investigated member of this sub-family, Thamnidium elegans, is shown in Fig. 111. Starting from observations conducted by BREFELD (IX.), the dependence of fructification, in this species, on the external conditions (chemical composition and concentration of the nutrient substratum; temperature) was examined by J. BACHMANN (I.). This worker demonstrated that, by controlling these conditions, it is possible to compel the fungus to produce either terminal sporangia or sporangioles exclusively, or both together, or again to remain barren of fruit.

The second sub-family of the Mucoraceae, namely, the Piloboleæ, is distinguished by the feature that the ripe sporangium-owing to its peculiar structure-is released, and even forcibly expelled from, the organ on which it has been developed. Of the species belonging to this group, mention may be made of Pilobolus crystallinus, which is often found on horse droppings.

Finally, the Mucoreæ constitute the third sub-family. They are distinguishable from the first by producing only a single kind of sporangia, which, unlike those of the second sub-family, do not separate from the sporangiophore before discharging their contents, but remain attached thereto after bursting. The discharge is effected in consequence of either the liquefaction or brittleness of the membrane of the ripe sporangium.

One of the genera in this third family, viz. the genus. Sporodinia, is characterised by the forked branchings of its sporangiophores and the suspensores of the zygospores. This form is represented in Fig. 107. On the other hand, in all the remaining Mucorec-which have been arranged into five genera by A. FISCHER (III.)—these organs are either not branched at all or at least not forked.

1. Sporangiophore, slightly (6) magnified;

2. Three pieces of same, more highly (120) magnified; a, terminal sporangium; c, sporangioles.

3. Stunted sporangiophore, exhibiting only sporangioles. Magn. 200.

4. Sporangioles that have separated from the sporangiophore. Magn. 200. (After Brefeld.)

The genus Phycomyces is characterised by the possession of spinous prolongations on the suspensores and sporangiophores, which latter are unbranched, olive-green in colour, and possess metallic lustre.

One species of this genus, Phycomyces nitens, is plentifully met with in empty oil-casks, on oil-cakes, in concentrated fodder works, and similar places, and puts forth stiff, upright sporangiophores 7 to 30 c.m. long and 50 to 150 μ in diameter. These become crowned with an initially orange, but finally black, globular sporangium, 0.25 to 1.0 m.m. across, exhibiting a cylindrical columella and filled with ellipsoidal, yellow-brown, thick-walled endospores, 16 to 30

Brefeld.

Newly burst sporangium. m is the membrane, z the matrix, sp the spores. Several of the latter have been

squeezed out of the sporangium. Magn. 300. (After Brefeld.)

μ long and 8 to 15 μ broad. It has not yet been ascertained whether this species feeds on the fats present in the aforesaid medium; in fact, the whole question of the decomposition of fats by mould fungi (§ 233), on which sundry observations have been communicated by R. H. SCHMIDT I.) and by RITTHAUSEN and BAUMANN (II.), is a matter still requiring closer attention.

The genus Mucor is distinguished by the absence of spiny branches on the suspensores, by the silky gloss of the sporangiophores, and by the liquefaction The contents of the ripe

of the ripe sporangium membrane. sporangium are only partly consumed in building up the spores, the remainder serving as a matrix wherein the individual ripe spores are embedded and separated from each other. Now this matrix is capable of distension, and holds the crowd of spores together, even in microscopical preparations; whereas on the other hand the sporangial membrane, being liquefiable, is either quite invisible in such (aqueous) preparations, or at most is only seen as a residual trace at the point of attachment to the sporangiophore. An attempt is made to portray this in Fig. 112. The outside of the sporangial membrane is found to be more or less closely set with crystals of calcium oxalate. A few of the species composing this genus are pathogenic, and therefore interesting to the pathologist, since they are able to set up mycosis (in this case Mucor-mycosis) or fungification of the body they have infested, or into which they have been artificially inoculated. Of these pathogenic species of Mucor- -some of which have not yet been properly examined and classified in a botanical sense-mention may be made of the following: Mucor pusillus, discovered by