Technical Mycology - The Utilization of Micro-Organisms in the Arts and Manufactures - Part II Eumycetic Fermentation: A Practical Handbook on Fermentation and Fermentative Processes for the use of Brewers and Distillers, Analysts, Technical and Agricultural Chemists, Pharmacists, and all Interested in the Industries Dependant on Fermentation

TECHNICAL MYCOLOGY.

SECTION X.

RUDIMENTS OF THE GENERAL MORPHOLOGY AND PHYSIOLOGY OF THE EUMYCETES.

CHAPTER XXXIX.

GENERAL MORPHOLOGY.

§ 217.—Articulation of the Thallus.

ALREADY in the first volume (§ 22) the algæ and the fungi were arranged in a single group, that of the Thallophytes, in contradistinction to all other plants, the latter being classed in the group of Cormophytes. The distinguishing characteristics of these two groups were stated to be the absence in the former, and the presence in the latter, of an articulation of the body of the individual organism into leaf and stem. At the same time, it was mentioned that the corporeal form assumed by the Thallophytes, and differing fundamentally from the cormus of the Cormophytes, has received the general name of thallus. That intermediate forms between these two types should exist, and that the thallus of the highest Thallophytes should approximate to the cormus of the lowest Cormophytes, is perfectly natural and in accordance with the general laws of phylogenetic evolution.

Although the fungi, the only class of Thallophytes with which we are now concerned, do not exhibit division into leaf and stem, their thallus is not entirely destitute of all articulation. True, in one of the two chief divisions of the fungus family, the Schizomycetes, the articulation of the thallus is practically undiscernible, the individual organisms taking the form of globular or oval cells, or straight or bent rods of variable length. Should any extensive development of the thallus occur, this may almost invariably be regarded as either a malformation preceding death—e.g. the branching of bacteroids (§ 195) and Bacterium aceti (§ 211)—or as an assemblage of several individual organisms giving rise to a deceptive appearance of articulation, as in the case of Cladothrix (§ 197). Again, in many species of bacteria, the colonies known as zooglœa seem to exhibit a more or less well-developed articulation; but these cannot be considered as a thallus, since they represent assemblages of many uniform cells, and not separate individual organisms.

The case is, however, different in the second division of the fungus family (§ 23), namely, the Eumycetes or higher fungi. These differ from the Schizomycetes in universally possessing the faculty of forming true branchings. This characteristic resides in the immediate and uninterrupted connection between the plasma of the branch (or the oldest cell of same) and that of the main stem from which the branch proceeds. The divergence of form and the luxuriance of this branching vary in the different orders of fungi, and, in general, increase the higher we get in the system and the nearer we approach the Cormophytes. In the lowest members, on the other hand, this tendency is often greatly simplified and restricted, thus approximating to the Schizomycetes.

FIG. 91.—Thallus of Mucor mucedo.

Shows the unicellular mycelium sprung from the spore; together with three organs of fructification, a, b, c, in different stages of development, raising themselves from the mycelium. Magn. about 10. (After Kny.)

A more careful examination of the thallus of the higher Eumycetes, even with the unassisted eye, will reveal something more than the existence of a more or less copious branching. It will soon be found possible to dissect the thallus into two parts (Fig. 91), which though intimately connected serve entirely different purposes: one, known as the mycelium, having charge of the nutrition and maintenance of the individual plant; whilst, on the other, or organ of fructification, devolves the task of reproduction, and therefore the maintenance of the species. This latter organ develops special cells or spores, which are mostly globular or oval, and from each of which under favourable circumstances, a new individual of the same species can be produced.

FIG. 92.—Mucor mucedo.

Germinating spore, which has already put forth two buds. Magn. 300. (After Brefeld.)

The mycelium may therefore be defined as the portion of the thallus spreading in or upon the nutriment medium and extracting nutriment therefrom. It proceeds from a spore. As soon as this latter comes under the influence of circumstances favourable to its germination, it absorbs water and other nutrient materials from the surroundings, swells up more or less, and usually puts forth one or more tubular buds (Fig. 92). These continue to develop in two directions, increasing in length, and forming lateral branches which in turn continue to act in like manner The name hypha is given to each of these branchings, and the whole group of hyphœ that have resulted from a single spore and serve to nourish the individual plant in question, is called the mycelium. The spore may also germinate by the process of gemmation described in § 219. In some fungi the spores can only germinate in the one manner, whilst in others the germination is restricted to the other type. The mycelium, in order to fulfil its task, must continue to penetrate towards more remote portions of the nutrient medium; and accordingly, the hyphæ must progressively increase in length. Now this growth is confined to the apex of the hyphæ, i.e. the part farthest from the centre of development. On the other hand, the parts nearest that centre quickly cease to extend and branch; consequently, the Eumycetes exhibit acrogenous growth. This behaviour constitutes another feature of difference between Eumycetes and Schizomycetes, since, in the latter, the growth of the cells is not restricted to one end only, but proceeds by the extension of the whole body.

Examination of the mycelia of a large number of species of Eumycetes, under a power of about 100 diameters, soon leads to a separation of these specimens into two groups: the one comprising species whose mycelium, however large and extensively branched, consists of only one single cell; whilst the other group contains the species wherein the mycelial hyphæ are subdivided into cylindrical parts of variable length by transverse walls (septa) perpendicular to the longitudinal axis. This fundamental and highly important difference constitutes the chief basis for the separation of the Eumycetes into two main subdivisions: Eumycetes with a unicellular mycelium on the one hand, and Eumycetes with a septated mycelium on the other. In mycelial structure and several other particulars, the members of the first subdivision bear a remarkable resemblance to certain unicellular algæ, for which reason they have received the name Phycomycetes, or algic fungi. On the other hand, the Eumycetes of the second group, with septated mycelia, bear the groupname Mycomycetes. In comparison to the others these latter stand on a higher plane of development, and are almost exclusively aerial; whereas the majority of the Phycomycetes still incline to subaqueous existence. The following scheme will easily fix the foregoing classification in the reader’s memory:—

§ 218.—The Typical Mycelium.

We will now consider the development of the mycelium of a Mycomyces from its spore, Fig. 93 helping to make this clear. Soon after the tubular buds have sprouted from the spore, a septum forms between the spore and each of the buds. The tube then increases in length, and develops in its interior a septum which divides it into two cells, the one nearer the spore (or centre of growth) being termed the inner cell, whilst the outer one is called the terminal or crown cell. Now, whereas the inner cell ceases to increase in length the crown cell continues to grow longitudinally, and in turn develops a septum, whereby an inner cell (of the second order) is again formed; and this operation is repeated at convenience. Meanwhile the inner cells are not inactive, since, although they do not increase in length, they throw out lateral projections, which develop into branches separated from the inner cell by septa. These branches grow longitudinally, and separate into a crown cell and a secondary inner cell by developing another septum, an operation repeated by the crown cells as often as external circumstances will admit. This faculty of the inner cells of the first order is also shared by those subsequently formed, so that we have lateral branches of the third, fourth, and other orders. The whole of these hyphæ or mycelial threads together constitute the mycelium. The serial order in which the inner cells begin to throw off lateral branches is, as a rule, in accordance with their age, the oldest starting first. Consequently, the development of the mycelium proceeds laterally from the spore (the basis) towards the periphery or apex of the mycelial thread, such a mode of growth being termed basifugal or acropetalous. Again, the lateral position of the branches in question is, as a rule, very uniform; those from the inner cells of odd-numbered orders all branching from one (e.g. the left) side, and all those springing from inner cells of even-numbered orders appearing on the opposite (e.g. right) side of the respective inner cells. When the branching from any given cell is confined to a single lateral offshoot the system is termed monopodial, the main and branch cell together being called a monopodium.

FIG. 93.—Mycelial development of a Mycomyces (the ordinary bread mould, Penicillium glaucum).

A, the ripe spore. B and C, the same, with respectively one and three tubular offshoots. In D each of these has become separated from the spore by a septum, s. In E each tube has become divided by the formation of a second septum, s', into a terminal cell (e) and an inner cell (b), whilst branching has commenced in two places. F shows each of the three tubes developed into a main branch (I. to III.), which in turn has thrown out lateral branches of the first to the third order (1, 2, 3). Magn. 400. (After Zopf.)

The progress of mycelial development in a Phycomyces naturally differs from that just described, inasmuch as no septa are formed under normal circumstances; consequently there is, in this case, no separation of the elongating tubular bud into inner and crown cells. In such event the resulting mycelium consists, as already stated, of a single, many-branched, tubular, non-septated cell, such as is shown in Fig. 91.

The foregoing statement that the inner cells do not play any further part in the mycelial growth of the Mycomycetes, inasmuch as they neither extend in length nor develop septa, may be taken as the rule. There are, however, exceptions, septation, accompanied by elongation, frequently occurring within the inner cells in the event of abnormal conditions of nutrition. This phenomenon is termed intercalary growth, or intercalary septation, to distinguish it from acrogenous growth.

If, in the absence of external causes of hindrance, the growth of the mycelium is able to proceed equally in all directions, a stellar system of radial, branched threads, with the spore as a centre, is the result. This form of growth was termed a typical mycelium by Zopf. The practical worker in a mycological laboratory can obtain such typical mycelia in a youthful condition, and consequently easy to survey, if he re-examines, after a lapse of one or two days, the plate cultures (§ 85) that have already been examined for the purposes of mycological analysis (e.g. of water, milk, beer). During the first investigation the spores of all kinds of mould fungi from the air will have settled on the solid nutrient medium, each of them then germinating to form a mycelium, and thus yielding, as it were, a self-prepared culture.

Mention must here be made of one of the various instances of irregular mycelial development, since it will have to be referred to on a subsequent occasion: this is the phenomenon of intergrowth. It is caused by one of the cells in a mycelium putting forth a branch into the interior of an adjoining cell, so as to displace the intervening septum. The invader may then become divided into cells within the plasma of the invested cell, with the result that an inexpert observer may easily be led to believe that endogenous spores are present. An example of this growth is represented in Fig. 94. Another will be found in a later section dealing with Dematium pullulans, and a third in the case of Oidium Ludwigii Hansen, occurring in mucilaginous discharges from trees (§ 248), and investigated by W. HOLTZ (I.). This was probably also the method of formation of the alleged spores observed by EDM. KAYSER (V.) within the hyphæ of an unknown mycelial fungus isolated by him from fermenting pineapple juice. Intergrowths also occur in the sporangia of several fungi.

In the case of a large number of fungi, the development of the mycelium ceases with the formation of the branched hyphæ, the ensuing process being the elaboration of organs of fructification. Fungi exhibiting this class of simple mycelial structure are classed under the generic name Hyphomycetes or Thread Fungi. The term Mucedinæ, occurring in the French and English literature, expresses about the same thing. It may be remarked in passing that several botanists, e.g. Strasburger, Noll, Schenck, and Schimper, in their botanical text-book, employ the name Hyphomycetes in a far wider sense, namely, to include the whole of the Eumycetes, the reason for this being that the production of hyphæ is characteristic of these fungi, and constitutes a fundamental distinction between them and the other divisions of the fungus family, the Schizomycetes in particular. Nevertheless, in the following pages we will apply the term in its more restricted sense.

In many of the other classes of Eumycetes, the development of the mycelium does not cease at the stage we have described as the typical mycelium, but extends further, to the production of aggregations, of the forms known as mycelial threads and mycelial films. A combination of these two forms constitutes the large bodies known in colloquial language as mushrooms or fungus; the botanist, however, terming them fungoid bodies. The capacity of forming such bodies, upon or within which the organs of fructification are situated, is confined to the most highly developed species of fungi. An example is given in Fig. 95. This, however, is only one variety (though appearing in numerous modifications) of the coalescence and intertwined growth of hyphæ, another form being that of the so-called sclerotium or hard mycelium. The well-known ergot of rye, which will be more closely described in the last section but one, forms an example of a sclerotium. This is constructed of closely intertwined hyphæ, furnished with a store of nutrient material, and constituting a hard permanent form, which, after a variable period of quiescence, awakens to active life, puts forth organs of fructification, and is then able to await and utilise the occurrence of favourable conditions, in order to effect the reproduction of the individual from which it has originated. An observation on the. artificial production of such permanent forms has been communicated by J. RAY (I.). Among the foodstuffs accumulated in the cells of the sclerotium, special importance attaches to glycogen (§ 253) as the source of easily liberated chemical energy and abundant disengagement of heat. This substance was first observed—without, however, being specially named—by A. DE BABY (1.) in the sclerotium of Coprinus stercorarius; and it was afterwards found, by W. ROTHET (I.), in that of Sclerotium hydrophilum. A thin section of sclerotium, or of a fungoid body—both of which are, as already stated, composed of a network of hyphæ—exhibits under the microscope an appearance similar to that of the parenchyma of higher plants, e.g. a section through the flesh of an apple. On account of this similarity, these networks of hyphæ have received the name pseudoparenchyma, which, however, is not intended to express any further likeness, whether in respect of the mode of formation or physiological purpose. It is perhaps superfluous to emphasise that the mycelia of this class of fungi consist merely of hyphæ when in the earliest stages of existence and consequently at such times are indistinguishable in this respect from the mycelia of the Hyphomycetes.

FIG. 94.—Botrytis cinerea.

Intergrowth. Each of the two penultimate cells of the depicted fragment of mycelium has grown into its neighbour, and there become separated into globular cells. The central cell of the mycelial thread has put forth abnormally developed organs of fructification. (After P. Lindner.)

FIG. 95.—Boletus edulis.

Longitudinal section (above) and transverse section (below) through the fruit stem (fungus) of Boletus edulis. Magu. ∞. (After Strasburger.)

§ 219.—The Gemmating Mycelium.

The application of the name typical to a mycelium growing in the manner described in the preceding paragraph, indicates the possibility of other methods of growth, manifesting themselves as modifications and simplifications of this form. Of these the most important, from our point of view, is the gemmating mycelium, the development of which proceeds in the following manner (Fig. 96): The germ cell, or mother-cell, puts forth a protrusion which, however, instead of enlarging to a tube as in the case of the incipient typical mycelium, assumes a form resembling that of the parent-cell and therefore termed a bud. The daughter-cell then becomes divided from the parent by a septum, which subsequently splits into two layers and enables the two cells to separate. In many instances the parent-cell puts forth only a single bud, but in others two or more. As soon as the daughter-cell has attained the size of the parent, it is then able to behave in turn like the latter, and itself put forth a bud (of the second order), from which again proceeds another bud (of the third order) and so on.

If the parent-cell—as is the case, for example, in most kinds of yeast—be globular, oval, or lemon-shaped, the daughter - cell will also usually be of similar form, and is then termed a short bud. Such globular buds are referred to in the older literature (and occasionally even now) as spherical yeast, more particularly in the case of Mucor. If, on the other hand, the parent-cell be of elongated form, the daughter-cells issuing therefrom will preferentially develop in a longitudinal direction from the start, and thus form elongated buds. Most of the species of Mycoderma afford examples of this type. Fungi with gemmating mycelia of this kind are therefore, in this respect, intermediate to the fungi with typical mycelia.

FIG. 96.—Gemmation of a Tortula in Beer wort.

At (a) one of the cells has just put forth a tiny bud. At the end of 1 1/2 hours (b) this has become considerably larger. After another two hours it has grown to half the size of the parent cell, and has already separated from the latter. Magn. 1000. (After Hansen.)

The above-mentioned double stratification of the septa between the cells produced in the foregoing manner, permits these cells to enjoy an independent existence, and consequently enables them to be separated from one another. In many instances this actually occurs, and consequently the nutrient medium wherein this takes place, will exhibit a comparatively large number of single cells. Conversely, in other instances, the successively developed buds remain connected together, forming a cellular aggregation (Fig. 97). In the older literature, such aggregations, when composed of globular cells, and therefore resembling a series of small knots (Lat. Torula), were generally named Torulæ. This was afterwards employed as the generic name for a number of species, some of which are capable of exciting alcoholic fermentation and will be described in a later section. An example of these is given in Fig. 98.

The form of the gemmae from one and the same species is also dependent on the temperature and the conditions of nutrition, as has been shown by E. Ch. Hansen in the case of beer yeasts and wine yeasts. These, when submerged in beer wort, develop mycelia constructed of short gemmæ; whereas, when cultivated on the surface of the liquid, and therefore in presence of abundance of air, they form mycelia composed of elongated buds. Further particulars of this will be found in § 246.

The formation of mycelia composed of gemmæ was first observed in the case of yeast fungi, and was regarded as a method of development peculiar to these organisms. BAIL (I.), however, in 1857, showed that this phenomenon also appears in certain species of Mucor (see Chapter xliv.) when submerged in a nutrient solution containing sugar. For more precise observations of this phenomenon in the case of Mucors, we are indebted to BREFELD (XVI.). With Mucor racemosus, the carbon dioxide collecting in the nutrient fluid acts upon the cells, by which it has been produced, in such a manner that the latter put forth none but spherical gemmæ, and therefore grow, not to a long and many-branched, unicellular mycelium, but to one composed of stumpy gemmæ. On the other hand, Mucor mucedo treated in the same manner does not produce similar gemmæ, though, according to BREFELD (XVI.), its spores, when placed in a nutrient solution rich in citric acid, swell up to large globules from whence proceed a number of similarly formed daughter-cells, which, however, finally perish. Numerous other fungi are also credited with the same capacity.

FIG. 97.—Saccharomyces pyriformis Ward.

Cell α, embedded in a hanging drop of gingerbeer gelatin and kept at a temperature of 15° C., threw out a bud (β) within 4 1\2 hours. At the end of another 14 hours three normal cells (γ) were present, which grew to the aggregation δ in another 10 hours. This in turn had developed into the colony ∊ in 13 1/2 hours more. (After M. Ward.)

FIG. 98.—Tonila.

Specimen grown in beer wort a represents a group of gemmating cells, the condition of which, after the lapse of an hour, is shown at b. Magn. about 1000. (After Hansen.)

Now, all the Eumycetes capable of forming mycelia of this description can be divided into three groups. To the first group belong all such as, under normal conditions of nourishment, develop exclusively in the form of mycelial aggregations of gemmæ. They are therefore known as budding fungi. This group comprises, without exception, all the Saccharomycetes concerned in the fermentation industries, together with the Mycodermœ, Torulœ, &c. The second group consists of such Eumycetes as, under normal conditions, are equally well able to develop either a filamentous mycelium, or one of gemmæ. These organisms also are occasionally termed budding fungi, examples of the class being afforded by certain species of Monilia (Fig. 99), Dematium, and others. Finally, the third group includes all the fungi which usually produce a filamentous mycelium, the other form being only developed under abnormal