spores. The sporangium with its endogenous spores has been
compared with an ascus, and on these grounds the group is placed
among the Ascomycetes—a very doubtful association. The group
has attained an importance of late even beyond that to which it was
brought by Pasteur’s researches on alcoholic fermentation, chiefly
owing to the exact results of the investigations of Hansen, who
first applied the methods of pure cultures to the study of these
organisms, and showed that many of the inconsistencies hitherto
existing in the literature were
due to the coexistence in the
cultures of several species or
races of yeasts morphologically
almost indistinguishable, but
physiologically very different.
About fifty species of Saccharomyces
are described more or less
completely, but since many of
these cannot be distinguished
by the microscope, and some
have been found to develop
physiological races or varieties
under special conditions of
growth, the limits are still far
too ill-defined for complete
botanical treatment of the genus.
A typical yeast is able to develop
new cells by budding when submerged
in a saccharine solution,
and to ferment the sugar—i.e.
so to break up its molecules that,
apart from small quantities used
for its own substance, masses of
it out of all proportion to the
mass of yeast used become
resolved into other bodies, such
as carbon dioxide and alcohol,
the process requiring little or
no oxygen. Brefeld regards the
budding process as the formation
of conidia. Under other
conditions, of which the temperature
is an important one, the
nucleus in the yeast-cell divides,
and each daughter-nucleus again,
and four spores are formed in the mother cell, a process obviously comparable
to the typical development of ascospores in an ascus. Under
yet other conditions the quiescent yeast-cells floating on the surface
of the fermented liquor grow out into elongated sausage-shaped or
cylindrical cells and branching cell-series, which mat together into
mycelium-like veils. At the bottom of the fermented liquor the
cells often obtain fatty contents and thick walls, and behave as
resting cells (chlamydospores). The characters employed by experts
for determining a species of yeast are the sum of its peculiarities as
regards form and size: the shapes, colours, consistency, &c., of
the colonies grown on certain definite media; the optimum temperature
for spore-formation, and for the development of the
“veils”; and the behaviour as regards the various sugars.
 |
From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer. |
|
Fig. 10.—Taphrina Pruni. Transverse section through the epidermis of an infected plum. Four ripe asci, a1, a2, with eight spores, a3, a4, with yeast-like conidia abstricted from the spores. After Sadebeck. st, Stalk-cells of the asci. m, Filaments of the mycelium cut transversely. cut, Cuticle. ep, Epidermis. |
The following summary of some of the principal characteristics of half-a-dozen species will serve to show how such peculiarities can be utilized for systematic purposes:
| Species. | Optimum Temperature for | Characters of | Sugars Fermented and Products, &c. | |||||
| Spores. | Veils. | Fermentation. | Cells. | Spores. | ||||
| S. cereviseae I. | 30° | 20°-28° | High | Rounded | Globoid |
| ||
| S. Pastorianus I. | 27°-5° | 26°-28° | Low | Rounded | Globoid | |||
| S. ellipsoideus | 25° | 33°-34° | Low | Rounded | Globoid | |||
| S. anomalus | 28°-31° | ? | High | Elliptical | Hat-shaped | Ditto, and evolves a fragrant ether. | ||
| S. Ludwigii | 30°-31° | ? | ? | Elongated | Globoid | Will not invert maltose. | ||
| S. membranaefaciens | 30° | ? | High | Elongated | Globoid | Inverts neither maltose nor saccharose. | ||
Two questions of great theoretical importance have been raised over and over again in connexion with yeasts, namely, (1) the morphological one as to whether yeasts are merely degraded forms of higher fungi, as would seem implied by their tendency to form elongated, hypha-like cells in the veils, and their development of “ascospores” as well as by the wide occurrence of yeast-like “sprouting forms” in other fungi (e.g. Mucor, Exoasci, Ustilagineae, higher Ascomycetes and Basidiomycetes); and (2) the question as to the physiological nature and meaning of fermentation. With regard to the first question no satisfactory proof has as yet been given that Saccharomycetes are derivable by culture from any higher form, the recent statements to that effect not having been confirmed. At the same time there are strong grounds for insisting on the resemblances between Endomyces, a hyphal fungus bearing yeast-like asci, and such a form as Saccharomyces anomalus. Concerning the second question, the recent investigations of Buchner and others have shown that a ferment (zymase) can be extracted from yeast-cells which causes sugar to break up into carbon dioxide and alcohol. It has since been shown by Buchner and Albert that yeast-cells which have been killed by alcohol and ether, or with acetone, still retain the enzyme. Such material is far more active than the zymase obtained originally by Buchner from the expressed juice of yeast-cells. Thus alcoholic fermentation is brought into line with the other fermentations.
Schizosaccharomyces includes a few species in which the cells do not “bud” but become elongated and then divide transversely. In the formation of sporangia two cells fuse together by means of outgrowths, in a manner very similar to that of Spirogyra; sometimes, however, the wall between two cells merely breaks down. The fused cell becomes a sporangium, and in it eight spores are developed. In certain cases single cells develop parthenogenetically, without fusion, each cell producing, however, only four spores. In Zygosaccharomyces described by Barker (1901) we have a form of the usual sprouting type, but here again there is a fusion of two cells to form a sporangium.
Cytology.—The study of the nucleus of yeast-cells is rendered difficult by the presence of other deeply staining granules termed by Guillermond metachromatic granules. These have often been mistaken for nuclei and have to be carefully distinguished by differential stains. In the process of budding the nucleus divides apparently by a process of direct division. In the formation of spores the nucleus of the cell divides, the protoplasm collects round the nuclei to form the spores by free-cell formation; the protoplasm (epiplasm) not used in this process becomes disorganized. A fusion of nuclei was originally described by Jansens and Leblanc, but it was observed neither by Wager nor Guillermond and is probably absent. In Schizosaccharomyces and Zygosaccharomyces, however, we have a fusion of nuclei in connexion with the conjugation of cells which precedes sporangium-formation. The theory may be put forward that the ordinary forms have been derived from sexual forms like Schizosaccharomyces and Zygosaccharomyces by a loss of sexuality, the sporangium being formed parthenogenetically without any nuclear fusion. This suggests a possible relationship to Eremascus, which can only doubtfully be placed in the Ascomycetes (vide supra).
Carpoascomycetes.—The other divisions of the Ascomycetes may be distinguished as Carpoascomycetes because they do not bear the asci free on the mycelium but enclosed in definite fruit bodies or ascocarps. The ascocarps can be distinguished into two portions, a mass of sterile or vegetative hyphae forming the main mass of the fruit body, and surrounding the fertile ascogenous hyphae which bear at their ends the asci. When the ascogonium (female organ) is present the ascogenous hyphae arise from it, with or without its previous fusion with an antheridium. In other cases the ascogenous hyphae arise directly from the vegetative hyphae. In connexion with this condition of reduction a fusion of nuclei has been observed in Humaria rutilans and is probably of frequent occurrence. The asci may be derived from the terminal cell of the branches of the ascogenous hyphae, but usually they are derived from the penultimate cell, the tip curving over to form the so-called crozier. By this means the ascus cell is brought uppermost, and after the fusion of the two nuclei it develops enormously and produces the ascospores. The ascospores escape from the asci in various ways, sometimes by a special ejaculation-mechanism. The Ascomycetes, at least the Carpoascomycetes, exhibit a well-marked alternation of sexual and asexual generations. The ordinary mycelium is the gametophyte since it bears the ascogonia and antheridia when present; the ascogenous hyphae with their asci represent the sporophyte since they are derived from the fertilized ascogonium. The matter is complicated by the apogamous transition from gametophyte to sporophyte in the absence of the ascogonium; also by the fact that there are normally two fusions in the life-history as mentioned earlier. If there are two fusions one would expect two reductions, and Harper has suggested that the division of the nuclei into eight in the ascus, instead of into four spores as in most reduction processes, is associated with a double reduction process in the ascus. Miss Fraser in Humaria rutilans finds two reductions: a normal synaptic reduction in the first nuclear division of the ascus, and a peculiar reduction division termed brachymeiosis in the third ascus division.
Various types of ascocarp are characteristic of the different divisions of the Carpoascomycetes: the cleistothecium, apothecium and perithecium.
