In some forms we find definite male and female sexual organs (Sphaerotheca, Pyronema, &c.), in others the antheridium is abortive or absent, but the ascogonium (oogonium) is still present and the female nuclei fuse in pairs (Lachnea stercorea, Humaria granulata, Ascobolus furfuraceus); while in other forms ascogonium and antheridium are both absent and fusion occurs between vegetative nuclei (Humaria rutilans, and probably the majority of other forms). In other cases the sexual fusion is apparently absent altogether, as in Exoascus. In the first case (fig. 9) we have a true sexual process, while in the second and third cases we have a reduced sexual process in which the fusion of other nuclei has replaced the fusion of the normal male and female nuclei. It is to be noted that all the forms exhibit the fusion of nuclei in the ascus, so that those with the normal or reduced sexual process described above have two nuclear fusions in their life-history. The advantage or significance of the second (ascus) fusion is not clearly understood.

The group of the Hemiasci was founded by Brefeld to include forms which were supposed to be a connecting link between Phycomycetes and Ascomycetes. As mentioned before, the connexion between these two groups is very doubtful, and the derivation of the ascus from an ordinary sporangium of the Zygomycetes cannot be accepted. The majority of the forms which were formerly included in this group have been shown to be either true Phycomycetes (like Ascoidea) or true Ascomycetes (like Thelebolus). Eremascus and Dipodascus, which are often placed among the Hemiasci, possibly do not belong to the Ascomycetes series at all.

From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer.
Fig. 9.—Sphaerotheca Castagnei. Fertilization and Developmentof the Perithecium. (After Harper.)

1, Oogonium (og) with the antheridial branch (az) applied to its surface

2, Separation of antheridium (an).

3, Passage of the antheridial nucleus towards that of the oogonium.

4, Union of the nuclei.

5, Fertilized oogonium surrounded by two layers of hyphae derived from the stalk-cell (st).

6, The multicellular ascogonium derived by division from the oogonium; the terminal cell with the two nuclei (as) gives rise to the ascus.

Exoascaceae are a small group of doubtful extent here used to include Exoascus, Taphrina, Ascorticium and Endomyces. The mycelium is very much reduced in extent. The asci are borne directly on the mycelium and are therefore fully exposed, being devoid from the beginning of any investment. The Taphrineae, which include Exoascus and Taphrina, are important parasites—e.g. pocket-plums and witches’ brooms on birches, &c., are due to their action (fig. 10). Exoascus and Ascorticium present interesting parallels to Exobasidium and Corticium among the Basidiomycetes.

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. sp, Epidermis.

Saccharomycetaceae include the well-known yeasts which belong mainly to the genus Saccharomyces. They are characterized by their unicellular nature, their power of rapid budding, their capacity for fermenting various sugars, and their power of forming endogenous 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.

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 Inverts maltose and saccharose
  and form alcohol 4-6 vol. %.
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.