Order 1. Hymenolichenes. To this order belong some gymnocarpic forms: Cora, Dictyonema, Laudatea.[15]
Order 2. Gasterolichenes. To this belong some angiocarpic forms: Emericella, Trichocoma.
Appendix to the Fungi.
Fungi imperfecti (Incompletely known Fungi).
1. The Saccharomyces-forms are Fungi which are only known in their yeast-conidial form. They are conidia of higher Fungi which can multiply to an unlimited extent by budding in nutritive solutions, and in this way maintain their definite size and shape. The budding takes place only at the ends of the conidia. The wall of the conidium forms at one or at both ends a small wart-like outgrowth, which gradually becomes larger, and is finally separated from its mother-cell as an independent cell, surrounded by a closed cell-wall (Fig. [182] a, b).
Fig. 182.—Beer-yeast (Saccharomyces cerevisiæ): a-b (× 400); c-f (× 750); c a cell in the process of forming spores; d a cell with four ripe spores; e the spores liberated by the dissolution of the cell-wall; f three germinating spores; g mycelium-like cell-chains. (× 1000: after Em. Chr. Hansen.)
Under very favourable conditions multiplication occurs so rapidly that the daughter-cells themselves commence to form buds, before they have separated from their mother-cell, with the result that pearl-like chains of cells are produced. When the yeast-cells have only limited nutriment, with an abundant supply of air, at a suitable temperature, an endogenous formation of spores takes place. The protoplasm of the cells divides into 1–4 (rarely a greater number) masses (Fig. [182] c, d, e) which surround themselves with a thick cell-wall, and in this state can withstand adverse conditions and periods of dryness lasting for several months.
The sporangia are not asci since they have no definite form, and a definite number, form and size of spores is not found. The spores in the different species and kinds occupy varying periods for their development, although exposed to the same temperature, a fact of importance in determining one from another. On germination the wall of the mother-cell is destroyed, and each spore gives rise to a new cell, multiplication taking place by budding (Fig. [182] f). The majority of Yeast-Fungi are able to produce alcoholic fermentation in saccharine fluids.
The most important of these Fungi is the Beer-yeast (Saccharomyces cerevisiæ) with ovate, ellipsoidal or spherical cells (Fig. [182]). It is a plant which has been cultivated from time immemorial, on account of its property of producing alcoholic fermentation in sugar-containing extracts (wort), derived from germinating barley (malt). Carbonic acid is also set free during this process. The “surface-yeast” (Fig. [182] a), which produces ordinary beer when the brewing takes place at higher temperatures, has cell-chains; “sedimentary yeast” (Fig. [182] b), used in the brewing of Bavarian beer, has spherical cells, solitary, or united in pairs. Both these and the following Yeast-Fungi include, according to Hansen, several species and kinds.