Part 179. The yeast fungi (Class Saccharomycetes). Alcoholic fermentation, or the conversion of a carbohydrate into alcohol and carbonic acid gas, such as takes place in the manufacture of beer and wine and in the raising of bread, is usually accomplished by means of yeast. This consists of unicellular fungi (Fig. 151, a-d). The usual method of reproduction differs from fission in that new cells arise as small protuberances or buds which eventually attain the size of the parent cell. Several resting spores are formed in a single cell (e, f) and these germinate by budding (g, h). There is reason to believe that yeast-plants represent merely a stage in the life-history of more highly developed fungi, which, however, have the power of perpetuating themselves indefinitely in the simple ways described, much as we have seen to be the case with wall-stain alga. Whatever may prove to be their true relationship to other fungi the species of yeast are conveniently placed provisionally in a class by themselves composed of unicellular forms, which reproduce only by budding and the formation of spores by internal cell-division.