Page:The yeasts (1920).djvu/26

 yeasts. These do not present a true mycelium at any time. They reproduce at times by elongation of their cells, which adhere together, forming structures resembling mycelium; but these never offer the complexity of a typical mycelium. In the category of yeasts belong the alcoholic ferments and all of the fungi more generally known under the name of yeasts.

These yeasts, which are often designated as "true yeasts" in contradistinction to "yeast-like fungi" derived from more highly developed fungi, are not distinguishable in any manner from the latter. The general form and the internal characteristics of the cells are the same in both cases. Physiologically, certain true yeasts differ only from yeast forms of molds by their resistance to anaerobic conditions and exceptional activity of the fermenting function, but very many yeast-like structures, derived from fungi more highly developed, are equally capable of producing alcoholic fermentation, and only differ, from this point of view, from true yeasts by a decreased activity of fermentation. On the other hand, a certain number of true yeasts are totally deprived of the fermenting function. It is understood then, how the early investigators were much confused when it became necessary to characterize the yeasts.

In the meantime, an essential difference which did not escape investigators existed between the yeast-like fungi and the yeasts properly so-called. Indeed, most of the true yeasts are distinguished closely from "yeast-forms " by their aptitude to produce resistant endospores at certain stages in their life cycles (unfavorable conditions), in the interior of their cells; the cells are then transformed into sporangia. De Bary, Rees, and Hansen first compared these sporangia to ascs of Ascomycetes, considering the true yeasts as autonomous fungi which live only in the form of yeasts and are incapable of developing a mycelium.

This conception is definitely admitted today, as we shall see when the origin and systematic relationships of the yeasts are taken up. The autonomy of the yeasts and their incorporation as a group of Ascomycetes have been demonstrated only since Hansen observed their life cycles in nature and since certain investigators have given evidence in the origin of the asc of certain yeasts, of the presence of