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 THE YEASTS

INTRODUCTION

What are Yeasts?

UNDER the name of yeasts have been generally grouped all microorganisms which, when placed in sugar solutions, decompose them into alcohol and carbon dioxide cause alcoholic fermentation. Knowledge with regard to the chemical properties of the yeasts has, to a great extent, preceded that with regard to their nature. The old word yeasts (Fr. levure = Latin lever) which emphasized their chemical properties dates from an epoch when no attention was given to their biological significance or nature. But today the name yeast has taken on a restricted meaning among botanists. In the botanical sense, yeasts are unicellular fungi of biochemical interest, spherical or oval in shape, and which multiply by budding. A yeast, then, is a fungus with special morphology. Be that as it may, the term is not applied to an indefinite group of fungi but to a natural one. Many fungi, more or less developed, living normally with a mycelium are able to reproduce by budding of their filaments, to form cells which have the shapes of yeasts. These multiply in their turn by budding and retain the form of yeasts for many generations. (Fig. 1.) The basidiospores of certain Basidiomycetes (Calcera viscosa) and ascospores of certain Ascomycetes (Sphaerulina intermixta Taphria) give rise to yeasts and it is only after living for a certain time in this form that the yeast cells elongate filaments and produce a mycelium. Among the Ustilaginales, the sporidia, which spring from the promycelium, exist also in the shape of yeasts; it is this state in which they develop, and which they constantly retain when cultivated in artificial media. The Mucors, when placed in sugar solutions, are able to dissociate their filaments into round bodies, or buds, in a similar manner as the yeasts. Dematium pullulans (Fig. 1), a mold with a well-differentiated mycelium, produces in a regular fashion, by budding of its filaments, numerous yeast conidia; when these are cultivated under certain conditions, they are transformed with difficulty into mycelium. Vegetation with forms like yeasts is, then, rather widespread among the fungi.

Aside from these fungi, in which yeast forms are merely stages of development, there are others which live constantly in the forms of