THE YEASTS 



INTRODUCTION 



What are Yeasts? 



UNDER the name of yeasts have been generally grouped all micro- 

 organisms which, when placed in sugar solutions, decompose 

 them into alcohol and carbon dioxide cause alcoholic fermen- 

 tation. 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 myce- 

 lium 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 vis- 

 cosa) 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 



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