62 



MORPHOLOGY AND CULTURE OF MICROORGANISMS 



favorable conditions, of developing into an ascus. Many unsuccessful 

 attempts have been made to connect the true yeasts genetically with 

 various forms of fungi such as Mucor, Ustilago and Dematium. At 

 present they must be considered as distinct species. 



Some yeasts have a tendency during fermentation to remain at the 

 bottom of the liquid; others form a thick foamy layer on top. These 

 are known respectively as bottom and top yeasts. No sharp distinction 

 can be made as there are intermediate forms. 



The vegetative reproduction in the genus Saccharomyces takes place 

 by budding, in Schizosaccharomyces by fission. 



FIG. 43. Yeast cell. (Original.) 



The extreme temperatures for budding lie between i and 47, vary- 

 ing with different species. The optimum temperature varies in the 

 same way between 25 and 35. The rate of multiplication under favor- 

 able conditions will range from one to several hours for the formation of 

 a new cell. 



When young, vigorous, well-nourished cells are supplied with abun- 

 dant air and moisture at a comparatively high temperature under con- 

 ditions that discourage budding (lack of nutriment) they form endo- 

 spores. These spores are usually about half the diameter of the mother 

 cell and from one to eight or more may occur in each cell. They may 

 be formed by cells before or after budding and may even change to asci 

 and form new spores. They are generally spherical or slightly ellip- 

 soidal, rarely kidney-shaped (S. marxianus) or furnished with a zonal 

 ring (S. anomalus) (Fig. 43). 



