CHAPTER IX 



MORPHOLOGY AND CLASSIFICATION OF THE YEASTS 



AND YEAST-LIKE FUNGI 



The term yeast is not one with an exact botanical meaning. Hen- 

 rici ^ has stated: "Many bacteriologists with little experience in 

 studying yeasts think that they know very precisely what a yeast is 

 and define it as a unicellular fungus multiplying by budding. Actu- 

 ally such a definition will apply to only a small proportion of the 

 organisms usually classified as yeasts and only to these when they 

 are maintained under constant conditions and not studied too 

 closely." Some yeasts do not multiply by budding and most yeasts 

 if cultivated for long periods of time in giant colonies will produce 

 fringes of mycelium. The following definition is fairly satisfactory: 

 Yeasts are true fungi whose usual and dominant growth form is 

 unicellular. This definition, however, does not exclude certain of the 

 primitive fungi which are unicellular but are not yeasts, and it does 

 not include certain forms sometimes considered as yeasts in which 

 a mycelium is commonly produced. 



Yeasts are phylogenetically a heterogenous group. Some of them 

 seem to be derived from the Basidiomycetes, so degenerate as to 

 have lost most of the characters of that group but retaining the 

 ability to produce exogenous spores, which are forcibly projected like 

 basidiospores. Others have degenerated from these and have lost 

 even this character. If we eliminate the yeasts which are probably 

 imperfect or degenerate Basidiomycetes, the rest fall into two nat- 

 ural groups, the sporogenous yeasts and those lacking ascospores. 

 The former produce ascospores as a result of conjugation or by par- 

 thenogenesis. Both groups seem to be primitive or degenerated As- 

 comycetes. 



Asexual Reproduction. The vegetative multiplication of the 

 yeasts is accomplished by budding, by fission, or by a combination 

 of these two processes. In simple budding the cell wall apparently 

 softens at one point and the protoplasm bulges it out. The bud 

 generally reproduces the form of the parent cell. When the bud is 

 mature it may be separated from the parent cell by constricting its 

 base. Some species may form several buds (multipolar budding) 



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