CHAPTER II. 



YEASTS.* 



MORPHOLOGY OF CERTAIN TYPES. 



DEFINITION AND BASES OF CLASSIFICATION. If the cloudy freshly 

 expressed juice of grapes or other fruits be passed through a centrifuge, 

 the sediment will be found to consist principally of amorphous particles 

 of dirt and plant tissue. If the clear juice is now allowed to stand in a 

 warm place for a few days it will ferment and the sediment thrown 

 down by the centrifuge may be shown by the microscope to consist prin- 

 cipally of unicellular microorganisms. 



These microscopic cells are called collectively "yeast" and belong 

 to various groups of fungi. Some of them are special vegetative forms 

 of Phycomycetes (Mucor), others of Ascomycetes (Saccharomyces, Asper- 

 gillus), while others are unknown in any other form and are classed as 

 Fungi imperfecti (Mycoderma, Toruld). They are widely distributed in 

 nature and some of them occur on all exposed surfaces and particularly 

 on moist organic substances containing sugar and acid. The true 

 yeasts (Saccharomycetes}, which are of the greatest importance indus- 

 trially, occur naturally on the raw material (S. ellipsoideus on grapes) 

 or are known only in the cultivated condition (S. cerevisia of beer). 



The true yeasts occur in the form of spherical or more or less elongated 

 cells varying in normal width from 2.5/4 to 12 fj.. The first classifications 

 were based on shape and size alone but these vary and depend so much 

 on cultural conditions that they are of little value in differentiating 

 species or varieties. 



The range of variation in shape and size, especially of the spores, 

 under given conditions of culture medium and temperature, is now used 

 only in conjunction with the reactions brought about in various solutions 

 to distinguish the various forms. 



The true yeasts are characterized by the formation of endospores 

 and are classed with the Gymnoasceft. Each cell seems capable, under 



* Prepared by F. T. Bioletti. 



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