CHAPTER IV 

 MORPHOLOGY AND CLASSIFICATION OF THE YEASTS 



Size, Shape, and Grouping of Yeast Cells. Yeasts are 

 usually somewhat larger than bacteria. This usually ren- 

 ders comparatively easy differentiation from the bacteria. 

 The yeast cell is usually spherical, ovoid, or ellipsoid in 

 shape, occasionally it is considerably elongated or even 

 cylindrical. In most species there is very little of the reg- 

 ularity of grouping which is characteristic of many types 

 of bacteria. The cells occur in masses or occasionally in 

 chains or filaments. Yeasts are typically unicellular 

 plants, and can by this means be differentiated from the 

 molds, although in a few species the chains of cells (fila- 

 ments) approach in appearance the hyphae of the true 

 mold. 



STRUCTURE 



The cell wall is relatively thin in young yeast cells but 

 may be considerably thickened in old. Occasionally the 

 cells are embedded in gelatinous masses. The chemical 

 composition of the cell wall has not been clearly estab- 

 lished. It is frequently said to consist of yeast cellulose, 

 although not identical in composition with the cellulose of 

 the cell walls of higher plants. The organism practically 

 never produces definite capsules and is never motile, that 

 is, flagella are not produced. 



Cell Contents. The contents of the cell are somewhat 

 more clearly differentiated than is the case with the bac- 

 teria. Furthermore, one may note marked differences in 

 the appearance of cell contents of young cells and older 



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