140 



GENERAL BIOLOGY OF MICRO-ORGANISMS 



Yeasts. The yeasts (Blastomycetes) are very closely related 

 to the molds. In fact some stages in the growth of molds resemble 

 very closely the normal development of a yeast. The yeasts, 

 however, do not grow out into long filaments but remain spherical 

 or ovoid. The cells vary from 2.5 to 12 ft in diameter. During 

 active growth they reproduce by budding, a smaller portion being 



FIG. 61. Wine and beer yeasts. A, S. ellipsoideus, young and vigorous; B, S. 

 ellipsoideus, (i) old, (2) dead; C. S. cerevisia, bottom yeast; D, S. cerevisice, top 

 yeast. (After Marshall.) 



pinched off from the parent cell. The true yeasts also form spores 

 inside the cell, from four to eight typical ascospores, showing 

 their very close relationship to molds. Yeasts are very important 

 in the fermentation industries. Very few of them are pathogenic. 

 Among themselves, the yeasts are subdivided into two groups, 

 (i) those which produce ascospores (saccharomycetes or true 



