1 -4 THE YEAST CELL 



usually incapable of acting on galactose. This genus does not store 

 glycogen. Saccharomyces are generally 4-spored. The Schizosac- 

 charomyces are 4- or 8-spored. 



Saccharomycodes (fig. 1-4) has a unique characteristic. The 

 cells multiply by terminal budding (fig. 1-3) which amounts to a 

 compromise between the budding and binary fission. The cells are 

 lemon- shaped with bud^ on each end and with a cross wall between 

 bud and mother cell. The bud in the Saccharomyces is never termin- 

 ated by a cross wall, it simply increases in size and is cut off by a 

 very narrow constriction. The spores of Saccharomycodes are not 

 starchy but turn yellow with iodine solution and are acid fast. 



There is no specific orientation of spores in either Saccharo- 

 myces or Schizosaccharomyces. In Saccharomycodes each cell be- 

 comes an ascus containing two pairs of spores (one pair at each 

 end). Each pair of spores copulates just as the cell germinates and 

 again produces the regular diploid cell. There is no opportunity 

 for haplophase multiplication. This means that there is no natural 

 haplophase in Saccharomycodes. 



The different species of Saccharomyces differ principally in 

 their ability to ferment different sugars. The sugars dextrose, 

 mannose, fructose, galactose, sucrose, maltose, lactose, melibi- 

 ose, and raffinose are generally used in a study of this type. Most 

 of the different species of Saccharomyces are distinguishable from 

 one another by their action on these sugars. The commonest is S. 

 cerevisiae (the Saccharomyces of beer). It ferments all these sug- 

 ars except lactose and melibiose, and ferments only one -third of 

 raffinose. 



Raffinose is a tri -saccharide with the following constitution: 



Melibiose 



Fructose + Glucose + Galactose 



Sucrose 



Any yeast which can hydrolyze sucrose should be able to fer- 

 ment one -third of raffinose leaving one molecule of melibiose. The 

 diagnostic fraction is the melibiose. S. carlsbergensis ferments 

 raffinose completely and therefore ferments melibiose also. S. cer- 

 evisiae ferments one -third of raffinose; it does not ferment melib- 

 iose but can ferment sucrose. These species are differentiated by 

 their ability to ferment melibiose. We have made hybrids between 

 them and have found that this difference is due to a single gene. 



Saccharomyces lactis alpha is able to ferment lactose but is not 

 able to ferment maltose. Kluvyer established the rule that there 

 are no Saccharomyces which can ferment both lactose and maltose. 

 S. lactis probably has its origin in contaminations in milk, it may 

 be relatively recent in origin and its origin may be associated with 



