400 



BOTANY 



PART II 



-Sacrharomi/i'es cerevisiae. A , yeast-cell ; B, C, 

 j'east-cell budding; D, ascus with four spores. 

 (After GuiLLiERMOND, X 1125.) 



f/f/^ 



a^ 



Order 7. Saccharomycetes (Yeast-Fungi) {^") 



The beer, alcohol, and wine yeast.s included in tlie genns Saccharomyccs are 

 simple unicellular Fungi wliich assume tlie form of spherical, oval, or cylindrical 



conidia containing a single nucleus. 

 They increase in number by bud- 

 ding (Fig. 331). No mycelium is 

 formed, though sometimes the cells 

 remain for a time united in cliains. 

 With free access of oxygen and at 

 a suitable temperature yeasts form 

 asci when the nutrient substratum 

 is exhausted ; the asci externally 

 resemble the conidia but contain a 

 few spores. In some yeasts a con- 

 jugation of two cells accompanied by a nuclear fusion has been observed. In 

 Saecharomyces Ludivigii the four sjiores in the ascus germinate and fuse in pairs 

 by means of a narrow conjugation- 

 tube ; the latter elongates into a 

 germ- tube from which yeast-cells 

 are abstricted. In the ginger-beer 

 yeast (Zygosaccharomyces) and in 

 Schizosaccharomyces the yeast-cells 

 conjugate by means of long tubes 

 before spore - formation. These 

 nuclear fusions possibly corre- 

 spond to those in the young asci of 

 other Ascomycetes. 



Physiologically these Fungi are 

 remarkable for their power of ex- 

 citing, by means of an enzyme 

 (zymase), the fermentation of sac- 

 charine solutions, alcohol and 

 carbon dioxide being produced. 

 The beer yeast '{Saccharomy'ccs 

 cerevisiae) is only known in the 

 cultivated form ; the wine yeast 

 (S. cUipsoideus), on the other 

 hand, occurs regularly in the soil 

 of vineyards in the spore-form ; 

 the latter is therefore always 

 present on the grapes and need 

 not be added to the grape-juice. 

 Other genera, in some of which a 

 mycelium is develoj)ed, belong to 

 this order. 



No evidence is at present forth- 

 coming to show that the Yeasts 

 are to be regarded as developmental forms of otlu'r Fungi. In various members 

 of tiie Exoasci, and Ustilagineae, however, yeast-like conidia wliich reproduce by 

 budding are known. Possibly the Saccharomycetes are reduced Ascomycetes, 

 or they may represent an independent group of very simple Fungi. 



t:c 



Fid. •Mi.—StiijmaluinijiKS I'Mni. UesciilitiDii in text. 

 A, spore; L'-7'', suceessive developini'ntal stages. D, 

 with si)ennatia escaping from the antlieriilia mi ; K, 

 with antheridia above and tlie lateral female organ ; 

 F, Peritheciiim with developing asci ; G, ripe ascus. 

 (After Thaxtki!.) 



