CHAPTER XII 

 SACCHAROMYCETACEAE 



The Saeeliaromycetaceae, or true yeasts, may be regarded as direct de- 

 rivatives of the Eremascaeeae where growth of the thallus by sprouting has 

 become almost universal. Some species, such as Saccharomy codes Ludwigii, 

 Deharyomyces Kloeckeri, and Zygosaccharomyces Priorianus, are still able to 

 form true hyphae on gelatin substrates, but the hyphae are unstable and with 

 a slight alteration of the medium break up into sprout mycelia. 



The sprout cells are generally spherical or ellipsoid and hyaline, the size 

 varying according to medium and age. Under unfavorable conditions they 

 store fat and glycogen and form a double membrane. These resting cells, 

 which are very resistant to environmental changes and carry the organism 

 over unfavorable periods, are probably chlamydospores. At germination each 

 cell ruptures the outer fragile wall and grows into a sprout mycelium. 



On the basis of their method of cell multiplication, the yeasts are often 

 divided into two tribes. In the Schizosaccharomyceteae, each cell elongates, 

 abjoints two equal daughter cells which round off, separate, and again abjoint. 

 At present only a few species are known. In the Saccharomyceteae, sprout 

 cells begin as small lateral protrusions of the motlier cell ; they are abjointed, 

 increase in size, and finally attain the appearance of the mother cell. This 

 tribe contains most of the true yeasts. The tribes are not absolutely distinct 

 since Saccharoniycodes Ludivigii usually divides as in the Schizosaccharomyce- 

 teae but occasionally multiplies by sprouting, although the sprout cells are 

 produced only at the poles of the mother cell, never laterally. If sprouting 

 or division occurs rapidly, the cells may cling together in small colonies or, 

 in occasional old cultures, in filaments. 



Under certain conditions, as in age, with the exhaustion of nutrient, or 

 on solid substrates as gypsum blocks, asci with ascospores are formed. In the 

 wild yeasts the number of ascospores varies from 1 to 12 ; in many industrial 

 yeasts, certain numbers predominate; e.g., in Schizosaccharomyces octosporus 

 4 or 8, in Saccharomyces cerevisiue 4, and in S. Pastorianus 2. The spores are 

 spherical to ellipsoid and either smooth or rough. Ordinarily the spore wall 

 has only one layer, but in SaccJiaromycopsis guttulatus, isolated from feces, 

 there are two layers, the outer of which ruptures on germination. 



In 3-day cultures of Schizosaccharomyces octosporus, the cells copulate 

 in pairs by short tubes (Fig. 57, 1-6). Where the cells are in short chains, 

 the separating wall between sister cells may dissolve (adelphogamy). Two 

 nuclei migrate into the copulation canal and fuse; the copulation canal 

 broadens and each of the two individuals develops into a barrel-shaped struc- 

 ture, where, within approximately half an hour, after three or more, rarely 



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