THALLOPHYTA: FUNGI 



123 



Yeasts present three different types of life cycles. The first may be 

 illustrated by Schizosaccharomyces odosporus, a fission yeast. Here the 

 vegetative cells are haploid, and eight spores arise in the cell formed by the 

 conjugation of two cells (Fig. 96). The zygote is the only diploid cell in 

 the life history, meiosis occurring when its nucleus divides. In the second 

 type of life cycle, the vegetative cells are diploid. Two ascospores unite 

 and the zygote, without undergoing meiosis, gives rise to vegetative cells 

 that multiply and finally produce ascospores. Meiosis occurs when the 

 spores are formed, and so they are the only haploid cells in the life history. 



E F G H 



Fig. 96. Schizosaccharomyces octosporiis. A to D, conjugation of two cells, the two nuclei 

 uniting to form a single nucleus; E to G, three successive divisions of the fusion nucleus to 

 form eight nuclei; H, formation of eight ascospores. (After Guilliermond.) 



The third type of life cycle, represented by Saccharomyces cerevisiae, is 

 more complicated. Here the vegetative cells are either haploid or diploid. 

 When two haploid cells conjugate, the zygote gives rise to a large number 

 of diploid vegetative cells by budding. Meiosis occurs when one of these 

 cells forms four ascospores. The spores give rise to haploid vegetative 

 cells that multiply by budding. These are smaller than the diploid 

 vegetative cells. 



Yeasts live in sugar solutions and are the principal agents in causing 

 alcoholic fermentation. They use as food only a small part of the sugar 

 that they absorb. The rest is broken down into carbon dioxide, ethyl 

 alcohol, and small amounts of other substances. This process of fermen- 

 tation is accomplished by the production of an enzyme called zymase. It 

 is most active in the absence of free oxygen and serves as a means of 

 releasing energy when the ordinary type of respiration cannot be carried 

 on. 



2. Protodiscales 



The Protodiscales, numbering less than 100 species, are internal para- 

 sites attacking seed plants, especially trees. The only genus is Taphrina. 

 A common species, Taphrina deformans, causes a disease of peaches 



