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PLANT MORPHOLOGY 



1. Protoascales 



The Protoascales include the yeasts and other simple forms, most of 

 which are regarded as degenerate Ascomycetes. They number about 500 

 species. These are mainly saprophytes but some are parasites on animals. 

 A few of the saprophytic; forms have a mycelium. In the yeasts, which 

 are unicellular fungi, a mycelium ordinarily is not developed. Yeasts are 

 of economic value in breadmaking and in the preparation of alcoholic 

 beverages. The best-known genus is Saccharomyces. Some yeasts repro- 

 duce by fission but most of them reproduce by budding (Fig. 95). A bud 



Fig. 95. Saccharomyces cerevisiae. Cells in the living condition, showing reproduction by 

 budding, X 1,500. 



arises as a small outgrowth, usually at one end of the cell. The nucleus 

 divides to form two nuclei, one of which goes into the bud. The bud 

 enlarges and becomes abstricted from the parent cell. It may either 

 separate at once or remain attached and produce another bud. In this 

 way short chains may be formed. 



In many yeasts, under conditions unfavorable for vegetative activity, 

 the contents of any cell may divide to form four or, in some species, eight 

 thick-walled spores, thus becoming a simple ascus. In some yeasts a con- 

 jugation of two cells precedes the formation of ascospores. The develop- 

 ment of an ascus directly from the zygote is a feature occurring only in the 

 Protoascales. 



There is considerable variation in the life history of different yeasts, and 

 even in the same yeast under different environmental conditions. Thus 

 the ascospores may enlarge to form vegetative cells that undergo a long 

 period of multiplication, or they may conjugate at once. The zygote may 

 become an ascus directly, or may give rise to vegetative cells that later 

 become asci. Under unfavorable conditions, vegetative multiplication 

 may be omitted. If no conjugation occurs, the ascospores are formed by 

 parthenogenesis. 



