The Fungi — Yeasts 371 



form of the cells, the types of spores foiTned, and the mode of 

 multiplication of the organisms. The character of growth on lab- 

 oratory media and the fermenting abilities of the cells constitute 

 the most important cultural and physiological characteristics em- 

 ployed in classification schemes. Yeasts may be classified either 

 as Ascomycetes or as Fungi Imperfecta 



MORPHOLOGY AND REPRODUCTION 



Yeasts may be round, egg-shaped, or elongated, and the mor- 

 phology is rather constant for a given species grown under stand- 

 ardized conditions. The thick walls of yeasts are not difficult to 

 demonstrate by ordinary staining methods. A considerable space 

 within the cell is occupied by a distinct nucleus which divides dur- 

 ing multiplication of the cell, with part of the nucleus entering 

 each of the offspring. Granules of various sizes are particularly 

 evident in older yeast cells. Vacuoles contain material, the com- 

 plete chemical nature of which has not been determined, but fat 

 and glycogen have been demonstrated. These vacuoles serve as 

 reserve food supplies that may be drawn upon during periods when 

 extracellular food substances are not available. Cytoplasm com- 

 prises the remainder of the cell substance. 



Reproduction of yeasts takes place asexually by budding and 

 BINARY FISSION, scxually by ASCOSPORE FORMATION, and by fusion of 

 two yeast cells to form zygospores. 



The most common method of reproduction is budding. In this 

 process a blister-like formation makes its appearance on the parent 

 cell. As the bud, or daughter cell, becomes larger, a strand of pro- 

 toplasm connects the mother and the daughter. Part of the parent 

 nucleus breaks away and enters the bud, and when the new cell 

 reaches a predetermined size, it breaks away and begins to produce 

 buds of its own. Daughter cells have been observed to produce 

 buds while still attached to the parent cell, resulting in the forma- 

 tion of clumps of yeast cells. Under ideal growth conditions cell 

 division may take place once in about every twenty minutes. 



Yeasts belonging to the Schizosaccharomyces (fission sugar 

 fungi) divide by transverse fission in which the two cells thus 



