28o A TEXTBOOK OF THEORETICAL BOTANY 



in Fuais among the Algae, and in the phenomenon of Apospory among 

 Ferns. 



The discoven^ of the conjugation between germinating ascospores in 

 Yeast has opened up very remarkable and valuable possibilities of artificial 

 hybridization, and already a considerable number of such hybrids have been 

 created in the search for improved capabilities of fermentation. 



Schizosaccharomyces octosporus 



We may contrast the account of Saccharomyces with the condition in 

 another member of the Saccharomycetaceae, Schizosaccharomyces octosporus. 

 In this species the vegetative cells are more or less spherical and, like all yeasts, 

 are capable of multiplication by budding. Under certain conditions, how- 

 ever, reproduction by a sexual process may occur. Two adjacent cells of 

 similar size put out processes (Fig. 272) which come into contact and fuse, 



Gametes 



Fig. 272. — Zygosaccharomyces sp. Cells producing 

 conjugating processes. 



forming a conjugation tube.* The nucleus from each cell passes into the 

 conjugation tube and there they fuse together. The two conjugating cells 

 now enlarge, and eventually a single, large oval structure is formed, which 

 is an ascus (Fig. 273). The zygote nucleus divides to form usually eight 

 nuclei, around which ascospores become organized. Sometimes the remains 

 of the conjugating cells can be distinguished after the formation of the 

 ascospores, in which case four appear to lie in each of these cells. 



In this species, therefore, it is clear that the ascus is a post-fertilization 

 structure agreeing in the method of its production with that of a typical 

 Ascomycete, and there is some evidence that meiosis occurs during the forma- 

 tion of the ascospores. Hence in this type the true ascomycetaceous nature 

 of the yeasts becomes more apparent. 



* Conjugation also takes place in some other yeasts, e.g., Zygosaccharomyces, of which 

 we give an illustration in the text. Note that the conjugating processes are much longer 

 than in Schizosaccharomyces, 



