64 



PLECTOMYCETES 



[CH. 



structure, the ascus. The fusion nucleus divides to form four or eight 

 daughter nuclei about which ascospores are organized (fig. 25). Sometimes 



the limits of the conjugating 

 cells can be distinguished 

 after the ascospores are 

 formed, two or four lying 

 in each of the original cells. 

 In the closely related 

 Sch. Pombe and Sch. fnel- 

 lacei copulation takes place 

 in a very similar way, but 

 the union of the conjugating 



Fig. 25. Schizosacckaromyces octosponis Beyrinck; conjuga- 

 tion and formation of ascospores ; after Guilliermond. 



cells is less complete than is usually the case in Sch. octosporus, and the 

 number of ascospores is regularly four. The mature ascus is thus dumb-bell 

 shaped, with two spoi^es in each enlargement. 



Also in 1901 Barker discovered the yeast Zygosaccharomyces Barkeri 

 and observed in it a conjugation similar to that of Sch. Pombe. 



Following on these observations a number of other cases of conjugation 

 among the yeasts have been recognized. Many species form two spores, 

 one at each end of a dumb-bell shaped ascus ; in others a single spore is 

 produced, the fusion nucleus passing from the conjugation tube into one of 

 the fusion cells, while the other remains empty. 



Such cases lead up to the state of affairs in Zygosaccharomyces Chevalieri 

 where conjugation is between two cells of different sizes, the whole contents 

 of the smaller passing into the larger cell which is then cut off by a wall. 

 In the larger cell nuclear fusion takes place and one to four ascospores are 

 formed. 



In Gtiilliennondia fulvescens conjugation is between a mature cell and 

 its bud. The whole contents of the bud pass back into the parent cell, nuclear 

 fusion takes place and a fresh bud is put out in which the single ascospore 

 develops. It has been suggested that this represents a rudimentary sporo- 

 phyte. The problems connected with meiosis hardly arise here if, as Wager 

 has shown for Saccharomyces, amitosis is the rule. 



In Debaryomyces globosus, copulation takes place sometimes between two 

 similar cells, sometimes, as in Gttilliermondia, between a mature cell and its 

 bud. In either case one or two spores are produced 



In most species with sexually formed asci parthenogenesis is not un- 

 common. In Schwanniomyces occidentals and in Torulaspora Rosei it has 

 become the rule. The cells in which the ascospores are about to be formed 

 put out processes which are directed towards neighbouring cells at the same 

 stage of development. But they do not fuse, and ascospores are formed in each 

 cell independently. In T. Rosei more than one process is sometimes put out. 



