SEXUAL REPRODUCTION 267 



therefore truly reproductive, and in that they are not formed readily 

 as soon as the culture has reached a certain age, but only under 

 exceptional circumstances. Unlike bacterial endospores, yeast asco- 

 spores are sexual spores, known to be haploid in most cases, and to 

 result from a union of gametes with subsequent reduction division. 

 Blastospores, arthrospores, conidia, and basidiospores are formed by 

 some yeasts, but general usage has justified the use of spore to mean 

 ascospore. Likewise sporogenous and asporogenous yeasts are really 

 ascosporogenous and anascosporogenous yeasts respectively, but 

 these latter terms are little used. To demonstrate yeast spores it is 

 usually necessary to subject a vigorously growing culture rather 

 suddenly to unfavorable conditions, as described in Chapter III. 



Yeast spores germinate when transferred to a favorable medium, 

 generally by simple imbibition of water and a gradual swelling to 

 assume the form of vegetative yeast cell. In one group the spore 

 has a double membrane and on germination the outer membrane is 

 ruptured and cast off, the inner membrane serving as the cell wall of 

 the new vegetative cell. In some cases the ancestry of the yeasts 

 in the molds is still further indicated by the spores sending out a 

 short filament of mycelium (promycelium) on germination, from 

 which vegetative yeast cells are formed by lateral budding. 



Sexual reproduction, first carefully studied by Guilliermond in 1902, 

 has been studied by him almost to the present time. He observed 

 isogamous conjugation and the immediate development of ascospores 

 in yeasts belonging to the genus Schizosaccharomyces (Fig. 12), 

 Later both isogamous and heterogamous conjugation were found in 

 certain other genera. Moreover Guilliermond observed a conjugation 

 of spores two by two within the ascus in Saccharomycodes Ludwigii, 

 confirming observations made by Hansen in 1893. Most of the 

 yeasts of industrial importance, however, were found to form their 

 ascospores with no evidence of conjugation. The process of spore 

 formation in these yeasts was thought to be parthenogenetic*. Since 

 1935, however, researches of Winge and Laustsen^ have demon- 

 strated that there is usually an actual conjugation many vege- 

 tative generations previous to spore formation, and that probably 

 no yeasts form their spores exclusively by parthenogenesis. They 

 demonstrated frequent fusion of nuclei, two by two within the ascus, 

 which resulted in the formation of diploid spores. This is com- 

 parable to the process of conjugation of spores within the ascus ob- 

 served by Hansen and by Guilliermond. This was found to be more 

 general than was thought previously. Moreover, if the ascospores 

 (or nuclei) do not fuse, the germinating haploid budding cells are 



