TEMPERATURE CONDITIONS 171 



Finally a fixed number of ascospores is possible, 4 in the former and 

 1 in the latter. Among the other yeasts, (Schwanniomyces, Torula, 

 etc.) it has been pointed out that the asc forms after an apparent 

 copulation. 



Sometimes the shape of the ascospore is characteristic: hat shaped 

 in Willia anomala, with rings in Willia Saturnus, with a knotty mem- 

 brane in Schwanniomyces. Thus, the existence of sexual phenomena 

 in the formation of the asc, the shape of the asc and ascospores are 

 sufficient to characterize the Nematospora, Monospora, Zygosaccharo- 

 myces, Debaromyces, Schwanniomyces and Willia. 



The method of germination of the ascospores often furnishes deli- 

 cate information for the determination of species. Hansen's re- 

 searches have made it possible to characterize .the Saccharomyces, in 

 which the ascospores undergo generally a copulation at the beginning 

 of germination, for they are connected two by two with a copulation 

 canal. Two new cells are formed by a process intermediate between 

 budding and fission. 



But for the great number of varieties, notably most of the indus- 

 trial yeasts, the shape and dimensions of the ascospores and their 

 germination do not offer any distinctive factor which may be utilized 

 for their identification. 



Temperature Conditions which Influence Scum Formation and 

 Microscopic and Macroscopic Characteristics 



The mode of formation and the appearance of the scums formed 

 by different yeasts make excellent characters upon which to separate 

 the yeasts. By this method, as has been discussed in Chapter IV, 

 two groups of yeasts may be distinguished. The yeasts of one group 

 form a scum at the beginning of fermentation; it is their mode of 

 vegetation; the scums are well developed, grayish, with folds, and 

 usually dry. They contain air. This group includes the genera 

 Willia and Pichia. For this group the temperature limits of budding 

 and scum formation are evidently the same. 



However, many of the yeasts do not form a scum at temperatures 

 close to their temperature limits. It has been stated that Klocker 

 has demonstrated the beneficial effect of alcohol in the medium on 

 scum formation in the genera Pichia and Willia. 



The yeasts in the other group form their scums very slowly after 

 the principal fermentation has terminated. The scums are rather 

 viscous, wet, and do not contain entrained air. Many of the yeasts 

 in this group form only a ring and some form neither ring nor scum. 

 The character of the scum of this group has served Hansen for sep- 



