114 PHYSIOLOGY OF YEASTS (Continued) 



mation goes on most rapidly and budding. Budding is less dependent 

 on the temperature. 



In determining these various temperatures and the time necessary 

 for the scum to form in six species (S. cerevisiae, Pastorianus, inter- 

 medius, validus, ellipsoideus, and turbidans) , Hansen noticed that scum 

 formation was slower at low temperatures than at high temperatures. 

 On the other hand, it may be stated that the maximum temperature 

 for scum formation is lower than the maximum for budding. The 

 temperature limits of scum formation vary with the species and fur- 

 nish important characteristics for the differentiation of these species. 



Physiological Conditions of Sporulation 



We shall see, in a later chapter, that sporulation is generally a 

 function of inanition of the yeast. It is often necessary that the 

 yeasts have accumulated from former culture media the reserve prod- 

 ucts necessary for the formation of ascospores. Under these condi- 

 tions, the cells begin to bud as soon as these reserve products are 

 exhausted. It seems from all this that the formation of ascospores 

 is determined exclusively by the lack of food. This is the conclusion 

 to which the investigations of Klebs l leads us. However, it has 

 been known for a long time that the yeasts are able to sporulate very 

 rapidly on certain solid media (gelatin added to wine, slices of 

 carrot or potato) and sometimes in liquid media during fermentation. 

 Sporulation seems, then, to have other causes. Klebs admits that in 

 the case of solid media, such as nutrient gelatin or slices of carrot, if the 

 yeasts are able to sporulate, it is only those cells which are in the 

 innermost parts of the colonies where they are prevented from using 

 the* medium. These find themselves in bad conditions of food supply 

 which explains their sporulation. In these colonies, the cells occupy- 

 ing the marginal portion of the colony will continue to bud and 

 multiply while the cells which are on the inside of the colony will be 

 reduced to conditions which favor the formation of spores. 



The investigations of Hansen, 2 to which we owe much of our in- 

 formation with regard to sporulation, have demonstrated, on the con- 

 trary, that this matter is much more complicated. If the lack of 

 food is one of the most important factors, it is by no means indis- 

 pensible. Indeed, Hansen has stated that contrary to the ideas of 

 Klebs, in cultures on gelatin or beer wort, the ascospores form in the 



1 Klebs, C. Allgemeine Betrachtungen. Jahrb. wissenschaft. Bot. 35, 1900. 



2 Hansen, E. C. Recherches sur la physiologic et la morphologic des fer- 

 ments alcooliques. II. Les ascospores chez le genre Saccharomyces. III. Sur les 

 Torula de M. Pasteur. IV. Maladies provoquees dans la biere par les ferments 

 alcooliques. C. R. du lab. de Carlsberg, 1, 1883, 5, 1902. 



