PARTICULAR TYPES OF BUDDING 113 



Certain yeasts, such as the Mycoderma (Mycoderma vini and cere- 

 visiae), are essentially aerobes, never producing fermentations and 

 forming on the surface of the media a scum which is very character- 

 istic, reminding one of fungi. This scum is gray and dry. Later, it 

 develops and becomes wrinkled. Many bubbles of air are found re- 

 tained between the cells. But the Mycoderma are not characteristic 

 yeasts; they do not form spores and their place in a classification is 

 quite uncertain. 



Hansen has distinguished two groups among the Saccharomycetes 

 or true yeasts. In one, which includes Willia and Pichia, the scum 

 appears very rapidly at the beginning of the culture. It is well devel- 

 oped, dry, and filled with globules of air which are retained between 

 the cells. This is the characteristic scum for the Mycoderma. For 

 this group, the scum is a normal method of vegetation. It is under- 

 stood, then, that budding is confused with the formation of the scum. 



In the other group, to which belong the majority of known species, 

 a scum may or may not be formed. When one is formed it appears 

 at the end of fermentation and under conditions which Hansen has 

 well established. On the other hand, this scum differs in the group. 



It is necessary, in order for the scum to form, that the surface 

 of the medium be quiet and in direct contact with air. Hansen has 

 recommended that a 24-hour culture be used to which- there is added 

 new wine. The culture should be shaken, after which a drop is carried 

 over to a flask half full of new wine and closed with a cap of paper. 

 At the end of a shorter or longer time, the principal fermentation is 

 finished and on the surface of the medium are seen little spots of 

 yeast. These remain isolated as little islands, until they join to form 

 a thin scum which is gray and mucous. If the flask is shaken, parts 

 of the scum break off and fall to the bottom; eventually a new scum 

 will form over the surface. During the formation of this scum, the 

 medium becomes a clear yellow. The scum, thus formed, differs 

 markedly from that formed by Mycoderma; it is less tenacious and 

 has a more viscous appearance. 



Other yeasts do not form scums but simply rings about the side 

 of the tube. With some both are formed. 



The formation of the scum is influenced by the temperature, as 

 the investigations of Hansen have determined. Hansen has shown that 

 certain limits of temperature exist outside of which the scum is not 

 able to form. Between these limits the time of formation is determined 

 by the temperature. The time is constant for a given temperature. 

 A certain optimum temperature, variable with each species, allows the 

 most rapid formation of the scum. It is an interesting fact to note 

 that there is no relation between the temperature at which scum for- 



