298 MICRO-ORGANISMS AND FERMENTATION. 



they form shield-like colonies readily distinguishable from 

 those of the Saccharomycetes. 



In this connection we may mention Hansen's observation 

 that some species e.g., S. Marxianus and S. Ludwigii can 

 develop a mycelium when grown on a solid medium, while 

 others are unable to do so. 



The characters which can be obtained in this way fluctuate 

 greatly, for both the behaviour of the living material and of 

 the substratum may be strongly modified by other external 

 influences. This fact was brought out by Will in a special 

 study of four species of bottom - fermentation beer-yeasts. 

 He discovered that when the inoculating material contained 

 cells from the film they exercised considerable influence on 

 the appearance of the colony. 



The giant colonies investigated by P. Lindner were prepared 

 by transferring a drop-culture containing a vast number of 

 cells to a spot on the nutritive gelatine. It gradually 

 develops a large rounded colony which can be photographed. 

 Even under these conditions the picture fluctuates for one 

 and the same species, according to the differing circumstances. 

 In certain cases, by the use of such growths, strongly marked 

 differences can be produced, as is the case with the usual 

 plate and streak cultures. Frequently, however, the differences 

 between these giant colonies are so minute that it is impossible 

 adequately to describe them. 



Aderhold, during an examination of gelatine-growths of 

 German eUipsoid wine yeasts, found that in puncture-cultures 

 and giant-cultures two types were distinguishable, one of 

 which showed colonies with funnel-shaped depressions and 

 with marked concentric lines, whilst the other showed conical 

 growths with indistinct concentric structure, but very pro- 

 minent radial streaks. 



A great number of yeast species liquefy nutrient gelatine. 

 This was proved by the author in 1890 with respect to 

 brewers' high - fermentation yeasts. Subsequently Will, 

 Wehmer, and others made the same observations with other 

 yeasts. 



Structure and Character of Yeast Cells. During the growth 

 of the cell the membrane gradually becomes more distinct. 



