286 MICRO-ORGANISMS AND FERMENTATION. 



constant. It appears from this that the transformation does 

 not depend on temperature or aeration alone, but also on the 

 nutrition and propagation of the cells. 



A comparison of these different factors has, however, shown 

 that they contribute unequally to the result. Both the nutrient 

 liquid and the aeration are only of importance in bringing about 

 vigorous new formations, and may, therefore, vary consider- 

 ably in strength without materially affecting the result. This, 

 however, is not true of temperature ; a fluctuation of a few 

 degrees is sufficient to prevent the variations described from 

 coming into existence. Hence, it follows, that temperature 

 plays the principal part in these transformations. 



As previously stated, these remarkable changes are only 

 brought about by a long-continued and violent interference 

 with the vital processes of the cells ; they do not occur so 

 long as development takes place in the normal manner. 



An example of the way in which the Saccharomyces cells 

 retain their power of forming spores under ordinary conditions 

 is supplied in breweries and distilleries. Here culture yeasts 

 have existed continuously for centuries, and untold generations 

 have been produced under conditions which would not allow, 

 .as a rule, of this function being brought into play, and yet the 

 power remains intact. 



Lepeschkin observed a well-developed mycelium formation 

 in Schizo-saccharomyces Pombe and mellacei, which he regarded 

 as a stable variation brought about by the alteration (mutation) 

 of certain cells. 



Hansen observed a remarkable variation when young 

 growths of S. ellips. II. and the wine yeast Johannisberg II. 

 were preserved for a few months in Freudenreich flasks in 

 shallow layers of wort at 0-5 C. A few cells of these bottom- 

 fermentation yeasts gave top-fermentation phenomena. The 

 further investigation showed that selection had taken place. 

 The top-fermentation yeast cells remained continuously as 

 top yeasts, the bottom-fermentation yeast cells as bottom 

 yeasts. A similar state of things was observed during the 

 examination of a large number of cells from old cultures of 

 brewery bottom-fermentation yeasts. On the other hand, 

 .similar cultures both of wild top-fermentation and of brewery 



