CHAPTER LIV. 



VARIABILITY AND HEREDITY IN SACCHAROMYCETES. 



273. Temporary Variations. 



BOTH the morphological and physiological characteristics are 

 subject to variation, not only among micro-organisms but also 

 among higher organisms, both vegetable and animal. Just 

 as the study of the Saccharomycetes first obtained precision 

 when the discovery of reliable methods of pure culture 

 furnished a sure starting-point for the investigations, so also 

 the era of trustworthy observations on variation dates from the 

 same period. 



We are indebted to the researches of Hansen for the foundation 

 of our knowledge of the variations of the Saccharomycetes. These 

 researches form two main groups : one comprising such variations 

 as must be considered temporary and dependent on conditions at 

 present unknown, whilst the other relates to variations occurring 

 under known conditions. The experimental researches and the 

 results obtained in connection with this latter group form the 

 most important part of Hansen's labours in this field. 



The variations may be classified into temporary and constant 

 from another point of view, temporary variations being those 

 continuing for only a limited time, at the expiration of which they 

 disappear, either spontaneously or under special treatment, whilst 

 the constant variations are those that cannot be restored to their 

 original condition by any treatment. 



We shall first discuss the temporary variations, citing dif- 

 ferent examples. The total number of these variations is naturally 

 enormous. 



Hansen's observations in this connection were commenced at 

 the time he introduced pure culture, and they include examples 

 from nearly every branch of the morphology and physiology of the 

 yeast cell, the important being given below. Thus he observed 

 that a yeast from the brewery gave a higher attenuation, defective 

 clarification, a strange flavour, &c., after it had been grown for 

 some time in the laboratory, but regained its original properties 

 on being returned to practical use. This observation has also 

 been made by other workers. Moreover he found (XL.) that 

 when Carlsberg bottom yeast No. i was grown on wort gelatin, it 



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