178 MICRO-ORGANISMS AND FERMENTATION. 



from an absolutely pure culture, are again separately cultivated 

 as pure cultures, it is possible to obtain growths which exhibit 

 great differences in their power of forming spores. By a 

 methodical choice of single cells HANSEN succeeded in obtain- 

 ing growths which, under the recognised conditions, completely 

 failed to yield spores ; on the other hand, he found that, when 

 starting from the same original growth, a yeast speck which 

 had sprung from a spore-yielding cell was chosen and further 

 developed, a growth was obtained which was forthwith capable 

 of yielding an abundance of spores. By such methodical 

 selection, three varieties were separated from this species, one 

 of which was characterised by its high capacity of forming 

 spores ; in the second this property had nearly disappeared, 

 whilst the third did not form spores at all. After numerous 

 cultivations in wort, the third form reverted, but only slowly, 

 to its original condition, in which it was able to form spores; 

 when it was cultivated in a solution of 10 per cent, dextrose in 

 yeast-water, however, this property was immediately re-acquired. 



In other species, varieties which have lost their power of 

 spore-formation completely, or in part, may make their appear- 

 ance, without any assignable cause, both on liquid and on solid 

 nutrient media. In some cases (e.g. Sacch. Ludwigii) that 

 power is restored if dextrose is added to the nutrient liquid. 



If a pure culture of brewery yeast is developed in a wort 

 which has not been aerated after sterilisation, it generally loses 

 its normal " breaking " and clarifying properties, under brewery 

 conditions, and this to a degree dependent on the species. 

 These new variations must often be cultivated through very 

 many generations in ordinary brewery wort before regaining 

 the original qualities of the species. As aeration brings about 

 changes in the chemical composition of the wort, it is evident 

 that this effect on the plasma is due to such modifications in 

 the nutrient medium. 



As an additional instance of the effect of the chemical 

 composition of wort in producing new varieties, we may 

 mention the observation, due also to HANSEN, that Sacch. 

 Pastorianus I., which imparts an unpleasant taste and smell to 

 beer-wort, is apt, if preserved in an aqueous solution of cane- 

 sugar, to lose this power for a time. 



