TEMPORARY VARIATIONS. 259 



curdy yeast may be produced when the yeast has been left for 

 some time in a desiccated condition. 



In 1886 the same observer (XXXIX.) reported an experiment 

 in which pure-culture bottom yeasts behaved like top yeast, but 

 reverted to the original type after several re-inoculations. It was 

 also found that typical top yeasts can behave like bottom yeasts 

 for several generations, the whole being therefore merely transitory 

 variations. A few instances had also been previously reported 

 (in 1884) by Hansen and Kiihle, in which stormy fermentation, 

 with the characteristics of top fermentation, was produced at once 

 in wort by samples of Carlsberg bottom yeast No. 2, which had 

 been kept in the brewery for several weeks, partly in beer, partly 

 in wort, and partly as washed, pressed yeast in an ice-chest. Even 

 in this case, however, the vegetation quickly reverted to the 

 original state. Similar observations have also been recorded in 

 later years, HENNEBERG (IV.)? f r instance, having examined at 

 the Berlin Experimental station a typical Dortmund bottom yeast 

 which, after acting satisfactorily for some time in the brewery, at 

 length began to form a head and deposit similar to top-fermenta- 

 tion yeast. Nothing could be ascertained as to the cause of this 

 variation. According to LINDNER (XXVIII.), this yeast eventually 

 reverted to its normal state, and was successfully used in 

 practice. 



The percentage content of the enzymes present in the cells is 

 also subject to variation, the cause being largely attributable to the 

 method of nutrition. There is, however, no proof available that 

 yeast can lose its capacity of producing enzymes so completely as 

 not to regain it under favourable conditions of growth, nor is 

 there any known instance of a yeast producing, under special 

 treatment, a new enzyme that it previously lacked. The assump- 

 tion put forward by DUBOURG (I.) and other French workers, 

 that, under suitable treatment, a yeast could be induced to form 

 an enzyme that it had not previously produced, has been shown to 

 be totally inaccurate by the experiments of KLOCKER (IV.). This 

 statement does not imply the non-existence in nature of species 

 in a state of transition in this respect, but only that all the species 

 hitherto closely examined have proved constant in their behaviour 

 towards sugars (see chap. Ixv.). Recently it was found by 

 WAKSCIIAWSKY ( I .) that Sacch. cerevisice I. and Schizosacch. Pombe 

 produce zymase only when grown on a fermentable nutrient 

 medium, this enzyme not being formed when the medium is 

 unfermentable. He also found that even under the former 

 conditions, Schizosacch. Pombe does not produce zymase when the 

 nitrogen in the medium is in the form of ammonium phosphate. 

 On the other hand, zymase is again formed wlien favourable 

 conditions of cultivation are restored, so that the case is 

 merely one of temporary weakness. Moreover, no one has yet 

 succeeded in depriving an alcohol-forming yeast of that property, 



