HANSEN'S EXPERIMENTS WITH YEAST. 265 



employing a deep layer of wort in making comparative observa- 

 tions on top- and bottom-fermentation phenomena. One test per- 

 formed with 150 cells showed that not a single bottom-yeast cell 

 was present. In order to solve the problem whether the low 

 temperature had produced modification, the vegetation used for 

 the culture at 0.5 C. was subjected to analysis, the result being 

 that, of 100 cells, one-half gave top fermentation, the other 

 bottom fermentation. On a series of flasks, charged with a thin 

 stratum of wort, being inoculated with cells from each category 

 and kept for 34 months at 0.5 0., it was found that no 

 reproduction occurred in the flasks containing the bottom- 

 fermentation cells, whereas, on the contrary, the top-fermenta- 

 tion cells exhibited decided reproduction. The cultures treated 

 in this way were next grown in test-tubes, with the result that 

 the bottom cells again gave rise to bottom fermentation, and the 

 top cells to top fermentation, thus demonstrating that no modifi- 

 cation, but only selection, had been effected by the experiment. 

 When Hansen described Sacch. turbidans in 1883, it was a 

 bottom yeast; and the formation of top cells the cause of 

 which is unknown occurred spontaneously during the period of 

 storage in the laboratory. In the course of a year the bottom 

 cells and top cells continued to behave as such respectively 

 through a long series of cultures ; and 1000 cells isolated from 

 each class all produced the same type of fermentation as that of 

 the class from which they originated. 



Experiments with the typical bottom yeast, Johannisberg 2, 

 showed that the cultures not infrequently contain 70 per cent, of 

 top-yeast cells, the isolated cells in this case also retaining their 

 characteristic fermentative habit through a long series of cultures. 



Whilst in the cases cited above a transition occurred from 

 bottom fermentation to top fermentation, the converse change 

 seems more difficult to bring about. In this connection Hansen 

 carried on several experiments with Sacch. validus ( = >S'. Past. ///.), 

 which is certainly a typical top yeast, but he only succeeded in 

 obtaining a few bottom cells not exceeding 3 per cent. in one 

 of the cultures. The vegetations from these cells retained their 

 character as bottom yeast through a series of generations, ex- 

 tending over two years, and under conditions favourable to the 

 production of top fermentation phenomena. 



In this manner the old question whether the top- and bottom- 

 fermentation yeasts are independent forms or not has been settled 

 by the demonstration that bottom cells can be developed from top 

 cells, and vice versd. This essentially modifies our previous con- 

 ceptions (see p. 124, vol. ii.). The two forms into which the 

 species is subdivided may exist for a long time, side by side in the 

 same nutrient medium, until the growth of one of them is favoured 

 by the environment, as was the case in the experiments with 

 Sacch. turbidans at 0*5 C., where the top-fermentation form 



