17-2 



THE YEAST CELL 



D family (fig. 14-2), most of whose members ferment galactose, 

 was a nonJfermenter of galactose; when cells of this culture (Db23B, 

 Chap. 14) grown on glucose were suspended in 4 per cent galactose 

 in a Warburg respirometer under anaerobic conditions no carbon 

 dioxide was produced. However, when it was planted in galactose 

 broth and allowed to stand for several weeks fermentation nearly 

 always occurred; it is a-^slow" fermenter of galactose. This was 

 due to the fact that a small fraction of its vegetative cells became 

 capable of fermenting galactose and in the presence of galactose 

 they have a selective advantage. This was shown by the following 

 experiments. 



280 



240 



200 



100 



120- 



80- 



40- 





S 



tiJ 



30 



60 



90 120 



150 



leo 210 



240 



270 



Minutes 



Fig. 17-1 Three-Hour Adaptation Period of Culture Lk when 

 Stationary Population is Exposed to Galactose. (£) — Enzyme 

 activity, (J = Log of number of cells per cubic centimeter. 



Cells from a colony growing on an ordinary glucose plate were 

 suspended in chilled galactose broth and diluted to contain about 

 5000 cells per cc. as determined by direct count with a haemocyto- 

 meter. Of this suspension, 0.2 cc. was placed on the surface of an 

 agar plate from which all excess fluid had been allowed to drain. 

 The solid medium contained 4 per cent agar and 4 per cent galactose. 

 A sterile bent rod was then rotated over the surface to produce an 



