11-13 



THE YEAST CELL 



and galactose is "slow" not because each cell slowly achieves 

 fermentative ability in the presence of galactose but that mutation 

 (at the rate of about one per million cells) produces a small popu- 

 lation which is selected and finally becomes large enough to pro- 

 duce gas in the tube. We may assume that the population from 

 which these mutants are produced is not incapable of fermentation, 

 but is not capable of producing sufficient gas to register a positive 

 test in the Durham tube (fig. 11-2). We can say this because a large 



Fig. 11-6 Magnified area about one ot the lar^e colonies shown 

 in Fig. 11-5 revealing the background ol tiny colonies surrounding 

 the large colony. 



number of cells is capable of producing gas from galactose v/hen 

 allowed to stand in a Smith tube above mercury (fig. 11-4). in the 

 absence of growth. This statement does not imply that a small 

 amount of enzyme is always present for on this point v/e do not have 

 conclusive evidence. The ambiguity here lies in the fact that when 

 the so-called "nonfermenter" cells lie in galactose and buffer solu- 

 tion above the mercury in a Smith tube, they produce gas slowly 

 with a long lag, i.e. they adapt to the presence of the sugars. V/hether 

 this means that they slowly produce the enzyme or it takes some time 

 for a small amount of enzyme to take action, we do not know. This 

 is a very critical question on which we do not as yet have a definite 

 answer. 



