22-11 



THE YEAST CELL 



tained on inoculating a small number of cells into a new tube of me- 

 dium without added pantothenate. Serial transfer on minus pan- 

 tothenate medium Increases the rate of growth even further. 

 The increase in adapted rate of growth is not lost after transfer 

 from medium containing pantothenate. 



Fig. 22-4 Growtli of pant othena te-dependeii t culture 21S4 in the 

 presence of various concentrations of pantothenate. 



The zero concentration curves are incomplete in the time cov- 

 ered by the graph. They are shown in fig. 22-4A and are discussed 

 in the section entitled, "Growth of the Recessive Culture in the Ab- 

 sence of Pantothenate". 



A second important aspect of the adaptation curves (fig. 22-4) is 

 that below the critical concentration of pantothenate, although there 

 is considerable irregularity, adaptation takes place sooner in the 

 higher concentration (2 > and 1> per liter) than in the 0> concen- 

 tration. 



These points bring up the following questions: 



1. Is the heritable change a gene mutation? 



2. Why does adaptation take place at 2> per liter or below, but 

 not above this point? 



3. Why is adaptation speeded up by increases of pantothenate 

 at concentrations below the critical level? 



4. If this proves to be a gene mutation, is it specifically induced 

 by the minute amount of pantothenate present or does it occur spon- 

 taneously and at random? 



