21-6 



THE YEAST CELL 



period. This is quite different from the standard practice of dis- 

 continuing the experiment after 72 hours. When the period of ob- 

 servation was extended for a longer time, cultures of S. cerevisiae 

 (as well as C. carlsbergensis) were found to be able to grow on 

 Burkholder's medium in the absence of pantothenate. This was 

 equally true of all the progeny of the hybrid, many of which had pre- 

 viously been described as "nonsynthesizers. " In some cases there 

 was a delay of more than 600 hours before growth began. We grew 

 the cultures in 6 x 3/4 -inch Kimble tubes and determined the amount 

 of growth by measuring turbidity on the Klett Photoelectric Colori- 

 meter adapted to take the larger tubes. This made it possible to 

 make readings over the entire period without discarding the cultures. 



CULTURE NO. 7 



Micrograms of Pantothenate p«r Liter 



Fig. 21-2 llie Helationship between Increments ot Panto- 

 thenate in Burkholder's Medium and the Amount ot Growth of Cul- 

 ture No. 7 at different time intervals. In some phases of the 

 curves a straight-line relationship exists tietween the amount of 

 growth and the amount of added pantothenate but in all the cul- 

 ture tuhes the growth eventually rose to approximately tlie same 

 level . 



THE INADEQUACY OF THE CRITERION, EQUAL INCREMENTS OF 

 GROWTH FOR EQUAL INCREMENTS OF VITAMIN 

 Culture No. 7 was planted in a series of media containing dif- 

 ferent concentrations of pantothenate and the data produced a fami- 

 ly of parallel curves (fig. 21-2). If the experiment were terminated 

 at an arbitrary time (standard practice in nearly all assay experi- 

 ments), curves could be obtained in which equal increments of vita- 

 min appear to produce equal increments of growth. The curves in 



