FOOD REQUIREMENTS 505 



indication of the amount of acid or alkali necessary to change the re- 

 action by a given amount. In addition, the redox potential varies with 

 the pH of the medium. A number of investigators have correlated the 

 redox potential of culture media with the growth and metabolism of 

 bacteria, but very little work along this line has yet been done in proto- 

 zoology. 



So far, the only detailed investigations are those of Jahn (1933b, 

 1935a), who has studied growth of Chilomonas pavameciu7n in relation 

 to the redox potential of the medium. In his first publication, Jahn 

 found that growth is accelerated by NaSH, while the addition of H0O2 

 to a peptone and acetate medium inhibited growth. On the other hand, 

 relatively rapid growth occurred when both peroxide and a high con- 

 centration of -SH were added to the medium. These results were ex- 

 plained on the basis of the redox potential. In his second article, Jahn 

 traced the continuous changes in the pH and Eh in cultures of C. para- 

 fnecium. The Eh of different media was found to drop as much as 

 300-460 mv. during the first few days of incubation, and Jahn suggested 

 that such changes may involve not only a lowering of the oxygen ten- 

 sion but also the accumulation of reducing substances in the medium. 

 After three to five days, depending upon the type of medium, the Eh 

 began to rise; this change was attributed to a sharp decrease in the 

 growth rate of the flagellates, with a corresponding decrease in oxygen 

 consumption. 



So far as the Protozoa are concerned, Jahn's results indicate that 

 there is much to be learned concerning detailed relationships between 

 growth and the redox potential of culture media. The exact effects of 

 changes in the redox potential are still unknown, and the relative im- 

 portance of the redox potential and the oxygen tension in different cases 

 is yet to be determined. Possible relationships to growth have been dis- 

 cussed by Jahn (1934). 



Growth in Relation to Temperature 



The importance of temperature relationships is obvious, and rigid 

 control of temperature is essential in many types of experimental in- 

 vestigations. The actual relationships between growth and temperature 

 are undoubtedly complex, since changes in temperature may not only 

 affect metabolic activities of the organism directly, but may also modify 



