432 Physiology 



direct analysis. In a medium reduced to bare essentials, it might be pos- 

 sible to recognize species capable of synthesizing required materials at a 

 rate so slow that growth could never reach the maximum attainable in a 

 rich medium. In the second procedure, with maximal growth as the goal, 

 slow synthesis of a particular factor might conceivably be overlooked. In 

 practice, however, the first method has certain limitations. Validity of the 

 results obviously depends upon purity of the reagents and cleanliness of 

 the culture vessels. In addition, contamination of the medium with dust 

 or with volatile materials from the atmosphere of a laboratory could be a 

 possibly serious source of error. Even minute contaminations might turn 







Fig. 8. 1. Hypothetical growth responses of a test organism to essential 

 and stimulatory growth-factors. 



the balance in favor of slight growth, with resulting faulty interpreta- 

 tions of experimental data. Hence, it is essential, in following the first 

 procedure, to take all possible precautions. In the use of media which 

 support maximal growth, the influence of minute contaminations would 

 be less likely to account for positive instead of negative results. Further- 

 more, the response to graded increments of a given growth-factor can be 

 traced over a wide range of growth. An approximately linear growth- 

 response to a vitamin or a mineral in concentrations ranging from zero 

 to an optimum (Fig. 8. 1) would indicate that the factor is essential. 

 Omission of a stimulatory substance, on the other hand, would decrease 

 growth but not prevent it completely. 



