GENERAL FOOD RELATIONSHIPS— METABOLISM 77 



Growth Accessory Substances.— Many bacteria are very 

 sensitive to slight environmental changes in reaction, 

 moisture, O tension, temperature; also to the addition or 

 withdrawal of small amounts of chemical substances. Such 

 chemical substances as favor growth are strictly speaking, 

 "growth accessory substances"— hemoglobin for bacteria of 

 the influenza group is a good illustration. Growth accessory 

 substances in the commonly accepted sense of minute quan- 

 tities of unknown chemical substances which markedly affect 

 growth have not, in the author's opinion, been demonstrated 

 with certainty. Extremely careful work to exclude all known 

 substances and conditions which might affect growth must 

 be carried out before one concludes that a growth accessory 

 substance is responsible. 



