GROWTH 255 



kinds of food offered and upon the temperature used, but 

 eventually also upon other factors, e.g., upon chemical 

 substances which might increase or decrease the crop, or 

 upon physical factors such as high osmotic pressures in 

 concentrated salt or sugar solutions. 



The most common factor in chemical respects will be the acidity, 

 or more correctly for most cases, the hydrogen ion concentration. 

 Frequently, acid is a product of fermentation, and these cases fall 

 under the general heading of the influence of fermentation products 

 upon growth. The same holds true with alkali formers. 



Some organisms, like many yeasts and molds, have the abihty to 

 adjust the medium to their optimum acidity, i.e., they can increase 

 the acidity if sugar is present, or eventually decrease it by complete 

 oxidation of the acid. These are not included in the treatment of 

 this chapter. 



Growth comes to an earlier end if toxic substances are 

 present in the medium. This is readily understood. It 

 merely means the addition of one more unfavorable 

 factor. But with the same substances which decrease 

 the crop, an increase is often observed if the concentra- 

 tion of the toxic compound is chosen correctly. In 

 other words, the stimulation of the rate of growth by 

 small amounts of poison, as shown p. 227, may result 

 also in a greater crop. 



Of the older research on chemical stimulation, the 

 papers by Koch (1912) and by Fred (1911) are of 

 special bacteriological interest. The most comprehen- 

 sive modern paper on this topic is that of Hofmann (1922). 

 By dosing the toxic compound exactly right, Hofmann 

 obtained not only a larger number of colonies from the 

 same amount of culture, but the colonies were also larger. 

 Of the six experiments recording the size of the colonies, 

 only one shall be reproduced here (Table 72) . 



