GROWTH 205 



only the concentration and not the kind of food is 

 changed, the growth rate is not greatly altered as the 

 following facts will bear out. 



The data of Penfold and Norris (1912) show that the 

 generation time of Bad, typhosum in peptone water 

 remains practically constant if the peptone concentration 

 is above 0.4% (Table 50). Let us try here to dis- 

 tinguish again between the energy formation and the 

 building material. Above a certain concentration of 

 fermentable material, the fermentation, i.e., formation of 

 available energy, remains constant. In this stage, the 

 growth process can be accelerated only by a better supply 

 of building material, and even there, it must by necessity 

 soon reach the maximum. There must be a concentra- 

 tion where neither the increase of fermentable material 

 nor the supply of building material causes a more rapid 

 growth. This condition is apparently reached in the 

 above case at 0.4% peptone. 



If, in this example, the nature of the fermentable 

 material, or of the building material is changed, we have 

 the possibility of an increase or decrease in the rate of. 

 growth. With peptone alone, the cells cannot exceed a 

 definite number of calories per unit time, owing to the 

 limited quantity of oxidase or endoprotease in each cell. 

 However, if sugar is added to the peptone medium, a 

 different enzyme begins to react, in addition to the 

 previous ones, and this may yield a larger number of 

 calories per cell in unit time, and therefore may enable 

 the cell to grow more rapidly. This is proved by 

 experiments of Penfold and Norris. The addition of 

 0.175% glucose to a medium containing only 0.1% 

 peptone lowers the generation time about 50%, i.e., it 

 doubles the rate of growth; with 1 % peptone this effect is 

 less marked. 



