GROWTH 203 



would not grow aerobically in broth when the inoculum 

 was small; when the oxygen tension was lowered, it 

 grew well, and without lag. 



Lindner (1919) has already claimed that the bios problem was 

 nothing but an oxygen problem. Yeasts were found to keep healthy 

 and ready to multiply if oxygen was excluded. This is strengthened 

 by the observation of Miss Copping (1929) that the oxidizing yeasts 

 {Mycoderma, Willia, Torula) are less in need of ''bios" than the 

 fermenting yeasts {Saccharomyces cerevisiae, apiculatus). Impor- 

 tant is her conclusion: 



" The presence of bios . . . accelerates the growth, . . . but 

 does not increase the final number of the cells as this is very probably 

 limited by the supply of nutriment in the medium." 



Bios, then, is not a food. It creates a condition favorable for 

 growth, it influences the environment, but it is not building material 

 or source of energy. 



Old cells do not maintain the reduction potential. Litmus milk 

 cultures of streptococci are decolorized, but gradually, the color comes 

 back. There must be a relation between old age and loss of reduction 

 potential. 



A single cell, old and with a low power of reduction, may not be 

 able to fight off the invading oxygen of the medium; it may be 

 killed by oxidation. A large number of the same cells will have the 

 oxygen distributed among them. There is less oxygen per cell. 

 The chances for estabhshing a reduction potential sufficient for 

 growth is more probable when the inoculum is large. As soon as a 

 few cells are working efficiently, the oxygen tension in the medium 

 will be lowered and the weaker cells, unable to fight off the oxygen 

 by their own mechanism, will then be able to repair their cell mech- 

 anism and grow normally after a while. 



The growth of large inocula of anaerobes in culture tubes under 

 aerobic conditions, the great help of cysteine in such cultures, and 

 the beneficial influence of CO 2, observed by Rettger and associates, 

 all point in the same direction. The author's experiments are not 

 sufficiently uniform as yet to consider this explanation proved. 



(d) CONCENTRATION OF FOOD AND GROWTH RATE 



The growth rate depends upon the amount of available 

 energy, as well as upon the kind and quantity of the 



