198 PHYSIOLOGY OF BACTERIA 



shown especially by Henrici (1928). K. A. Jensen found 

 (1928) that young cells of Bad. coli may be so sensitive 

 that they die if transferred to a new medium. These 

 hypersensitive cells are almost invisible, transparent 

 ^'shadow forms" appearing about three hours after 

 transfer, and changing back to normal after about six 

 hours. 



The ready growth of young cells transplanted to a new 

 medium from a rapidly growing culture, and the slow 

 growth of cells from an old culture simplify the problem 

 of the growth curve greatly because the reason for the 

 initial lag phase appears to be the same as the reason for 

 the decreasing rate of growth. Lag is not typical for 

 new transfers, but for old cells. Thus, the lag phase need 

 not be discussed here as a problem in itself; it will be 

 studied in the next chapter with the decreasing growth 

 rate. 



Many efforts have been made to express the entire growth period 

 of bacteria and similar organisms in a limited volume by one mathe- 

 matical formula. Several formulas have been developed empirically 

 which fit the facts fairly well. Some formulas agree better with 

 certain sets of data while others fit better for observations with other 

 organisms. 



It will be shown in the following pages that there is more than 

 one cause for the cessation of growth in bacterial cultures. In fairly 

 early stages, the exhaustion of oxygen in the lower strata of the 

 medium must necessarily influence greatly the growth rate of most 

 microorganisms. In the last stages of slow growth, the accumulation 

 of fermentation products is hkely to be another cause of growth 

 retardation. It will be seen, from the following discussions, that the 

 main cause of growth retardation is still entirely unknown. It seems 

 impossible that these different causes of retardation of growth which 

 become effective at different stages of development can be incor- 

 porated into one mathematical equation. It also seems that the 

 empirical equations which fit the facts approximately will be of little 

 help to find the still unknown causes of retardation of growth. 



