KINETICS OF GROWTH 93 



this phase the slope changes back to zero, evidence that when this point 

 is attained [phase 3] either no further divisions take place, or that the 

 increase in cell numbers is exactly balanced by the disappearance or 

 death of individuals. The transition period, often brief and rather sharp, 

 would imply that : 



(a) either fewer and fewer cells are still in a position to divide, or 



(b) that the division rate of the individuals becomes progressively 

 less. 



During the final phase [4] the death of the cells is predominant ; from 

 the point of view of studies on growth this phase is of no interest. 



A considerable amount of time has been devoted to the first and 

 originally quite enigmatic phase, referred to as the latency and lag 

 period. As a result of various investigations, mostly before 1930, the 

 attitude towards the problems it presents can be summed up as follows. 



In many cases the cause for the existence of a latency and lag period 

 resides in the cells used as inoculum. It can be completely eliminated 

 by inoculating from a culture which itself is in the exponential phase. 

 This has been demonstrated by Chesney (i) for bacterial, and by 

 Phelps (2) for protozoan cultures. The careful observations of Henrici 

 (3) have shown that the morphology of a bacterial species dififers 

 markedly according to the phase of the culture. It is therefore clear that 

 the initial lag must be ascribed to a transformation of aged cells into 

 "embryonic forms" (3, p. 140). Depending upon the age of the culture 

 from which the inoculum is prepared, this change may require anywhere 

 from minutes to many hours. 



Similarly, a period of preliminary adjustment generally follows upon 

 a change of environmental conditions, such as composition of the cul- 

 ture medium, change in temperature of incubation, etc. In that event a 

 lag phase can be observed even if the new cultures are inoculated from 

 a culture in the exponential phase, and the length of the lag depends upon 

 the extent and nature of the change in conditions. 



Growth curves of microorganisms are as a rule accurately reproducible. 

 Chiefly responsible for this feature are ( i ) the rigorous control which 

 the experimenter can exercise over the environmental conditions, and 

 (2) the small size of the organisms. The small size eliminates many 

 complications connected with the accessibility of foodstuffs. In higher 

 plants and animals transport of nutrients, often over considerable dis- 

 tances, may be a controlling factor for growth ; in microorganisms this 

 is not the case. Furthermore, the small size insures that large numbers 

 of individuals are always involved. Hence a statistical treatment of 



