THE LAO PHASE 



83 



stationary phase, c to d, during which the organisms cease to multiply at maximal 

 rate, so that their increase in number becomes less and less, till ultimately it 

 ceases ; the number present in a unit volume remains approximately constant for 

 an appreciable length of time. During this phase the number of freshly formed 

 bacteria roughly counterbalances the number of those that are dying. (4) The 

 phase of decline, d to e, during which the organisms gradually diminish in number, 

 till finally the culture becomes sterile. 



So far we have been considering the viable bacteria only. If, however, a count 

 is performed of the total number of organisms alive and dead in the culture, a 

 different curve is obtained. It will be seen that this curve runs more or less 

 parallel to the curve of the viable bacilli till the period of decline sets in, after 

 which the two diverge, the total curve remaining practically stationary, or rising 

 very slightly. It will be noticed, moreover, that the total curve is throughout 



yo 



6-5 



80 



5 



^7-5 



70- 



6 5 - 



60 



5-5 



Time, in Hours 



Fig. 15. 

 Continuous line = Total number of bacteria alive or dead. 

 Interrupted line = Number of living or viable bacteria. 



Bomewhat higher than the viable curve. The probable explanation of this will 

 be given later. 



We must now consider each of the phases in detail. 



The Lag Phase 



It is important at the outset of any discussion on growth to define our terms. 

 Failure to do this, has been responsible for much confusion in the past. Growth 

 may occur either in size or in numbers. The individual cell may get progressively 

 larger, or it may divide and give rise to two daughter cells. Growth in size may be 

 referred to as cell enlargement, growth in numbers as cell multiplication. 



The observations of early workers in this field (Miiller 1895, Eahn 1906, Lane- 

 Claypon 1909, Coplans 1910, Penfold 1914, Ledingham and Penfold 1914, C!hesney 

 1916, Slator 1917, Buchanan 1918) were made almost exclusively on cell multiplica- 

 tion. They found that the lag phase, during which Uttle or no multiplication occurs, 

 was affected by a number of factors, such as the size and age of the inoculum, the 

 frequency of transplantation of the parent culture, the nature of the organism, 

 the composition of the medium, and the temperature at which the culture is 

 incubated. Generally speaking, the lag phase tends (a) to be long when a small 



