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GROWTH 545 



may be divided into the following phases: (1) a stationary period, (2) / 

 a lag period of increasing rate of growth, (3) a logarithmic period of 

 constant relative rate of growth, (4) a period of declining rate of growth, 

 (5) a period of equilibrium of numbers, and finally (6) a period of de- 

 clining numbers. The duration of these phases, and even the presence or 

 absence of some of them, depends on the age of the inoculation and the 

 nature of the environment. The stationary and the lag phases may be 

 eliminated when the inoculation has been taken from a culture during 

 the logarithmic period. Very large inoculations may exhaust the food 

 supply or make the environment toxic from the excretion products, 

 before any appreciable growth can take place. 



The understanding of population-growth studies may be clarified by 

 the aid of a hypothetical example, Figure 133. If ten organisms were 

 seeded into a limited amount of a suitable culture medium from a 

 population in the logarithmic phase of population growth, they would 

 grow at a constant rate, doubling their number at the end of each 

 generation time (curve A). If the conditions of growth were identical, 

 the rate of growth of the inoculum would be the same as it was in the 

 parent population. After a time the environment will no longer be 

 effectively constant, and the rate of growth will decrease. This may come 

 about by the lapse of more time between generations (curve B), or by 

 only a part of the animals being capable of reproducing (curve C). It 

 is apparent that if there had been different periods of increasing genera- 

 tion time, or if different numbers had been permitted to reproduce, it 

 would have been possible to make curves B and C coincide at all points. 

 Therefore, it is not possible to decide from the shape of the growth 

 curve alone, the cause of the slowing of the growth rate. A third possi- 

 bility, which would give a curve of the same general shape, would be 

 a selective encystment or the death of some of the animals, which would 

 reduce the number of individuals capable of reproduction. Such con- 

 siderations emphasize again the necessity of information from the use of 

 more than one criterion for the analysis of growth. 



The stationary or equilibrium period, when the population does not 

 change in numbers, is indicated by the curve D and the period of de- 

 clining population by curve E. The stationary period is usually a dynamic 

 equilibrium wherein the birth and death rates balance each other, but it 

 could be static if all of the cells encysted or became otherwise inactive. 



