Growth and Form 97 



survival. If now such cells are exposed to a fresh nutrient medium in which 

 growth is once again possible and survival no longer a problem, they must 

 reverse the course of events mentioned above. But this involves extensive 

 protoplasmic reorganization and a further synthesis of enzymes and other 

 macromolecules. All this takes time; hence the lag phase. 



THE SYNTHESIS OR ASSIMILATION OF RAW MATERIALS FOR GROWTH. 



Each original cell must stock up on the chemical subunits (amino acids, 

 sugars, fatty acids, vitamins, etc.) that are needed to construct enough 

 protoplasm to make two cells. If the nutrient supply were rich and these 

 compounds already present in the environment, the job would be simple 

 and the lag phase correspondingly short. If the nutrient supply were poor, 

 the missing subunits would have to be synthesized from simpler materials 

 in the environment. The job, then, would be more difficult and the lag 

 phase correspondingly longer. The lag phase can also depend on how 

 many cells are actually present, for each is a little factory geared to the 

 production of the subunits, and the more factories there are, the faster 

 they can be synthesized and the shorter will be the preparatory period. In 

 the jargon of the bacteriologist, the cells "condition the medium." More- 

 over, like a well-functioning assembly line, each original cell that has 

 stocked up on enough subunits to create two new cells also insures the 

 continuity of supply so that the two in turn will, without delay, be able to 

 collect or synthesize enough subunits to make four, and so on. Thus, no 

 further lag is apparent and growth can proceed apace. To summarize, 

 then, the lag phase is influenced by: 



1. The richness of the nutrient supply. 



2. The number of cells preparing for growth. 



3. The "physiological state" of the cells. 



THE EXPONENTIAL PHASE 



We now come to the stage of active growth and reproduction and 

 the peculiar S-shaped curve. To understand it we need merely consider 

 the properties of populations that are growing actively under optimal 

 conditions: 



1. The more organisms there are, the more there can be. In other 

 words, offspring must have parents and the more parents, the more off- 

 spring. 



2. Most of the organisms in such a population reproduce during 

 their life cycles. Very few are barren. 



3. The generation time is constant. In other words, the time for 

 each organism to emerge, mature, and reproduce is approximately the 

 same as long as the population continues to grow. 



