Growth and Form 99 



000,000,000,000,000 cells. A mass of bacterial cells this size would weigh 

 8 million pounds. We would quite literally be up to our ears in bacteria.^ 



Time for Organisms To Double Their Mass 



Escherichia coli 20 min 



Fly larva 13 hr 



Silk worm 68 hr 

 Rabbit at birth 6 days 



Pig at birth 6-7 days 

 Sheep at birth 10 days 



Guinea pig at birth 18 days 



Horse at birth 60 days 



Man at birth 180 days 



The same arguments hold for higher animals. As anyone knows who 

 has had experience with rabbits, they may differ from bacteria to the 

 extent that you need at least two ( of the appropriate sexes ) to get more, 

 but they are the same as bacteria in that the more rabbits you have the 

 more rabbits you will surely get. Therefore, rabbit colonies or any other 

 animal population, when well fed and given enough space, display 

 S-shaped curves of growth. It should be noted that two rabbits do not 

 necessarily, and in fact hardly ever do, produce only four rabbits and 

 four only eight. The fact that with each generation the number of organ- 

 isms more than doubles would not change the shape of the curve but 

 would merely make it steeper. 



THE STATIONARY PHASE 



As already mentioned, toward the end of the exponential curve, 

 more and more cells begin to die and those that survive take longer and 

 longer to reproduce. Ultimately the rate of cell death increases so much 

 and the rate of cell growth decreases so much that cells die as fast as new 

 ones appear and the total number of living cells remains constant. This is 

 the stationary phase. 



Why does the population stop growing? What limits growth? There 

 are at least two general causes: 



1. The supply of an essential nutrient becomes exhausted. If an or- 

 ganism requires a particular nutrient for growth and the supply gives out, 

 the organism will cease growing no matter how much there remains of 

 any other nutrient. Whether the exhausted nutrient happens to be a 

 vitamin or an amino acid or a source of energy, or a metal ion or oxygen, 

 the result will be the same. The cessation of growth may be subtle, as in 



1 See the section at the end of the chapter for a detailed discussion of growth 

 kinetics. 



