CHAPTER V 

 THE RATE OF MULTIPLICATION 



The study of any living unicellular species first con- 

 cerns itself with reproduction. Multiplication of 

 individuals is, to all visible perception, the predominat- 

 ing business of micro-organisms. Multiplication rate 

 is commonly used as the criterion of suitability of a 

 culture medium and of a food material, and as the 

 criterion of whether a given environmental factor in- 

 fluences the race. Multiplication rate, rather than size 

 or any other character, came to be the criterion of life 

 in these organisms because of its tangibility and con- 

 venience. 



The rate of multiplication is the measure of how 

 much time the individuals are allotted in which to 

 grow. It is obviously a limiting factor in the acquire- 

 ment of body size. Its ease of measurement presents 

 the opportunity to investigate quantitatively how size 

 varies in correlation with the rate of multiplication. 



1. Environmental Factors 



Cultural conditions. For many species of unicellu- 

 lar organisms cultural conditions have been found un- 

 der which the rate of multiplication is fairly constant. 

 The rate has been universally studied by reckoning the 

 average number of multiplications from counts of the 

 number of individuals present upon successive days 

 or at other convenient intervals. This method gives 

 correctly the mean rate, but obscures most of the varia- 

 tions from the mean. Yet modifications of the mean 

 rate were nearly always found in successive days or 

 weeks or months. Were these changes inherent, or 



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