The Rate of Multiplication 73 



due to conditions, or were some of the changes due to 

 each? 



It is recognized by most of those who have cultivated 

 unicellular species that the more constant the rate of 

 multiplication is, the more constant have been the con- 

 ditions of cultivation as judged simultaneously by 

 other criteria. For each race of organisms, therefore, 

 it is probably useful to study only the modifications ob- 

 served when the rate is most constant in control cul- 

 tures. 



In the past it has often been assumed that similarity 

 of multiplication rate existed throughout a species. 

 This assumption is known to be untrue in some cases; 

 differences of race must, therefore, be recognized in 

 order that reproducible results may be compared and 

 not confused. 



Constant conditions. Techniques have now been 

 devised whereby a great many species of unicellular 

 organisms may be cultivated so that uniform rates of 

 multiplication result. 1 Uniformity of multiplication 

 means, of course, that the number of organisms pres- 

 ent at each equal increment of time shall be a uniform 

 multiple of the number at the previous period. Such 

 a relationship is a geometric progression; when the 

 logarithm of the number present is plotted against the 

 time, a linear relationship results. Certain limitations 

 must be understood; for it would be possible to have 

 reproduction occurring in rhythms, as so often occurs 

 in green organisms, when the numbers may not in- 

 crease for some hours or days which are occupied in 

 growth of individuals. Roach ('26) studied such an 



'In bacteria: Barber ('08), M'Kendrick and Pai ('11), Penfold ('14), 

 Chesney ('16). In yeasts: Slator ('13, '21), Clark ('22), Peskett ('24, '25, 

 '27), Balls and Brown ('25), Richards ('28a). In ciliates: Cutler and 

 Crump ('23, '24), Greenleaf ('26), Myers ('27), Beers ('28b). In green 

 organisms: Hartmann ('21), Roach ('26), Jahn ('29). In sporozoa: 

 Taliaferro ('25), Hartman ('27). 



