42 MORPHOLOGIC VARIATION 



if it be agreed that different factors may limit growth in different 

 cases, i.e., that every culture is a special case like the pneumococci, 

 and that there is no generally applicable law. This is probably 

 true. Thus Brown found that in ordinary flask cultures of yeast, 

 oxygen was definitely the limiting factor; if the cultures were well 

 aerated, food seemed to be the limiting factor, the yield being greater 

 in wort of higher specific gravity. Clark believed that in aerated 

 yeast cultures the accumulation of alcohol was the limiting factor, 

 but the work of Balls and Brown would indicate that under such 

 circumstances the concentration of dextrose determines the rate of 

 growth. Rogers and Whittier have found that in cultures of Strepto- 

 coccus lactis the accumulation of acid is normally the limiting factor; 

 if this be removed by keeping the hydrogen ion concentration con- 

 stant, then growth proceeds to a higher level which is determined 

 by the oxygen concentration, a still higher level being reached if 

 the culture is aerated. Probably similar different limiting factors 

 will be discovered in each case, and if they are removed the final 

 factor will be found to be the concentration of foodstuff. At least 

 there is no sound basis for believing in the production of specific 

 autotoxines. 



Many theories have been proposed to explain the period of 

 latency at the beginning of the growth curve. Chesney has well 

 divided these into two groups, those which seek the origin of lag in 

 the medium, and those which find the cause in the inoculated cells. 



Rahn found that cultures seeded into a medium in which the 

 same organisms had previously grown and had been killed by heat, 

 had little or no lag, although there was a definite latent period in 

 control cultures on new medium. There was a definite lag, however, 

 when organisms were recultivated in a medium from which the 

 preceding crop had been removed by filtration. He concluded that 

 the maximum growth rate could only be attained at a definite con- 

 centration of some heat stable, non-filterable substance formed by 

 the bacteria themselves. 



This idea that the lag is due to the lack of some substance 

 secreted by the bacteria has also been held by various other authors, 

 but more particularly by Robertson, who has referred especially to 

 a similar phenomenon in the growth of protozoa. He lays especial 



