1 82 GROWTH 



lag phase. If the seeding be excessive, the retarding factors 

 may prevent any measurable growth.* Priestley and Pearsall f 

 look upon the logarithmic phase as the natural rate of increase 

 where increase in mass is an exponential function of time ; 

 the retardation phase, in which the growth rate is directly 

 proportional to time, is due to adverse operatnig factors, 

 such as the accumulation of end products, which are the 

 outcome of the earlier unrestricted growth. In addition to 

 the obvious products, carbon dioxide and alcohol, of the 

 activity of yeast, oxygen and sugar may also influence its 

 growth. 



The influence of oxygen is important, and this notwith- 

 standing the fact that yeast is capable of action in anaerobic 

 as well as in aerobic conditions. The growth exhibited in 

 these different circumstances is not the same : when grown 

 anaerobically, yeast cells quickly acquire a static condition of 

 equilibrium with regard to the medium in which they are 

 suspended ; J the lack of oxygen, especially prior to the be- 

 ginning of gemmation, arresting the reproduction function. 

 It is the small amount of oxygen initially present in the wort 

 which is considered to explain the fact that in fixed condi- 

 tions the maximum cell increase is independent of the num- 

 ber of cells of seed yeast per unit volume of yeast. Thus 

 Horace Brown § found that up to 65 or 70 per cent, of complete 

 aeration, the cell increase is a linear function of the available 

 free oxygen at the beginning of growth. In other words, 

 during the period of active reproduction in a suitable medium 

 in which access to oxygen is limited to that initially present 

 in solution in the liquid and in conditions of culture which 

 eliminate the competition factor, the number of yeast cells 

 present at any moment is directly proportional to the time. 

 Reproduction can, however, take place to a small extent 

 in anaerobic conditions : Horace Brown found the limit to 

 be 6-5 cells for each initial cell of seed yeast. This is ex- 



*Slator: "Biochem. Journ.," 1918, 12, 248. 

 f Priestley and Pearsall : " Ann. Bot.," 1922, 36, 239. 

 J Adrian Brown : " Journ. Chem. Soc," 1905, 87, 1395. 

 § Horace Brown : " Ann. Bot.," 1914, 28, 197. 



