GROWTH OF THE YEAST PLANT 117 



seeding be heavy, but not excessive, the logarithmic phase may 

 disappear and retardation set in immediately after the lag 

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

 prevent any measurable growth.* Priestley and Pearsallf 

 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 pro- 

 portional to time, is due to adverse operating 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 under anaerobic 

 as well as aerobic conditions. The growth exhibited under 

 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 sus- 

 pended ; J the lack of oxygen, especially prior to the begin- 

 ning 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 under fixed con- 

 ditions the maximum cell increase is independent of the 

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

 Horace Brown found that up to 65 or 70 per cent of com- 

 plete aeration, the cell increase is a linear function of the avail- 

 able free oxygen at the commencement 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 under 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 under anaerobic con- 

 ditions to a small extent : Horace Brown finds the limit to be 

 6*5 cells for each initial cell of seed yeast. This is explained 

 by the fact that before reproduction takes place, the cells of 

 the seed yeast absorb and fix oxygen which renders possible 



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



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



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



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



