GROWTH 211 



Brown (1925). The logarithms of the yeast crop plotted 

 against time should form a straight line if the growth rate 

 is constant (p. 192). Fig. 20 shows a constant growth 

 rate, after one hour's lag, to the seventh hour, and then 

 changes abruptly to another constant growth rate which 

 is only about one-tenth of the previous. We notice 



IS 2.0 Hours 



Fig. 20. — Logarithmic growth curve of a yeast culture, showing an abrupt 

 change of the growth rate after exhaustion of the sugar supply. 



that this change occurs when the sugar concentration 

 falls below 0.09%. The yeast then uses alcohol as the 

 only source of energy (see p. 176), and this source is not 

 nearly so good. The 0.5 gm. sugar per liter which remain 

 is an unassimilable reducing compound (see p. 175). 

 Other experiments of Balls and Brown show exactly 

 the same sudden change of growth rate at the point 

 when the sugar is exhausted.. 



SUMMARY OF FACTS 



The growth rate is not very greatly influenced by the 

 concentration of food. In a peptone solution without 



