GROWTH 251 



SUMMARY 



With increasing amounts of food, the crop, i.e., the 

 final number of cells in a culture, usually increases. 



With a liberal supply of building material, increase of 

 energy food will cause no acceleration in growth, but a 

 longer growth period, and, therefore, a larger crop. This 

 is limited by accumulation of fermentation products or 

 of inhibiting cell secretions. 



With all food components increasing, the crop will 

 still be limited by fermentation products or cell secre- 

 tions. The law of diminishing returns becomes quite 

 evident. 



Many species require a fairly well defined minimum 

 concentration of food for growth. 



(e) TEMPERATURE AND ENDPOINT OF GROWTH 



Our knowledge of the growth mechanism is too limited 

 to permit general predictions regarding the influence 

 of temperature changes upon the final crop. For tem- 

 peratures above the optimum, it is easily seen that the 

 crop must decrease with increasing temperature. This 

 is verified by the experiments of Ward (p. 216). 



In those cases where growth is limited by accumulation 

 of fermentation products, a prediction seems possible. 

 Since the fermentation products act as poisons, the same 

 concentration will act more strongly at higher tempera- 

 tures, or a smaller amount at high temperature will 

 have the same inhibiting effect as a larger amount at a 

 lower temperature. Consequently, a smaller crop should 

 be expected at higher temperatures. 



Where lack of food is the limiting factor, our basis 

 for prediction is much less sound. The preceding 



