GROWTH 245 



give a small crop, but a poor soil is either a fair one deficient in plant 

 food, or one which contains compounds injurious to growth, such as 

 acid, or alkali. Such poor soil is not comparable with our case, 

 which allows liberal amounts of a poor food. 



With animals, a large supply of poor food will bring about under- 

 sized growth. But the reason is different. The capacity of the 

 stomach and intestine is limited, and if they are filled to capacity 

 with a food low in nutriment, the cells of the growing animal do not 

 obtain all the nourishment they could utilize. The body does not 

 develop to its maximum size, because the rate of growth is abnormally 

 slow, and sexual maturity develops before the normal size is reached : 

 After this stage, there is hardly any more growth, even with better 

 food. 



This analogy applies to our problem only if we assume that there 

 is such a thing as an adult stage in bacterial cultures. The compari- 

 son with animals can be carried quite a long way. In this case, 

 we would have to assume that each cell, regardless of its rate of 

 growth, secretes into the medium some substance which, in sufficient 

 concentration, will prevent further growth. In a well nourished 

 culture, this limiting concentration is reached only after perhaps 

 one billion cells per c.c. are formed. Poorly nourished cells secrete 

 this harmful compound at the same rate, and the Hmiting concen- 

 tration is reached long before the more slowly growing bacteria 

 have had the chance to reach the billion mark. 



This explanation would be satisfactory if the existence of this, 

 peculiar secretion could be proved. This seems possible; a full grown 

 culture fed on ammonia as the only source of nitrogen should then 

 show no further growth if asparagin or peptone were added. No 

 such experiment is known to the author. 



If, however, there is new growth, then the cause of cessation of 

 growth cannot have been a secretion; it must be in the cell mechan- 

 ism. Then, the cell can use only a Umited amount of poor food, 

 though there is plenty of it available, but will continue to grow with 

 a better food, i.e., if the cell works under higher pressure, or with 

 higher intensity. Only an experiment can decide between these 

 two possibilities. 



(d) INFLUENCE OF THE CONCENTRATION OF FOOD 



We must again distinguish between energy-producing 

 food and building material. To the energy-producing 



