MECHANISM OF COMPETITION IN YEAST CELLS 71 



tion has led us to the very same assumptions that are at the bottom 

 of Pearl's logistic equation of growth (see Chapter III, equations (8) 

 and (9)) . This equation is one that gives us the S-shaped curve start- 

 ing from the point that growth depends on a certain potential geo- 

 metric increase which at every moment of time is realized only in a 

 certain degree depending on the unutilized opportunity for growth 

 at that moment. 



In the equation of Pearl the unutilized opportunity for growth is 

 expressed in terms of the population itself, i.e., as the relative num- 

 ber of the still vacant places. This presents a great advantage as we 

 shall see later on. The unutilized opportunity of growth often de- 

 pends on various factors, and to translate the number of "still vacant 

 places" into the language of these factors may become a very difficult 

 task. 



(3) Let us now analyze this problem. What is the nature of those 

 factors of the environment which depress the growth of the yeast 

 population and finally stop it? Of course they may be different in 

 various cases, and we have in view only our conditions of cultivation. 

 The nature of the factors limiting growth in such an environment has 

 been explained mainly by the investigations of Richards. When the 

 growth of yeast ceases in a test tube under almost anaerobic condi- 

 tions, there still exists in the nutritive medium a considerable amount 

 of sugar and other substances necessary for growth. A simple ex- 

 periment made by Richards ('28a) is convincing: if at the moment 

 when the growth ceases in the microcosm yeast cells from young 

 cultures are introduced, they will give a certain increment and the 

 population will somewhat increase. Consequently, there is no lack 

 of substances required for growth. The presence of a considerable 

 quantity of sugar at the moment when the growth ceases has been 

 chemically established, and in our experiments this is even more 

 apparent than in those of Richards, as our initial concentration of 

 sugar was 5 per cent and his only 2 per cent. 



If the growth ceases before the reserves of food and energy have 

 been exhausted we must eyidently seek an explanation in some kind 

 of changes in the environment. This question has been studied by 

 Richards and led him to conclude that the decisive influence here is 

 the accumulation of ethyl alcohol. As has already been mentioned, 

 when yeast cells grow in test tubes under almost anaerobic conditions 

 the decomposition of sugar into alcohol and carbon dioxide serves 



