MECHANISM OF COMPETITION IN YEAST CELLS 77 



the struggle for existence between two species in a mixed culture. 

 The simplest way to do this is again to begin by an analysis of the 

 kinetics of growth. Let us examine the experiments of 1931. In 

 Table 1 (Appendix) data are given on the anaerobic growth of the 

 volume and of the number of cells in the two species of yeast : Sac- 

 charomyces and Schizosaccharomyces, cultivated separately and in a 

 mixed population in two independent series of experiments. One 

 hundred and eleven separate microcosms were studied in these two 

 series, and every figure in Table 1 (Appendix) is founded on three 

 observations. Figure 13 represents graphically the growth of the 

 yeast volume. We can see that the growth of Schizosaccharomyces 

 under anaerobic conditions is exceedingly slow. Let us note also 

 that its population attains a much lower level than that of Saccharo- 



vr 



Saccharomyces 



a a fa\ Mixed population 



u 

 Schtzosaccfiaromycer 



so wo no no 

 Hours 



Fig. 13. The growth in volume of Saccharomyces cerevisiae, Schizosaccharo- 

 myces kephir and mixed population in two series of experiments. Anaerobic 

 conditions. From Gause ('32b). . 



myces. The volume of the mixed population is also smaller than the 

 volume of the pure culture of Saccharomyces. 



The parts taken up by each of the species in the yeast volume of a 

 mixed culture have been evaluated in the following manner. First 

 of all, a calculation was made of the average number of cells per unit 

 of yeast volume for the separate growth of Saccharomyces and Schizo- 

 saccharomyces (see Appendix, Table 1). It appears that the mean 

 number of cells occupying a unit of yeast volume varies in the course 

 of the growth of the culture, as Richards has already established. 

 However, these variations are not great, and for further calculations 

 average values for the entire cycle of growth can be taken. Accord- 

 ing to the first series of experiments, in Saccharomyces 16.59 cells in a 

 square of a Thoma counting chamber correspond to one unit of yeast 



