MECHANISM OF COMPETITION IN YEAST CELLS 75 



of cultivation the cessation of growth of the population of yeast cells 

 begins before the exhaustion of the nutritive and energetic resources 

 of the medium. The direct cause of this cessation is the accumula- 

 tion of ethyl alcohol which kills the most sensitive members of the 

 population — the young buds. This critical concentration of alcohol 

 is not of an absolute character, and in a first approximation we can 

 say that the cessation of growth is connected with the establishment 

 of a definite ratio between the concentrations of waste-products 

 (alcohol) and the nutritive substances (sugar). We now have to 

 answer the question raised earlier: what factors will furnish us with 

 the terminology for expressing the "number of vacant places" or "the 

 unutilized opportunity for growth" in the population of yeast cells 

 under our conditions of cultivation? Since the growth of population 

 ceases with the establishment of a certain ratio alcohol/sugar a 

 thought might appear that we ought to connect the unutilized oppor- 

 tunity for growth somehow with the ratio. However this would be a 

 false deduction from correct premises. We can see at once that we 

 have to deal here with two different things. (1) Should we wish to 

 make a purely theoretical calculation of the level of saturating popu- 

 lation in our microcosm, we would certainly be obliged to take into 

 consideration the ratio between the concentrations of alcohol and 

 sugar, and to try to calculate the moment when this ratio attains a 

 definite value. But certainly we should at once have to introduce 

 numerous corrections, as various other factors have also an influence 

 here. (2) The conditions of the problem before us are quite different. 

 We know beforehand at what level the population ceases to grow, and 

 what is the corresponding value of different factors of the environ- 

 ment. We wish only for different moments of time preceding the 

 cessation of growth to translate "the unutilized opportunity for 

 growth" into terms of the limiting factor. Such limiting factor is 

 always alcohol destroying the young buds. However considerably 

 other factors of the environment and the condition of the cells them- 

 selves should alter the absolute value of the critical alcohol concen- 

 tration, this does not essentially change the matter. Consequently 

 "the unutilized opportunity for growth" or "the number of still vacant 

 places" can simply be determined by the difference between the critical 

 concentration of alcohol at the moment of cessation of growth, which is 

 characteristic for the given conditions and established experimentally in 

 every case, and the concentration of alcohol at a given moment of time. 



