74 THE STRUGGLE FOR EXISTENCE 



These data were criticized by Klem ('33) who carried out experi- 

 ments with wort and not with William's synthetic medium, which 

 Richards worked upon. Klem did not obtain any depression of 

 growth by adding a small quantity of alcohol corresponding to the 

 quantity which is usually accumulated in his cultures at the moment 

 when the growth ceases. According to Klem, it is only at a concen- 

 tration above 3 per cent that alcohol begins to depress growth, 

 and only concentrations of about 7 per cent have a distinctly 

 hindering influence. The experiments which I have made with 

 yeast decoction and 5 per cent sugar confirm the data of Richards 

 and not those of Klem. Figure 12 presents the results of several 

 experiments. The level of the maximal population in the control 

 was taken as ,100, and the levels of the maximal populations in the 

 cultures with this or that per cent of alcohol (added before the yeast 

 was sown, all other conditions being equal) were expressed in per cent 

 from the population level in the control. This figure shows that even 

 1 per cent of alcohol in our conditions lowers the maximal level of 

 population considerably. As we have already seen (Fig. 11, bottom) 

 at the moment the growth ceases in our cultures the concentration of 

 alcohol is near to 2 per cent (with the usual composition of me- 

 dium). This concentration is undoubtedly sufficiently high to be 

 responsible for the cessation of growth. 



Klem expressed an interesting idea, namely that the cessation of 

 growth is connected with the reaching of a definite relation between 

 the concentration of the waste-products and the nutritive substances, 

 i.e., alcohol and sugar. In other terms, the critical concentration of 

 alcohol checking growth is by no means of an absolute character. 

 With a small concentration of sugar, a comparatively weak concen- 

 tration of alcohol hinders growth. But if the quantity of sugar be 

 increased, this concentration of alcohol will no longer be sufficient 

 for checking growth which will continue. Klem's opinion is per- 

 fectly justified and many experimental data confirm it. But, as he 

 himself remarks, the ratio alcohol/sugar left at the moment growth 

 ceases, also varies within rather wide limits. (A critical analysis of 

 Figs. 53-54 on pp. 80-81 of his paper ('33) shows that even with 

 concentrations of sugar from 1 to 5 per cent the ratio alcohol/sugar 

 left does not remain constant, and that Klem's calculations are 

 not quite exact.) 



(4) All we have said may be resumed thus: under our conditions 



