RETARDING INFLUENCE OF CROWDING 121 



ments. The results are given graphically in Figure 4. For the ex- 

 periments in which the medium was changed every 24 hours the 

 Paramecia in 5, 20, and 40 drops are shown to have divided 2.4, 

 6.4, and 7.4 per cent more rapidly, respectively, than did those in 2 

 drops. When the medium was changed every 48 hours, the per- 

 centages for the same volumes were 5.3, 9.3, and 9.25. The results 

 are given throughout as averages for 4-day periods. 



From these experiments Woodruff concludes, "The rate of repro- 

 duction of specimens from pure lines of Paramecia when bred under 

 identical conditions of temperature and culture medium is influenced 

 by the volume of the culture medium (within the limits tested in the 

 experiments) and the greater the volume, the more rapid is the rate 

 of division." The slight discrepancy with the 40-drop cultures 

 changed every 48 hours is unexplained, but the suggestion is offered 

 that the bacteria always found in such cultures, and which are used 

 as food by the Paramecia, develop so rapidly under these conditions 

 that they may exhaust their own food or produce sufficient excretion 

 products to be injurious to the associated Paramecia. Otherwise, 

 Woodruff believes that by his culture methods, which included cross- 

 inoculations between the different cultures, he has eliminated the 

 bacteria as agents causing the observed difference in rate of Para- 

 mecium division. 



The conclusion that the recorded effects are due to the accumula- 

 tion of Paramecium waste products rests on three lines of evidence. 

 In the first place, as we have just seen, the rate of division is higher, 

 for the periods and amounts tested, the larger the amount of avail- 

 able medium. Second, the rate averaged 8 per cent greater in the 2- 

 drop cultures changed daily than in similar cultures changed every 48 

 hours. The other cultures similarly showed a 6 per cent increase if 

 changed daily. Finally, culture media in which Paramecia had flour- 

 ished for 10 days before removal were shown to have a depressing 

 effect upon the reproductive rate of Paramecia replaced in it, as 

 compared with the effect of an infusion which had contained no 

 Paramecia but which otherwise was as nearly comparable as the two 

 could be made. 



Woodruff (19 13), as a result of further experience, concluded that 



