FEEDING PARAMECIUM KNOWN BACTERIA 149 



tics, and the letters given in this description will be used in the 

 following account. 



During the period from August 18 to August 27, 1920, the 

 bacteria A, B, and C were fed in pure cultures and in the mix- 

 tures AB, AC, BC, and ABC. A chance mixture made by 

 inoculating sterile infusion with timothy hay was used as a 

 control. The letter M has been given to this type of mixture 

 throughout the description of the feeding experiments. 



The average division rate for M for the entire period was taken 

 as the mean from which the percentages of high divisions, low 

 divisions, and deaths were computed. Percentages of mean and 

 zero divisions do not appear to be of any great help in interpreting 

 results, and have, therefore, been omited from all accounts and 

 tables which follow. Throughout this account the term 'inter- 

 val' is employed to indicate the short groups of three days which 

 were used in computing the averages of division rates, etc., 

 whereas the term 'period' is applied to the longer durations of 

 time during which the animals were subjected to a given set of 

 conditions. Woodruff ('11) has defined a rhythm as "a minor rise 

 and fall of the fission rate, due to some unknown factor in cell 

 metabolism from which recovery is autonomous." It is in this 

 sense that the term is used in this paper. 



The bacteria A, B, C and their mixtures proved to be unsatis- 

 factory food for Paramecium. In no case did the division rate 

 exceed 1.5. The death rate was high in all save A, and it was 

 impossible to get an active metabolism by the use of these bac- 

 teria in pure culture or in mixtures. Meanwhile, the control, M, 

 maintained a high metabolic rate. 



On August 22, 1920, feeding with the pure cultures, J, K, L, 

 and the mixtures JK, JL, KL, and JKL, was started. This 

 group of cultures was under observation until August 31, 1920, 

 during which time the result was comparable to that gained in 

 attempting to feed A, B, and C. Although the results of feeding 

 mixtures were slightly better than those obtained by the use of 

 pure cultures, the slight advantage thus gained could not be 

 continued, and the animals eventually died. 



