EFFECTS OF CHANGES OF MEDIUM. 185 



amount of KaHPC^ to be added to the normal medium, as men- 

 tioned above, the pH was taken of the different solutions and 

 found to vary from 7.8 in the medium plus N/iooo K 2 HPO 4 to 

 7.1 in the medium plus N/6ooo K 2 HPO 4 , and the greatest division 

 rate was found among the individuals living in the medium to 

 which had been added a solution of N[$ooo K2HPO 4 and which 

 showed a pH of 7.2. 



The results of experiments in this connection are shown in 

 Table IV. Two experiments were conducted with five series of 

 different ages. The experiments were identical except for the 

 fact that in one the potassium solution was made up with 

 distilled water and added to the normal medium, which then 

 had a pH of 6.8, while in the other experiment the potassium 

 solution was made as usual with spring water and the resulting 

 medium showed the normal pH of 7.2. In every experiment the 

 distilled water series showed a lower division rate than that of 

 the spring water series and also than that of the control series 

 except in one instance. The youngest series 131 B in the 2Oth 

 generation divided 3.8 divisions less per line than the control 

 series and the oldest series 128 in the 295th generation was also 

 depressed and divided 3. divisions less per line than the control. 

 Series 133 in the I22d generation was the only series that was 

 able to live in the unfavorable medium without showing depres- 

 sion. Here the protoplasm was in the early stage of maturity 

 and apparently could adjust itself more easily to the changed 

 conditions. Undoubtedly an acid condition creates an unfavor- 

 able environment in which the youngest and oldest individuals 

 are unable to maintain their normal activity. The method of 

 distillation of the water may be an additional factor in causing 

 depression. However, when the solutions are made with spring 

 water and have the same hydrogen ion concentration as has been 

 the case in all the experiments with this one exception, some 

 other explanation must be sought to explain the variation in the 

 response of protoplasm of different ages. 



2. Experiments with Di-sodium Phosphate. 



In all of the experiments in which Uroleptus mobilis was kept 

 in a medium to which sodium phosphate was added, the amount 

 13 



