EFFECTS OF CHANGES OF MEDIUM. 183 



Two young series 131 B and 129 B in the 2Oth and 58th 

 generation respectively were transferred daily for ten days to the 

 potassium-hay-flour medium. Both series as a result of the 

 treatment showed a lowered vitality and a slower division rate 

 than the control series. In the 8oth generation however, 129 B 

 was definitely stimulated when again treated daily with the 

 potassium-hay-flour medium, dividing 4.4 divisions per line more 

 than the control series. Series 133 was treated in the same way 

 in the iO4th, I22d, I92d, and 2i2th generation for ten days 

 each and at each period the division rate was increased over 

 that of the control. 



Series 131 was treated three times with the potassium-hay-flour 

 medium. In the 22oth generation it was but slightly stimulated 

 during the ten days in the medium. In the 28oth generation 

 the division rate rose to 4 divisions more per line than the control 

 series and finally in the 3OOth generation, the rate dropped to 

 but .4 divisions per line more than the control series. This was 

 a vigorous series throughout all of the experiments and probably 

 the usual old age reaction was only beginning in the 3OOth 

 generation. As has been clearly shown by Calkins (5) in his 

 study of this species, the various series differ in the length of 

 life and also in general vitality. Some series live a long life with 

 a moderate division rate which diminishes gradually during old 

 age, others have a high division rate, respond definitely to changes 

 in the environment and then die out quickly. Series 133 seems 

 to have been more like the latter type while series 131 always 

 showed a more moderate division rate which was less easily 

 changed by variations in the environment. 



The oldest series (128) in the 295th generation showed a slight 

 depression in its vitality when treated with the potassium-hay- 

 flour medium. These results seem again to indicate that potas- 

 sium used as a constant addition to the normal medium, causes 

 the same changes in vitality as when it is used as an initial dose; 

 the youngest lines are depressed, the mature ones are stimulated 

 while the old lines are only slightly effected if at all. 



The hydrogen ion concentration of the potassium-hay-flour 

 medium used in all the above experiments was 7.2 which is that 

 of the normal medium. When testing for the most favorable 



