104 



THE STRUGGLE FOR EXISTENCE 



products had already attained a critical value. Now with Para- 

 mecia our object is that the concentration of the waste products 

 should still be very far from the critical threshold at the moment when 

 the food is exhausted. 



First of all we turned our attention to the hydrogen ion concentra- 

 tion (pH), which in the light of the researches of Darby ('29) can 

 be of great importance for our species. When Paramecia are culti- 

 vated in Osterhout's medium, pH is near to 6.8 and unstable, where- 



no 



ISO 



no 



so 



"Half- loop" medium 



K=ior 



■*—* a— s a 



K'6V 



Faureha jeparate/y 

 i 



- T 1 * 



• • • • • 



P.caudatum separately 



~ One loop medium 



K=I9? D 



^ Raureha separately 



• K'/3Z 



■ * 



P.caudatum jeparate/y 



• • 



10 



/v 



it 



18 



20 



Days 



Fig. 23. The growth of the "volume" in Paramecium caudatum and Para- 

 mecium aurelia cultivated separately on the buffered medium ("half-loop" 

 and "one-loop" concentrations of bacteria). From Gause (\34d). 



as the reaction in our wild cultures is commonly near to 8.0. There- 

 fore we, like Johnson, buffered Osterhout's medium by adding 1 cm 3 



m 

 of — KH 2 P0 4 to 30 cm 3 of diluted salt solution, and bringing the 



reaction of the medium with the aid of — KOH to pH = 8.0. At 



the same time we isolated new pure lines of Paramecia out of our wild 

 culture, as the Paramecia which had been cultivated for a long time 

 on Osterhout's medium could not stand a sudden transfer into a 

 buffered medium. 



