538 GROWTH 



able for the maintenance of P. aurelia. She concluded also that the 

 paramecia could not live on dissolved substances, but were dependent 

 on particulate food. Glaucoma ficar/a was grown on a number of single 

 species of bacteria, yeast, and flagellates by D. E. Johnson (1936). 

 B. prodigiosus was the most satisfactory food organism. The results 

 depended largely on the food being small enough for ingestion. 



Recent studies have been directed toward determining the food ele- 

 ments required by Protozoa and toward devising synthetic media in which 

 the Protozoa could be grown in bacteria-free, pure cultures. While it 

 it not possible to separate studies on growth and nutrition except for con- 

 venience, this chapter will be limited to studies occupied primarily with 

 the analysis of growth. The broad problem of nutrition will be covered 

 elsewhere. Diff^erent species have different nutritional requirements, 

 and the failure of some protozoologists to realize this fully has led to 

 confusion in the literature on growth. Very few data are available which 

 give the growth of the bacteria, as well as that of the Protozoa, present 

 in mixed cultures. Considerably more labor would be involved in secur- 

 ing this information, but the methods have been worked out and the 

 information gained would justify the work. It is now possible to grow 

 pure cultures of a variety of Protozoa in bacteria-free synthetic media. 

 Some of the nutrient conditions limiting growth will be examined briefly. 



Tolerance to changes of osmotic pressure was found by Loefer ( 1938) , 

 in attempts to adapt fresh-water Protozoa to artificial sea water, to be 

 limited. Yocom (1934) was more successful. Loefer (1939) found 

 that tolerance to diluted Van Hoff solution developed over several 

 generations. Changes in the oxidation-reduction potential have been 

 measured in Chilomonas Paramecium cultures by Jahn (1933), and his 

 results suggest that when the medium is poised at the optimum rH, 

 growth will be most rapid. 



The increased growth of Protozoa at the surface of mass cultures 

 shows their sensitivity to oxygen. Aeration will often extend the growth 

 to deeper levels. Inadequate amounts of oxygen limit the growth of 

 Polytoma uveila, and sufficient oxygen must be provided before the 

 effects of other nutrients may be evaluated (Rottier, 1936; Mond, 

 1937). Reich (1936) believes oxygen concentration more important 

 in Amoeba cultures than acidity. Jahn (1936) aerated bacteria- 

 free cultures of Glaucoma pyrijormis and Chilomonas Paramecium in 

 a hydrolyzed casein medium with air, and air freed of carbon dioxide. 



