FOOD REQUIREMENTS 501 



centrations were much less favorable. In Colpidium campylum, Bond 

 (1933) observed comparable relationships between growth rate and 

 concentration of yeast autolysate. Optimal concentrations of peptone for 

 C. striatum (Elliott, 1935b) lie between 1.0 and 3.0 percent, while for 

 Glaucoma ficana (D. F. Johnson, 1935a) the optimum is about 1.5 per- 

 cent. In both species of Colpidium the effects were apparent after twenty- 

 four hours of incubation and became more marked in older cultures. 

 Loefer's (1936b) observations on Paramecium bursaria indicate similar 

 relationships, although the limits are somewhat narrower than for 

 Colpidium and Glaucoma. For example, one of the peptones tested was 

 optimal in 0.5 -percent solution, whereas no growth of P. bursaria oc- 

 curred in a concentration of 1.4 percent. Phelps (1936) observed that in 

 the logarithmic phase the growth rate of G. piriformis was, within wide 

 limits, practically independent of the food concentration. In later history 

 of the cultures, however, the relationships were comparable to those ob- 

 served by Elliott and D. F. Johnson. Rottier (1936b) described, in 

 Polytoma uvella, a direct relation between the growth rate and the con- 

 centration of peptone (0.2 to 1.0 percent), and of asparagin (0.2 to 2.0 

 percent), significant differences being noted after about five days of 

 incubation. 



The effect of a substance added to an adequate medium also varies 

 with concentration, as would be expected. Johnson (1935a) reported 

 for Glaucoma fcaria in peptone medium, maximal acceleration by 0.5- 

 percent yeast extract and inhibitory effects of concentrations above 2.0 

 percent. In Paramecium bursaria (Loefer, 1936b), the optimal concen- 

 tration of the same yeast extract was 0.03 percent, and growth was defi- 

 nitely inhibited in a 0.5-percent solution. Dextrose was most effective for 

 P. bursaria in a concentration of 0.5 percent. Reich (1936) has obtained 

 similar results with Mayorella palestinensis. The effects of added sodium 

 acetate on P. uvella (Rottier, 1936b) vary in the same fashion, the maxi- 

 mal effect being produced by 0.8-percent acetate. The optimum is much 

 lower in Euglena stellata (Hall, 1937d), in which 0.05-percent sodium 

 acetate was most effective in both light and darkness. 



Growth in Relation to pH of the Medium 



It has been known for many years that the acidity or the alkalinity of 

 the medium bears some relation to the growth of Protozoa, and investi- 

 gations on bacterized cultures have determined the optimal pH and pH 



