478 Physiology 



the cytoplasm during nuclear division soon passes into the culture me- 

 dium, there to accelerate later fissions. When the inoculum contains more 

 than one organism, the liberation of more catalyst would cause mutual 

 stimulation of fission, or allelocatalysis. Accordingly, the fission-rate va- 

 ries more or less directly with size of the inoculum. An apparent allelo- 

 catalytic effect has been reported for bacterized cultures of certain ciliates 

 (258, 436), Chilomonas Paramecium (393), and Mayorella palestinensis 

 (474). The case of C. Paramecium has been questioned (178) because in- 

 terpretations were based upon terminal counts without any information 

 concerning the earlier history of populations. 



Various explanations have been proposed for the Robertson effect. Cut- 

 ler and Crump (78, 79, 80, 81) believed that Robertson's findings resulted 

 from failure to control the bacterial flora of his cultures. The importance 

 of the bacterial concentration also has been stressed by Johnson (258) 

 who showed that, in cultures of Oxytricha fallax, the initial concentra- 

 tion of bacteria may determine whether a culture is to show a Robertson 

 effect. Another possibility is that the initial pH of Robertson's poorly 

 buffered medium was not optimal for his ciliates, which could change the 

 pH toward the optimum (64). On this basis, two ciliates should produce 

 such a change more rapidly than one and cause an allelocatalytic effect. 

 Jahn (240) has suggested that, in similar fashion, the oxidation-reduction 

 potential of the medium might be responsible for an allelocatalytic effect. 



An inverse relationship between growth-rate and initial density was 

 observed by Woodruff in Paramecium aurelia and P. caudatum. The more 

 rapid reproduction with lower initial densities was attributed to less rapid 

 accumulation of waste products (580). A comparable relationship has 

 since been reported for cultures of P. aurelia, P. caudatum, and Pleuro- 

 tricha lanceolata (167), Stylonychia pustulata, P. caudatum (88), and 

 Euglena sp. (235). This so-called W^oodruff effect is variously attributed 

 to the accumulation of waste products in the medium, exhaustion of a 

 scanty food supply, and changes in oxygen tension and pH away from the 

 optimum. 



Initial pH of the culture medium 



The observed relations to growth indicate that pH of the medium 

 influences utilization of food and synthesis of protoplasm, perhaps 

 through effects on solubility and ionization of substrates and on permea- 

 bility of the organism to components of the medium. Activities of extra- 

 cellular enzymes also may be influenced by pH of the medium. The 

 "internal" pH may be relatively independent of environmental pH, since 

 immersion of Amoeba dubia in liquids at pH 5.5 and 8.0 induces no 

 change in cytoplasmic pH (54) from the normal level of about 6.9 (479). 

 However, the activity of a proteinase from Tetrahymena pyriformis varies 

 with pH of the medium (312). Likewise, the rate of oxygen consumption 



