542 GROWTH 



Euglena gracilis can be grown in the dark for extended periods. Without 

 light, some of the organisms require more compHcated food substrates, 

 and the experiments demonstrating this have been summarized by Hall 

 (1939). The amount of sunlight may exert a seasonal effect on the 

 division rate of Protozoa (Richards, 1929). Reflected light stimulated 

 multiplication of Varamecium caudatuni in the red, but had a depres- 

 sing action in the violet, according to Zhalkovskii (1938). Filtered, 

 transmitted light had a greater depressing effect than reflected light. 

 The difference was believed to be due to the polarizing effect of the re- 

 flected light. Heritable changes in the size and form of Chilodon un- 

 cinatus have been produced by McDougall (1929) with ultra-violet 

 radiation. Giese (1939) found that ultra-violet of 2,654A injured the 

 nuclear material of P. caudatuni and that the damage was less readily 

 repaired than was the damage to the cytoplasm caused by 2,804A. 



The ease with which the acidity of the culture medium may be meas- 

 ured is responsible for a considerable volume of information (Table 6). 

 The early measurements of Peters (1904, 1907) and Fine (1912) were 

 made by titration. The advent of simple methods for the measurement of 

 the hydrogen-ion concentration was welcomed by the protozoologists, and 

 Bodine (1917) related the old and the new methods. Pruthi (1926) 

 found a sequence in hay infusion of Holphyra, Plagiopyla, Colpidium, 

 Amphileptus, and Paramecium. The first two do not persist beyond pH 7.5 

 and the paramecia did not appear before the pH reached 7.0. Mass cultures 

 and some synthetic media change during the growth of the organisms, and 

 the change in pH of mixed cultures is probably more the result of bacterial 

 action than that of the Protozoa. Eddy (1928) believed that the changes 

 in pH were not of importance in themselves, but rather the result of other 

 effects. Phelps (1931) attributed the changes to the food supply, and 

 Johnson (1935,1936) stressed the effect of bacterial action. The ac- 

 celerative effects of stimulants depend on the acidity of the medium, 

 and there now seems no question but that there is an optimum pH range 

 for different media and Protozoa, and that beyond the optimum range 

 growth is less and may be entirely inhibited. Elliott ( 1935b) found that 

 sodium acetate stimulated Colpidium at pH 6.8-7.5, and butyrate at a 

 pH less than 7.0. The size of P. bursaria has been shown by Loefer 

 (1938a) to depend on the acidity of the tryptone and proteosepeptone 

 culture media. 



