RESPIRATORY METABOLISM 393 



type of evidence is well founded in fact — anaerobes do not grow in media 

 of high Eh value. However, if the Ei, value of a suitable medium is low- 

 ered through displacement of air with Ho or N,, or by various chemical 

 reagents, or by the growth of an aerobic organism, then the anaerobic 

 forms are capable of growth. According to this theory, anaerobes and 

 aerobes differ in their ability to grow at various points along the Eh scale, 

 in a manner comparable to that which is exhibited by various acido- 

 philic and basiphilic forms in growing at various points along the pH 

 scale. Of course, there are intermediate-range and wide-range forms in 

 respect to both pH and Eh. The fact that the toxic effects of lack of O2, 

 or of supernormal Oo tensions, are not equal in all species supports this 

 idea, but these data, of course, are subject to other interpretations. 



Investigations of the role of oxidation-reduction potentials among the 

 Protozoa have never passed the preliminary stages. The possible im- 

 portance of such a study was pointed out by Jahn (1933b, 1934), in 

 connection with experiments on -SH compounds, on the toxic action of 

 methylene blue and on possible relationships with auto- and allelocataly- 

 sis. Measurements of the Eh of Chilomonas cultures (Jahn, 1935b), of 

 hay infusions (Efimoff, Nekrassow, and Efimoff, 1928), and of digestive 

 contents (Jahn, 1933a) have been made. However, until more data be- 

 come available, most of these results are difficult to interpret. It was de- 

 termined that Chilomonas would grow in mixtures of NaSH and HjOo 

 only if the concentrations of these were balanced so that the medium 

 just failed to reduce methylene blue. This might indicate a microaero- 

 philic tendency for Chilomonas. Other experiments with Chilomonas 

 indicated that it could live, but could grow only slowly, however, in 

 media in which methylene blue was reduced. Neither of these ideas is 

 contradictory to its known habits in laboratory cultures. It was also 

 demonstrated (Jahn, 1935b) that casein-acetate broth cultures of Chi- 

 lomonas, when exposed to the air, developed potentials of -20 mv. at 

 pH 7.55, a point at which methylene blue is about half reduced. The 

 chief difficulty in interpreting experiments pertaining to the effect of Eh 

 on growth is that it is necessary to change Oo tension in order to change 

 Eh. This makes an experiment containing only one variable seemingly im- 

 possible to execute, and the theory, therefore, has not been amenable 

 to experimental approach. 



The fourth theory of the effect of O. on anaerobes — that of an inac- 



