494 



FOOD REQUIREMENTS 



vestigated by Elliott (1938). The growth of E. gracilis was markedly 

 accelerated at pH 5.6, while effects at lower and higher pH values were 

 much less significant. In the colorless euglenoid, K. hall/, and in C. 

 str'mtufu no acceleration of growth was observed at any pH. Thus the 

 effects of the auxins may be correlated with the presence of chlorophyll, 

 as well as the pH of the medium. Elliott (1937b) has shown further 



TIME 



TIME 



Figure 128. Growth phases in a hypothetical population. In the curve at the left, 

 logarithms of numbers are plotted against time; on the right, numbers are plotted against 

 time for a comparable population. Successive growth phases are numbered from 1 to 7. 



that such acceleration may also be dependent upon light, since there was 

 no effect on growth of E. gracilis in darkness. 



An accelerating effect of pimelic acid upon the growth of Colpidium 

 campylum has been noted by Hall ( 1939c) ; concentrations ranging from 

 10"^" to 10"^ gm. per cc. were effective in gelatin and in peptone media. 

 These results are comparable to the findings of Mueller (see Koser and 

 Saunders, 1938) with the diphtheria bacillus. Certain preliminary ob- 

 servations (Hall, 1938a) may indicate a possible growth-factor status 

 for pimelic acid, but a definite conclusion is not yet warranted and pimelic 

 acid may be considered, at least for the present, a growth stimulant for 

 C. campylum. 



Glucose caramel, as used by Pringsheim (1937b, 1937c), may also 

 be classified as a growth stimulant. Pringsheim insists that this substance 



