506 FOOD REQUIREMENTS 



the action of other environmental factors. Conversely, changes in vari- 

 ous environmental conditions may modify the temperature relationships 

 of a given species. 



In one of the few investigations carried out on pure cultures, Jahn 

 (1935c) has demonstrated an interesting temperature relationship in 

 Euglena gracilis. In darkness, the optimal temperature for this species 

 in a peptone medium was about 10° C. When sodium acetate was added 

 to the medium, not only was growth accelerated, but the optimal tem- 

 perature was shifted to about 23°, a point approaching the optimum 

 for growth in light. Another instance in which the temperature rela- 

 tionships vary with other environmental conditions is represented by 

 the thermal death time of E. gracilis (Jahn, 1933a), which appears to 

 be a function of the pH, the greatest resistance to a temperature of 40° C. 

 being noted at pH 5.0, and a greater susceptibility above pH 7.0 than 

 below. 



Growth in Relation to Light and Darkness 



Little or nothing is known concerning the relation between light and 

 the growth of colorless Protozoa. On the other hand, the importance of 

 light is obvious in the case of the chlorophyll-bearing species, and the 

 relation to photosynthesis probably accounts for a number of the known 

 effects of light. Dusi (1937) has noted certain interesting peculiarities 

 of several Euglenidae. In constant light, E. gracilis grows well in pep- 

 tone medium, but poorly in inorganic media. On the other hand, E. 

 klebsii grows perfectly in inorganic medium under constant illumina- 

 tion, while E. viridis is incapable of growing under such conditions, 

 even in peptone medium. No explanation for such specific differences in 

 light relationships is yet available. An apparent relationship between 

 light and the optimal temperature for growth has been noted by Jahn 

 (1935c), who reported that the optimal temperature for growth of E. 

 gracilis in darkness lies near 10° C, whereas the optimum in light for 

 the same species is approximately 25°. The presence or absence of light 

 is also a factor which must be considered in interpreting the effects of 

 carbon compounds on growth. Thus Jahn (1935d) has observed that 

 the accelerating effects of several organic acids on the growth of E. 

 gracilis are relatively much greater in darkness than in light. Succinate, 

 on the other hand, produced a slight acceleration in darkness, but was 



