480 Physiology 



between the pH limits 2.1 and 9.9 (Table 8.8). Survival for at least short 

 periods may be possible within a wider range, such as pH 2.3-11.0 for 

 Euglena gracilis (2), 2.0-9.65 for E. gracilis var. bacillaris (331), and 1.4- 

 9.6 for Polytomella caeca (346). Euglena mutabilis apparently can survive 

 in polluted waters at pH 1.8 (306), and in pure cultures, for at least 12 

 days within the range, 1.4-7.9 (84). Growth throughout most of the gen- 

 eral range seems to have been observed only in Euglena gracilis and 

 Polytomella caeca, and the specific range varies considerably in other 

 species. 



The pH optimum also varies from species to species and within one 

 species under different conditions. Unfortunately, it is sometimes uncer- 

 tain just what a reported "optimum" means in terms of protozoan growth. 

 The apparent optimum may depend upon the time of observation, as in 

 Euglena gracilis which showed heaviest growth at pH 6.6 after 8-9 days, 

 but at pH 1.1-1 A after seven weeks (237). Present knowledge of growth- 

 pH relationships should be extended by tracing growth curves in media 

 at different pH levels. Most of the available information does not elimi- 

 nate the possibility that within reasonable limits, a pH above or below an 

 apparent optimum may retard growth without modifying the eventual 

 density of population. 



The growth of Astasia longa in acid media throws some light on such 

 questions (507). Growth in peptone medium at pH 3.7, for example, is 

 rapid for the first fev; days and then ceases for a period of 3-5 weeks. 

 Later, a second period of growth produces populations comparable in 

 density to those obtained much sooner at higher pH levels (Fig. 8. 5). 

 This resumption of growth apparently cannot be attributed to the slight 

 rise in pH (0.1) during incubation. Only the first phase of growth is 

 observed in a medium at pH 3.1 and second transfers in medium at the 

 same pH show no significant growth after four months. A delayed growth 

 phase seems to be limited to distinctly acid media since it has not ap- 

 peared within the pH range, 6.0-9.6. A particularly interesting feature 

 of these populations is the early increase in acid media, even at a pH 

 level which inhibits later growth. The data suggest the possibility 

 that inocula from a healthy culture may contain enough critical re- 

 serves to insure a 20- to 25-fold increase in number, in an unfavorable 

 environment. This reserve apparently is exhausted before the flagellates 

 are completely adjusted to the new environment, and in media which 

 are not too acid, a period of "adaptation" precedes the resumption of 

 growth. 



Two periods of logarithmic growth separated by an appreciable sta- 

 tionary phase — Monod's phenomenon of "diauxie" — have been observed 

 also in bacteria grown on a mixture of two carbohydrates (401). In such 

 cases, it has been assumed that the first phase of growth ends with 



