548 GROWTH 



Jahn (1934) protozoan populations; and for other animals, Pearl 

 (1925), Gray (1929), Gause (1932), Johnson (1937), Hammond 

 (1938), and Park (1939) may be consulted for reviews and bibliog- 

 raphy. 



Jones (1928) followed the population growth of an inoculation of 

 200 P. niultimJcronucleatum in 70 cultures at 80° F., with counts of 

 0.5 ml. samples made periodically. The pH of the medium was also 

 measured. A maximum crop of 10'' paramecia were obtained in 700 ml. 

 cultures. Growth stopped when the pH decreased to 5. With hay-flour 

 infusions, two cycles of growth were found (1930). The first was 

 terminated by the high acidity; when the acidity returned to about 

 pH 7, the second growth cycle commenced. During a three-day period 

 Jones (1937a) found that the number having died at the close of the 

 first growth cycle exceeded the maximum number present during the 

 second period. The death of the animals was believed to be due to toxic 

 excretion products, which were neutralized by the materials liberated from 

 the cytolysis of the dead animals. Death was apparently disruptive, as no 

 intact dead animals were observed. With large one-gallon cultures, the 

 decline of the populations was related to the decline of food; and, by 

 periodically renewing the food, the cultures could be maintained for 

 four years. 



The growth of Euglena gracilis in mass cultures was used by Jahn 

 (1929) to test the allelocatalytic theory of Robertson. The organisms 

 were derived from a single cell isolation and grown in an autotrophic 

 mineral-salts medium, with temperature and light controlled. Jahn's 

 larger inoculations gave a population growth with two cycles (Fig. 134) . 

 No evidence for allelocatalysis was obtained. The relative rates of growth 

 were computed (Jahn, 1930) and found to give a decreasing sigmoid 

 curve. Jahn emphasized the difference between the absolute rate of 

 growth [dy/dt) of the total number and the relative rate of growth 

 {dy/ydt), or division rate, of the organisms, without entering into the 

 discussion of the relative growth rate as such. 



Phelps (1935) measured the population growth of bacteria-free G. 

 pyriforniis in 700 ml. cultures of a mineral salts-yeast extract medium in 

 one-liter flasks. The length of the stationary and the lag phases were 

 proportional to the age of the seeding, and seeding from populations 

 in the logarithmic phase gave no stationary or lag phases. Increasing 



