GROWTH 551 



second equilibrium level decreased slightly from the eighth day to the 

 sixteenth day, when a second and shorter growth cycle commenced. The 

 second cycle passed through a brief equilibrium period and then de- 

 clined to about the same level as that which followed the first growth 

 cycle. Population growth curves are given for 5". mytillus, P. aurelia and, 

 in a later monograph (1935), for Glaucoma scmt'dans, Didinium nasu- 

 tuni, Bursaria truncatella, and P. bursaria. Some of these will be dis- 

 cussed in the next section. One set of data is interesting from the view- 

 point of population growth, that for P. aurelia and P. caudatum, grown 

 separately in a standardized medium which was changed every twenty- 

 four hours (Fig. 134). The equilibrium phases showed that there 

 were over twice as many P. aurelia produced as P. caudatum. Gause then 

 measured the sizes of the animals and computed the mean volume of 

 each and the total volume of population. The volume curves showed that 

 very nearly the same volume of protoplasm was produced by each 

 species, with the same medium and conditions of culture. 



P. caudatum was grown in a balanced salt medium, with one unit of 

 concentration, and with five units' concentration of bacteria, by Johnson 

 (1935). The growth curves are sigmoid and show no stationary phase 

 and only a short lag phase. The equilibrium number was maintained 

 with no decline for seven days. The number of animals produced in the 

 greater concentration was more than five times the number in the lesser 

 concentration. In the lower concentration a single animal divided more 

 times than did a group of animals, while in the greater concentration a 

 group divided more rapidly, for about three days, when the population 

 figure from the single animal seeding passed the group curve to reach 

 a higher equilibrium level. 



Mond's (1937) estimates of both the bacteria and the infusorian 

 populations point the way to more adequate studies of protozoan growth. 

 Populations of Colpoda duodenaria were maintained in aeration flasks 

 for four months by Taylor and Strickland (1938). By continuous feed- 

 ing, densities of 6 X 10*^ per milliliter were produced. The size of the 

 population fluctuated with the amount of food available and could be 

 modified as the experimenters wished. Over the whole period the num- 

 ber of Protozoa produced from a given amount of food was constant. 

 Excretion products did not limit the growth, but the continuous aeration 

 may have ameliorated the effects of the waste products, so that the 

 conditions are not comparable with unaerated cultures. 



