FOOD REQUIREMENTS 495 



does not serve as a carbon source in his cultures of plant-like flagellates, 

 and that it should be considered a "growth factor." On the other hand, 

 it has not yet been demonstrated that glucose-caramel is essential to the 

 life of Protozoa, and until such evidence is available the substance should 

 not be classified as a growth factor. 



Growth in Cultures as a Population Problem 



The growth of microorganisms in cultures has been described by 

 Buchanan (Buchanan and Fulmer, 1928) in terms of seven phases: 

 (Fig. 128): (1) Initial stationary phase, during which there is no in- 

 crease in population; (2) lag phase (phase of positive growth accelera- 

 tion), in which the division rate increases to a maximum; ( 3 ) logarithmic 

 growth phase, during which the maximal rate is maintained; (4) phase 

 of negative growth acceleration, in which the division rate decreases 

 steadily; (5) maximum stationary phase, in which the population re- 

 mains practically constant; (6) phase of accelerated death, in which 

 the total population begins to decrease; and (7) a so-called logarithmic 

 death phase, during which the population decreases at a more or less 

 constant rate. 



Little is known about the history of protozoan populations, and com- 

 plete growth curves seem to have been traced for only two species in 

 pure cultures — Varamecimn bursaria (Loefer, 1936b) and Polytoma 

 (Provasoli, 1938c). Loefer's growth curves, comparable to the numbers 

 curve in Figure 128, show in general the phases recognized by Buchanan. 

 Since counts were made at intervals of twenty-four hours or more, an 

 initial stationary phase was not detected in several of the cultures. Total 

 population histories covered from twenty to forty days in different media. 

 Provasoli's curve for Polytoma also shows the general growth phases. 

 Phelps (1935, 1936) traced Glaucoma piriformis well into the maximal 

 stationary phase and observed in most cases the first five of the con- 

 ventional growth phases. On the basis of such evidence, it may be assumed 

 that the growth of Protozoa in pure cultures follows the general trends 

 observed in populations of bacteria and yeasts. 



More information concerning growth of protozoan populations is 

 needed, since interpretation of experimental results may depend upon 

 such knowledge. For example, the addition of a given substance to a 



