552 GROWTH 



The growth curves of protozoan populations are sigmoid and re- 

 semble closely in form those of other populations. The growth curves 

 of some Protozoa show all phases. The growth of different Protozoa 

 depends on environmental conditions, and for details the reader should 

 consult the original publications. The size of a protozoan population 

 depends primarily on the amount of available food. Waste products 

 do not limit the growth, as they do with yeast populations, and are in- 

 hibitory only in very dense populations. However, yeast populations 

 contain more organisms than the protozoan populations — Paramecium 

 (Jones, 1928) 10'^; Glaucoma (Phelps, 1936) 7.25 X 10'; Colpoda 

 (Taylor and Strickland, 1938) 60 X 10'; yeast (Richards, 1932) 335 

 X 10^ per ml.; bacteria (Steinhaus and Birkeland, 1939) to 2.5 X 10^ 

 — and laboratory populations of yeast are far less dense than those 

 produced in aerated and cooled commercial fermenters. The total volume 

 of protoplasm (number of individuals, X mean size) should be con- 

 sidered, and metabolic rates known, when comparing populations of 

 different organisms. Under identical conditions P. aurelia and P. cauda- 

 tum produced nearly the same total volume, although there were over 

 twice as many of the smaller P. aurelia. 



So far no selective mortality has been reported for protozoan popula- 

 tion growth, although this is well known in yeast populations. The 

 decline may occur because fewer of the Protozoa reproduce or it may be 

 due to a slowing of the rate of cell division. Jahn (personal communica- 

 tion) believed the latter true for his Euglena populations. Jones reported 

 a disruptive mortality in his P. multimicronucleata populations. No 

 evidence of differences in the sizes and their distribution among Protozoa 

 — which would reveal how homogeneous the populations are from time 

 to time during the population growth — has been given in recent studies, 

 with carefully controlled conditions (e.g., bacteria-free cultures, on syn- 

 thetic media) . Can Protozoa become resistant to an unfavorable medium 

 and remain able to reproduce? Is encystment always governed by food 

 concentration (Taylor and Strickland, 1938), or do other factors have a 

 role? To what extent can an equilibrium population be maintained by 

 en- and excystment? The lack of information on these and many other 

 problems should attract more students of physiology and of growth to 

 protozoology. 



