GROWTH 533 



were followed by Eddy (1928). Counts were made with a Sedgewick- 

 Rafter cell, but the results were not published, beyond general statements 

 of sequence and dominance. Light had no effect on the sequence. Tem- 

 perature exerts its influence by way of the bacteria serving as food for 

 the Protozoa. Oxygenation of the culture increased the growth, espe- 

 cially at the bottom of the culture. Too great concentrations of carbon 

 dioxide were deleterious and could be buffered by including soil in the 

 culture. The sequence was effected by the quantity and type of the in- 

 fusion material. Dominance of a species was believed to depend on favor- 

 able growing conditions for that species, rather than on the rate of re- 

 production [cf. Woodruff, above). 



Unger (1931) has listed the sequence of Protozoa for two years in 

 five cultures started from five different plants. 



Laboratory cultures, not restricted to a single species, show a regular 

 series of population growths and declines for different species. The na- 

 ture of the culture, its bacterial flora, and the reproductive potential of 

 each species regulate the period of intensive growth, and the accumulat- 

 ing excretion products of the animals bring about the decline of the 

 population. The growth cycle of a species may modify the medium so 

 that it becomes favorable for the growth of the next following species. 

 Limiting conditions are oxygen and carbon-dioxide concentrations, pH, 

 and temperature, and these will be discussed later. Remarkable flower- 

 ings of Algae and Protozoa in the ocean and in lakes have been re- 

 ported and are apparently on a more intense scale than occurs in labora- 

 tory cultures. Some bacteria are inadequate as food sources; others are 

 poisonous for some Protozoa; and the rise of a population of these 

 bacteria would eliminate the susceptible Protozoa in the culture. Poisonous 

 bacteria have been reported by Hargitt and Fray (1917) and by Kidder 

 and Stuart (1938). The Protozoa commonly studied in the laboratory 

 are apparently less frequently found in natural habitats. The growth of 

 protozoan populations in mixed mass cultures is different from that of 

 most other organisms, as no equilibrium is reached and maintained; 

 instead, extinction seems to be the rule. 



AUTOCATALYSIS AND AlLELOCATALYSIS 



The theories of T. B. Robertson have greatly influenced the study 

 of growth, and the first of these has been concerned primarily with the 



