544 GROWTH 



on well fed and on starved Varamecimn, and showed that the inadequate 

 growth of the Didin'mm restricted to a diet of starved Paramecium was 

 due to a qualitative deficiency, rather than to a shortage of food. Mond 

 (1937) reported that Infusoria grown in known concentrations of 

 Bacillus coli and B. subtilis grow in a linear relation to the available 

 food. The same amounts of bacteria were used for each division of the 

 Infusoria. Such studies will permit the determination of the amounts 

 of energy used for the growth process and for maintenance of life. When 

 enough data become available, Wetzel's (1937) methods may be used 

 and the resulting data would aid in evaluating his theory of growth. 



The knowledge of the nutritional requirements of the Protozoa is 

 increasing rapidly and suitable methods for growing bacteria- free pure 

 culture of a number of species are now available. It will be difficult to 

 decide what is the optimum culture medium for a given species. The 

 lack of trace elements may appear only after a period of years. Super- 

 optimal media will give an increased rate of growth which may not be 

 best for the species (McCay, 1933). Pearl's (1928) generalization that 

 the length of life is inversely correlated with the rate of living must be 

 remembered when experimental conditions are devised either to yield a 

 maximum amount of Protozoa in a given time, or to provide an op- 

 portunity for the study and perpetuation of the species under the most 

 favorable conditions. 



The Growth of Population 



Adequate measurement of population growth should include the fol- 

 lowing information, as well as the number of organisms present at a 

 given time, per unit of environment: food concentration; the concentra- 

 tion of excretion products, pH, rH, oxygen, and carbon-dioxide concen- 

 trations; temperature; the amount of light, when light-sensitive organ- 

 isms are used; and the effects of other species, when mixed populations 

 are used, on the species measured and on its environment. Few studies 

 approach this degree of completeness. The earlier studies made no attempt 

 to measure the food concentration, when bacteria were the main source 

 of organic food. 



A few Protozoa inoculated into a limited amount of an adequate 

 culture medium, soon begin to increase in numbers and continue to do so 

 until a maximum yield is produced. The course of the population growth 



