GROWTH 33 



There is no reason to suppose that mutants produced in the laboratory- 

 differ fundamentally from those isolated in nature. 



The potentiahties of a fungus are limited by its genetic constitution. 

 The realization of these potentialities may be denied or favored by the 

 external environment, and only as the environment is suitable do these 

 inherent factors find expression. Diversity, rather than uniformity, in 

 behavior among species and isolates is the rule. 



Only a small amount of inoculum is used in most studies. It is impor- 

 tant to learn if the age, history, or kind of inoculum has any effect on the 

 subsequent development of the fungus. All these factors may influence 

 the rate and amount of growth and other functions of the fungi. Young 

 and vigorously growing inoculum is most suitable, since old cells as a 

 general rule are slow to start growth. Apparently one of the first func- 

 tions a cell loses is the power of division. From this standpoint such cells 

 are "dead," although they may be still capable of performing many vital 

 functions, such as respiration. Difficulty is frequently experienced in 

 making subcultures from old cultures. Certain species are difficult to 

 maintain in culture unless they are frequently subcultured. In general, 

 these species do not readily form resting cells. Among these are various 

 species of Pythium and Phytophthora, test-tube cultures of Choanephora 

 cucurhitarum, and others. 



In experimental work of the highest precision neither the temperature 

 nor the medium upon which the inoculum is grown may be neglected. 

 Zikes (1919) investigated the generation time of six strains of yeast and 

 found that the storage temperature of the inoculum affected the time 

 required for cell division. These original cultures were grown at 8°C. and 

 25°C., and subcultures were incubated over a range of temperatures. 

 When the inoculum which was grown and stored at 8°C. was subcultured 

 at low temperatures, the generation time was less than that of the culture 

 grown and stored at 25°C. At temperatures above 25°C. the generation 

 time of the high-temperature yeast was less than that of the low-tempera- 

 ture yeast. In some way, yeast cells cultured over long periods of time 

 at a certain temperature become adapted to this temperature, and when 

 such cells are transferred to other temperatures, the influence of the 

 original temperature of incubation persists for a time. It is evident that 

 some change in the internal environment has occurred. 



Comparable studies on the filamentous fungi are rare. From Fawcett's 

 data on the rate of linear growth of four citrus pathogens it appears that 

 the same phenomenon takes place with some filamentous fungi. Fawcett 

 grew the inoculum at 20°C., and on subculturing at 7.5°C. the linear rate 

 of growth increased with time, as is shown in Table 5. 



Many fungi have latent abilities to synthesize various essential metab- 

 olites. In the virtual absence of these compounds in the medium and 



