GROWTH 25 



takes place at the expense of reserve materials and protoplasm formed by 

 and stored in the vegetative mycelium. 



PHASES OF GROWTH 



Growth in the fungi, as in other organisms, follows a definite pattern. 

 The way this development takes place depends upon the species and the 

 environmental and nutritional conditions. In the present discussion, it 

 will be assumed that the external conditions are favorable and that growth 

 takes place in a limited volume of medium. 



Unicellular organisms. The bacteriologists have long been interested 

 in the mathematical analysis of the phenomenon of growth. The student 

 is referred to Buchanan and Fulmer (1928) and to Rahn (1932, 1939) for 

 further information on this subject. Among the fungi, the yeasts have 

 somewhat the same type of development as the bacteria. Since bacteria 

 multiply by fission and the yeasts (except Schizosaccharomyces) by 

 budding, we cannot expect the growth pattern of yeasts to fit exactly the 

 same formulas which have been developed for bacteria. But, in a general 

 way, yeasts follow closely the phases of growth shown by bacteria. 

 These phases of growth are as follows: (1) Stationary phase. When cells 

 are inoculated into a medium, there is a period of time following inocu- 

 lation when there appears to be no change in number. The stationary 

 phase may be long or short depending upon the age and vigor of the 

 inoculum, the medium, and other factors. (2) Phase of accelerated growth. 

 Not until cell division is established and new protoplasm is being formed 

 from the constituents of the medium may growth be considered as begun. 

 This phase is characterized by an increase in the rate of cell division, i.e., 

 the generation time is decreasing. (3) Exponential or logarithmic phase. 

 This phase is clearly defined for bacteria and approached by yeasts. It 

 is characterized by a constant generation time. If the logarithms of the 

 cell numbers are plotted against time, the curve is a straight line. (4) 

 Phase of declining acceleration. As the nutrients become exhausted, or 

 as toxic by-products accumulate, the average generation time increases. 

 A combination of these and other factors results in a lessened rate of 

 growth. If fresh medium were continuously supplied and toxic by-prod- 

 ucts removed, it is possible that this phase would never be attained. (5) 

 Maximum stationary phase. This marks the attainment of maximum 

 weight, or numbers of living cells. It is quite likely that the death of old 

 cells is balanced by new growth. The duration of this phase is dependent 

 upon the organism and upon the composition of the medium at this time. 

 (6) Phase of decline or autolysis. Sooner or later, following attainment of 

 maximum development, autolysis sets in. As the cells die, the cellular 

 enzymes begin to digest the various cell constituents. Only the more 

 resistant portions of the cell remain. Microscopic examination at this 



