THE DYNAMICS OF GROWTH ' 



occurs only at hyphal tips (272, 273, 280). Interior cells of the myce- 

 lium do not normally contribute to the new growth directly, although 

 they supply nutrients to peripheral cells, especially to aerial structures 

 in still culture. Growth in any one time period is therefore a function, 

 not of the total number of cells present, as in bacteria and yeasts, but 

 of the number of hyphal tips and of the rate at which these tips are 

 supplied nutrients. 



Some of the chemical events during the phase of rapid growth are 

 illustrated in Figure 3; these include especially utilization of carbo- 

 hydrate, nitrogen, and phosphate. Metabolic products, e.g., acids, 

 may or may not appear in the medium at this time. Characteristically, 

 respiratory activity is at a maximum (Chapter 7). 



Few studies have considered critically the relation of reproduction 

 to growth phases; casual observation and the data of Robinson (248) 

 on Sporodinia grandis indicate that in still culture spores usually de- 

 velop at the end of the phase of rapid growth; spores may, however, 

 appear earlier (209). 



The third and last phase is, as shown in Figure 3, characterized usu- 

 ally by a decline in mycelial weight and in the appearance of nitrogen 

 and phosphate in the medium. The loss in weight varies from neg- 

 ligible to extreme; a common pattern is a rapid loss of weight for a 



40 



Time, hours 



Figure 2. The growth of Neurospora crassa as a function of time. Curve 1, loga- 

 rithmic plot; curve 2, cube root plot; curve 3, linear plot. Redrawn from Emerson 

 (86), by permission of the Williams and Wilkins Company. 



