THE DYNAMICS OF GROWTH 9 



media, and ammonia, in high-nitrogen media, are the compounds most 

 frequently involved. 



In better balanced and more dilute media the principal factor in 

 cessation of growth is exhaustion of the carbohydrate supply (15, 74, 

 79), as shown by the data of Figure 3. 



The extent of autolytic breakdown, once growth has ceased, varies, 

 as mentioned above, with species and cultural conditions. Autolytic 

 breakdown of Aspergillus niger mycelium is much more extensive in 

 nitrate than in ammonium salt media, possibly because of the alkalin- 

 ity of the nitrate medium (15). However, autolysis of a thermophilic 

 Streptomyces sp. is more rapid at acid than at neutral pH (158). 



So much for growth in liquid media. A fungus colony on an agar 

 surface follows a different course. Typically, a period of little or no 

 growth is succeeded by the establishment of a constant growth rate 

 which is maintained unless toxic metabolites accumulate (36, 93, 308). 

 The growth rate tube of Ryan et al. (253) may be used to show this 

 linear relation of extension to time; the data of Figure 4 are typical. 

 Growth on soil should follow the same course if measured by horizontal 

 spread only; specialized rhizomorphs of Armillaria mellea grow several 

 times as rapidly as unorganized mycelium (111). 



Neurospora crassa and other fungi respond to certain toxicants, e.g., 



3 6 



Time, days 



Figure 4. The linear growth of several fungi in growth rate tubes. Curve 1, 

 Neurospora crassa; curve 2, Rhizopus oryzae; curve 3, R. nigricans; curve 4, Sordaria 

 fimicola; curve 5, Fusarium oxysporum lycopersici; curve 6, Monilinia fructicofa. 



