28 



PHYSIOLOGY OF THE FUNGI 



almost useless in others. At least these measurements can be made in an 

 objective way. In this method, the diameter, radius, or area of a colony 

 is used to express the amount of growth, while the daily increase repre- 

 sents the rate of growth. It is obvious that this method neglects the 

 thickness of the colony. Worley (1939) has proposed to take the thick- 

 ness of the mycelium into account when growth is measured by this 

 method. Such measurements are difficult and neglect the mycelium 

 buried in the agar. The rate of linear growth of some fungi has little 

 relation to the composition of the medium. The rapid extension of 

 mycelium on water-agar medium may serve as a familiar example. 



It has been frequently assumed that fungi grow at a constant rate when 

 maintained under constant environmental conditions. This assumption 

 is not necessarily true, for the growth of Aspergillus rugulosiis and many 

 other fungi is self-limited under cultural conditions. Two factors may 

 contribute to cause nonuniform rates of growth: (1) the change in con- 

 centration of nutrients due to diffusion and utilization; (2) the excretion 

 of inhibitory metabolic products into the medium. 



The same fungus may have a constant rate of growth at one tempera- 

 ture and not at another. The rate of growth is frequently not constant 

 when fungi are cultured at temperatures higher than optimum. Fawcett 

 (1921) found the rate of growth of Phytiacystis citrophthora, Phytophthora 

 terrestris, Phoviopsis citri, and Diplodia natalensis to decrease with time 

 when these fungi were cultivated above the optimum temperature. Some 

 of Fawcett's data which illustrate this phenomenon are given in Table 4. 



Table 4. The Effect of Temperature upon the Rate of Growth 



OF Three Fungi 

 The daily increase in the average radius of the colonies is given in milhmeters, 

 (From the data of Fawcett, Univ. Calif. {Berkeley) Pubs. Agr. Sci. 4, 1921.) 



If the rate of growth under a given condition does not change with 

 time, this method is useful and simple. It permits observation of the 

 same culture for the duration of the experiment. Ryan et al. (1943) have 

 proposed the use of an ingenious growth tube in which linear growth can 



