34 



PHYSIOLOGY OF THE FUNGI 



after a shorter or longer period of incubation, a fungus may begin to 

 synthesize these essential metabolites, and growth then takes place in a 

 normal way. This is especially true of the yeasts with respect to vitamins. 

 Many fungi lose their pathogenicity Avhen cultured for a long time on 

 laboratory medium. Host passage frequently restores pathogenicity. 

 The indiscriminate use of inoculum from a variety of substrates and of 

 different ages may introduce unexpected variation in experimental work 

 and should be guarded against. 



Table 5. Daily Increase (in Millimeters) in Diameter of Colonies 



OF Four Fungi 

 Inoculum grown at 20°C.; subcultures incubated at 7.5°C. (From the data of 

 Fawcett, Univ. Calij. {Berkeley) Pubs. Agr. Sci. i, 1921.) 



Species 



Phythiacystis citrophthora 

 Phytophthora terrestris . . . 



Phomopsis citri 



Diplodia natalensis 



1st 

 day 



0.04 

 0.02 

 0.01 

 0.05 



2d 

 day 



0.4 

 0.14 

 0.16 

 1.9 



3d 

 day 



0.6 

 0.21 

 0.83 

 2.1 



4th 

 day 



0.8 

 0.7 

 0.9 



5th 

 day 



1.2 

 0.8 

 1.0 



External factors. Among the external factors which influence the 

 growth of fungi, temperature plays an extremely important role. Tem- 

 perature affects almost every function of the fungi. For each fungus 

 there is a temperature below which it will not grow, the minimum tem- 

 perature. Likewise there is a temperature above which growth ceases, 

 the maximum temperature. These two temperatures indicate the tem- 

 perature range of an organism. A few fungi are capable of growing below 

 0°C., but for most species the minimum temperature is to 5°C. The 

 maximum temperature varies from 27°C. ior Phacidium infestans (Pehrson, 

 1948) and Sclerotinia cameUiae (Barnett and Lilly, 1948) to 45 or 50°C. for 

 Aspergillus fumigatus (Thom and Raper, 1945). The maximum tempera- 



