42 PHYSIOLOGY OF THE FUNGI 



metabolism. Chemical agents may favor or prevent similar morphologi- 

 cal changes. Trichophyton ruhrum produces two metabolic products of 

 unknown constitution which inhibit the transformation of Candida 

 albicans to the mycelial form (Jillson and Nickerson, 1948). The addi- 

 tion of excessive amounts of inositol to the culture medium causes 

 Ophiostoma (Ceratostomella) muUiannulatum to grow almost entirely in 

 the form of conidia (Fries, 1949). The morphology of the vegetative 

 mycelium and sporangia of various species of Phytophthora was found to 

 depend upon the medium used (Leonian, 1925). 



The form of mycelial growth of many species, when grown on agar 

 media, is an aid in identification. The colony form may be altered 

 beyond recognition when cultures are grown in agitated liquid medium. 

 In general, spherical colonies or balls form in agitated medium. Burk- 

 holder and Sinnott (1945) investigated colony form of a large number of 

 species when subjected to agitation. 



The acidity of the medium affects the size and shape of the vegetative 

 cells of some fungi. In a medium so acid as to allow only very slow 

 growth the cells often become swollen or nearly spherical in shape, much 

 like chlamydospores, but the wall remains thin (Fig. 7). This may be 

 accompanied by excessive branching. 



Unusual environmental conditions often affect the morphology of both 

 vegetative and reproductive structures. The environment which exists 

 in coal mines is unnaturally uniform with respect to temperature, mois- 

 ture,'and absence of light. Basidiomycetes growing on old mine timbers 

 either fail to fruit or produce odd-shaped sterile fruit bodies (Fig. 8). 



SUMMARY 



Normal growth results in an increase in cell number and mass. Limited 

 growth may result from either of these two processes alone. Growth is a 

 phenomenon which requires time for its various manifestations. Growth 

 follows a pattern which differs from species to species, but the general 

 sequence of phases is much the same for all fungi. Growth studies are 

 based upon measuring both the amount and the rate of growth. The rate 

 and amount of growth are controlled by the internal and external environ- 

 ment. The potentialities of a fungus are limited by its genetic constitu- 

 tion, but the expression of these potentialities is controlled by external 

 factors such as temperature, light, composition, and concentration of the 

 medium. Even the size and shape of the culture vessels used affect the 

 rate and amount of growth. 



The amount of growth can be estimated by visual comparison or meas- 

 ured by determining the diameter of a colony or by harvesting the myce- 

 lium and weighing it after drying to constant weight. The amount of 

 yeast growth may be measured by counting the numbers of cells produced, 



