THE MEASUREMENT OF GROWTH 11 



sity of non-mycelial growth phases is rendered more likely by the vari- 

 ety of stimuli which induce the phase in different fungi (Table 1). 



Several theories, reviewed elsewhere (152, 262), have been proposed 

 to explain dimorphism in the fungi pathogenic to animals. The most 

 attractive concept is that synthesis of new protoplasm and cell division 

 are separate processes. Filaments form when growth occurs under 

 conditions not permitting rapid cell division. The studies of Nickerson 

 and co-workers (152, 211, 212, 214, 215, 217) indicate that a supply of 

 reduced organic sulfhydryl compounds is essential for rapid cell divi- 

 sion and hence for maintenance of the unicellular condition. At least 

 in principle, all the stimuli known to affect dimorphism in the patho- 

 genic fungi could act through their effect on sulfhydryl content of the 

 cells. Whether this will apply to the non-pathogenic fungi listed in 

 Table 1 is not known. 



The yeast phase of Blastomyces dermatitidis and Paracoccidioides 

 {Blastomyces) brasiliensis is multinucleate, that of some other di- 

 morphic fungi uninucleate (82). The yeast and mycelial forms of 

 Blastomyces spp. differ in their respiratory capacities (213). 



Dimorphism in yeastlike fungi suggests, of course, that the true yeasts 

 may have arisen by a more or less permanent acquisition of the uni- 

 cellular habit. It is also perhaps legitimate to ask whether the fila- 

 mentous habit in the true fungi and the algae is an expression of the 

 same metabolic idiosyncrasy as the filamentous habit of Blastomyces 

 and Histoplasma. 



5. THE MEASUREMENT OF GROWTH 



As mentioned earlier, the meaning of growth is determined by the 

 method of measurement chosen. Some methods are useful only for 

 particular organisms or for special problems; no method is so general 

 that it can be recommended for all. 



The most widely used and within limits the most satisfactory meas- 

 urement of growth is by determination of dry weight of the mycelium. 

 The principal limitation is that weight may reflect accumulation of 

 polysaccharides or other reserve materials rather than synthesis of new 

 protoplasm. Oxidative assimilation (Chapter 7) is often very active 

 in the fungi under conditions which restrict protein synthesis. 



Various procedures may be used for the determination of dry weight. 

 Often a coherent mycelium can be removed from a liquid medium, 

 washed, and dried in a tared container. More commonly filtration is 

 required; tared filter paper, protected from atmospheric moisture, or 

 porous discs (ceramic or sintered glass) held in a demountable suction 



