Chapter 11 



PHYSIOLOGIC SPECIALIZATION AND VARIATION 



AMONG FUNGI 



The concept that physiological differences exist between the 

 members that together constitute a given species of fungus prob- 

 ably has its origin in bacteriology. In the early years of bacteri- 

 ology there were two opposing schools of thought, one of which 

 held to the monomorphic hypothesis and the other to the poly- 

 morphic or pleomorphic hypothesis. Adherents of the mono- 

 morphic hypothesis believed in fixity and immutability of species; 

 adherents of the polymorphic hypothesis, in variability in mor- 

 phological and physiological characteristics. Billroth (1874), 

 representing an extreme of the polymorphic group, for example, 

 believed that only one species of bacteria, "Coccobacteria 

 septica" existed. 



To the person who compares a considerable number of isolates 

 of any one fungus, especially when grown on artificial media, it 

 quickly becomes apparent that the species is variable and that 

 differences exist between the several isolates. Although these dif- 

 ferences may be so minute as to be morphologically indistinguish- 

 able, they are none the less real and of tremendous importance, 

 especially as they concern pathogenic species. In fact, problems 

 of virulence of species, of their aggressiveness, of the outbreak 

 of epidemics, and of the breeding of crop plants that are resistant 

 or immune to attack, all hinge upon the fact that these differences 

 are meaningful and must be taken into account. Questions con- 

 cerning the origin of these differences have been regarded too 

 largely as of only academic interest. Actually no useful purpose 

 is served by assigning them to the academician. 



Definition of terms. The concept embodied in the term 

 physiologic species has changed since Schroeter (1879) first sug- 

 gested that physiologic specialization in fungi exists. He observed 

 that Paccinia graminis, growing on wheat, failed to produce in- 



257 



