LIFE CYCLES, SEXUALll Y, AND SEXUAL MECHANISMS 47 



Haploid Cycle wuh Risiricikd Dicarvon 



A prcdoniinnnrlv haploid cycle, which differs from the one dis- 

 cussed above by the separation in space and time of plasmogamy and 

 caryogamy, is characteristic of members of the higher Ascomycetes 

 such as Neiirospora. At the time of the fusion of the sexual cells or 

 organs one or more dicaryotic pairs of nuclei are formed, and these, 

 by repeated mitotic divisions in the ascogenous hyphae, provide 

 paired nuclei for a large number of ascal primordia within which 

 caryogamy and meiosis occur. The multiplication of associated nu- 

 clei, though often extensive, is nevertheless restricted both in time 

 and by the complete dependence of the ascogenous hyphae upon the 

 haploid mycelium. The nature of the dicaryotic phase here would 

 suggest for this type of cycle an evolutionary position intermediate 

 between the exclusively haploid cycle and the more complex cycles 

 to be found among the Basidiomycetes. 



Haploid-Dicaryotic Cycle 



The predominant life cycle in the Basidiomycetes, excluding 

 many of the smuts, differs from the cycle just discussed by the unre- 

 stricted and independent growth of the dicaryotic phase. Both the 

 haploid, or homocaryotic, phase and the dicaryotic phase are com- 

 pletely independent and capable of indefinite vegetative growth and 

 are terminated by dicaryotization and fruit body formation respec- 

 tively. The termination of each phase depends upon the achievement 

 of certain requirements which is, in each case, largely a matter of 

 chance. The cycle may therefore be considered to comprise two 

 roughly equivalent phases and terminate in a single diploid nuclear 

 generation, the fusion or definitive nucleus in the basidium. 



A number of cases have been described among these forms in 

 which differentiated spores produced by the dicaryotic mycelium 

 re-establish the haploid phase (Brodie, 1931; Nobles, 1942). This oc- 

 curs through the separation of the members of conjugate pairs of 

 nuclei in the formation of uninucleate conidia or oidia. These spe- 

 cialized cells appear to attain their greatest effectiveness as fertilizing 

 (dicaryotizing) agents, although germination in low percentage does 



