LIFE CYCLES, SEXUALIl V, AND SEXUAL AlECIL\NISiMS 43 



of rhc peculiar hcnctirs offered by tlic fungi because of the variety 

 of sexual and cultural characteristics which they possess. Equally 

 important, of course, is an awareness of the limitations to their use. 



No comprehensive, up-to-date source of such information is now 

 available. This paper presents, in outline form, a comparative review 

 of sex in fungi which might serve as a preliminary sketch for a source 

 of this kind. 



Essential sexual processes may be defined as those processes req- 

 uisite to and including the juxtaposition and fusion of compatible 

 nuclei and the subsequent sorting out of genetic factors in meiosis. 

 These processes impose a cyclic progression of which plasmogamy, 

 caryoo-amy, and meiosis are the irreducible cardinal events. The cycle, 

 however, may be basically varied in three different ways: (1) by 

 variations of the temporal relationships between the cardinal events 

 by the intercalation, at different stages, of essential processes of 

 growth; (2) by the imposition of genetic restrictions upon universal 

 compatibility; and (3) by variations in the mechanical means of ac- 

 complishing the cardinal events. 



These three modes of variation determine three distinct facets of 

 sexuality, all separately definable but inextricably interrelated in the 

 living plant: (1) life cycle, (2) basic pattern of sexuality, and (3) 

 the sexual mechanism per se, respectively. Each facet is understand- 

 able only as a time-integrated and dynamic process. A detailed exam- 

 ination of each facet brings to light a number of facts w^hich are little 

 known but which are of considerable biological interest and are 

 essential to an appreciation of the broad implications of sexuality in 

 the fungi. 



LIFE CYCLES 



The fungi are commonly considered organisms which are essen- 

 tially haploid — perhaps with nuclear fusion occurring now and again 

 to give rise to a diploid phase which persists for a single nuclear gen- 

 eration. Although this is true of many species, particularly among the 

 more primitive groups, the regular occurrence of exceptions to this 

 simple pattern among the lower groups and the various complexities 

 of the life cycle patterns characteristic of the more highly evolved 

 forms make such a generalization meaningless. Life cycles among 

 fungi run the gamut from completely haploid at the one extreme to 

 completely diploid, minus the immediate products of meiosis, at the 



