12 The North American Cup-Fungi 



although the spores of the inoperculate forms are often multi- 

 cellular. 



5. Alternation of Generations 



Assuming that the sex organs do function in the cup-fungi 

 and taking as an illustration the best known form, Pyronema 

 omphalodes, it is interesting to trace the alternation of genera- 

 tions as we know it in the fungi. The sporophytic generation 

 would begin with the pairing of the nuclei in the ascogonium 

 and continue through the ascogenous hyphae up to the reduction 

 division in the ascus. The gametophytic generation, initiated 

 at the conclusion of reduction division, is continued through the 

 ascospores, mycelium, and up to the formation and fusion of 

 the trichogyne with the antheridium, and the pairing of the 

 nuclei in the oogonium. 



6. Heterothallism and Homothallism 



While heterothallism has been observed and established in 

 the Phycomycetes and some of the higher Basidiomycetes, it is 

 only recently that it has been demonstrated in the ascomycetes, 

 including some of the species of operculate Discomycetes. 

 Dodge states that in Ascobolus magnificus he was unable to 

 obtain sex organs from one spore culture while they were 

 abundantly formed on the mixing of two strains. In this case 

 the sex organs were not produced in a definite line where the 

 strains come together but the mycelia mixed so that they were 

 produced throughout the culture but more abundantly where the 

 two strains came first into contact. 



Under the direction of Dodge, Edwin Betts has recently 

 demonstrated heterothallism in Ascobolus carbonarius (Am. Jour. 

 Bot. 13: 427-432. 1926). While it was found impossible to 

 produce ascocarps from single spore cultures, they could be 

 produced by the intermingling of the mycelia from different 

 spores, even those from the same ascus. From this it was con- 

 cluded that sex factors are segregated at the time of the asco- 

 spore formation in the ascus and that probably four of the spores 

 are of one sex and four of the other. 



It has also been shown that in some species of ascomycetes 

 which have only four spores in the ascus, two nuclei, one of each 

 sex, may be included in the same spore. When such spores are 

 germinated they produce a homothallic or monoecious mycelium, 

 while a spore originally containing only a single nucleus will 



