1915] 



ATKINSON PHYLOGENY IN THE ASCOMYCETES 335 



way of the descent of the Ascomycetes from the Florideae. 

 Some (Bessey, E., '13, p. 151) have attempted to overcome this 

 difficulty by suggesting the homology of the ascus and tetra- 

 sporangium. But this effort leads to so many suppositions 

 and supporting hypotheses because of the fundamental dif- 

 ference between the process of spore formation in the ascus, 

 and the processes of carpospore or tetraspore formation, that 

 the descent of the ascus fungi from the red algae would re- 

 quire a far more labyrinthian course than would be necessary 

 in deriving them from the Phycomycetes. 



NOTE III 

 IS NUCLEAR FUSION IN THE ASCUS OF A VEGETATIVE OR SEXUAL 



NATURE f 



It is unfortunate that there is such great divergence of 

 opinion in the interpretation of the nuclear phenomena in the 

 archicarp and ascogenous threads. These conflicting results 

 are probably, in a large measure, due to the difficulties pre- 

 sented in the minute size of the nuclei. The divergence of 

 opinion relates primarily to the question as to whether the 

 fusion nucleus of the ascus is the result of two successive 

 nuclear fusions, the first taking place in the ascogonium and 

 the second in the ascus, or whether the nuclear fusion in the 

 ascus is the only one. 



The principle of a single nuclear fusion, that in the ascus, 

 interprets this act as the final stage in the process of fertiliza- 

 tion, by the fusion of two nuclei of more or less remote an- 

 cestry. At some time prior to ascus formation these two 

 nuclei may possibly become associated in pairs into a syn- 

 karion and multiply in the ascogenous threads by conjugate 

 division, or the synkarion and conjugate division may be post- 

 poned to the ascus hook and the complicated series of fusions 

 between the ultimate and antepenult cells of the crozier, or 

 proliferations of the young ascus with accompanying con- 

 jugate divisions of the synkarion. 



Dangeard ( '94) first described the presence of two nuclei in 

 the young ascus, and their fusion, in several species {Borrera 

 ciliaris, Peziza vesiculosa, Helvetia ephippium, Geoglossum 



