No. 460.] STUDIES ON PLANT CELL.— V. 253 



consequently they are not sisters. The two nuclei in the ascus 

 then fuse. The origin of the original pair is not known. 



No satisfactory explanation of this fusion in the ascus has 

 been advanced. The conditions in the Ascomycetes are not the 

 same as in the Basidiomycetes. There is no series of paired 

 nuclei in the ascogenous hyphae and no evidence of a delayed 

 fusion of gamete nuclei following a sexual act nor of nuclear 

 fusion§ associated with the apogamous development of a sporo- 

 phyte generation. On the contrary, a sexual act ' with the 

 ■ fusion of gamete nuclei has been clearly established in some 

 forms preliminary to the development of the ascocarp and the 

 nuclear union in the ascus is plainly a supplementary phenom- 

 enon. Wager and Harper point out analogies to the account of 

 Chmielewski ('90b) for Spirogyra, considered in a previous part 

 of this section, which described a double nuclear fusion in the 

 zygospore. Thus the primary, sexually formed nucleus of the 

 zygospore is reported to divide into four secondary nuclei, two 

 of which break down while the remaining two unite forming the 

 second and final fusion nucleus of the spore. It is hard to see 

 how these second nuclear fusions can be sexual and Groom ('98) 

 is perhaps correct in considering them superimposed on the sex- 

 ual act, but their physiological significance is not clear. 



Some recent papers support in general Harper's investigations 

 on the ascus. Guilliermond (: 04a ; : 04b) describes the devel- 

 opment of the ascus and ascospores in a number of forms. In 

 an unnamed species of Peziza he found, however, that the ascus 

 developed from the terminal cell of the ascogenous hypha which 

 received two nuclei (that fuse) of the four that are found at the 

 tip. Maire (: 03a ; : 03b) has reported a similar history for 

 Galactinia succosa. Both Maire and Guilliermond note the 

 resemblance of these conditions to the nuclear associations in 

 the young basidium and Maire does not hesitate to consider the 

 two nuclei in the tip of the ascogenous hypha as much reduced 

 synkaryons, (paired nuclei) appearing for a very short period 

 just previous to the nuclear fusions in the ascus. Maire fol- 

 lows Dangeard in denying the sexual processes described by 

 Harper in the Ascomycetes and would allign the events in the 

 ascus with those in the basidium. Guilliermond agrees with 



