82 



PLASMODIOPHORALES 



flows into tlie female, and its nucleus migrates to- 

 wards that of the female gamete. Furthermore, the 

 latter gamete carries a positive charge and has a 

 low oxidation-reduction potential, while the male 

 gamete is the opposite in these respects. Kambly 

 ('39), however, was unable to confirm these results 

 of Abe, and found no marked physiological differ- 

 ences between swarm cells of various species. Gil- 

 bert ('35) and Stosch ('37) likewise reported that 

 the male gamete may be distinguished during fusion 

 by the migration of its nucleus toward that of the 

 female. Such differences have not been reported for 

 the Plasmodiophorales as far as the author is aware. 

 In the Myxomycetes the gametes fuse by their pos- 

 terior ends, while in the Plasmodiophorales, accord- 

 ing to Cook ('33), they fuse at the anterior ends. 

 However, so little is known about gametic union in 

 this order that it is premature to regard the latter 

 type of fusion as characteristic of the Plasmodiopho- 

 rales. 



Comparison of the two groups on the basis of 

 time and place of sex segregation is also impossible 

 at present, because little is known about sexuality 

 in the Plasmodiophorales. No monospore cultures or 

 infections have yet been made to determine whether 

 the species are homo- or heterothallic. If, as Cook 

 ('33) maintained, the gametes are the direct prod- 

 uct of uninucleate spores and no division occurs in 

 amoebae and zoospores, sex segregation obviously 

 takes place during one of the meiotie divisions be- 

 fore or during sporogenesis. Otherwise, it is pheno- 

 typically determined in the haploid generation, and 

 the species are accordingly haplosynoecious. In the 

 Myxomycetes also, there are but few data relating 

 to sex segregation. Skupienski ('17) believed that 

 in D. difforme it occurs during one of the divisions 

 in the zoospores. Miss Clay ley reported its occur- 

 rence at the second meiotie division in the zoospores 

 of D. tiif/ripes. Schiinemann confirmed her rejjort of 

 haplophenotypic sex segregation in tliis s])ecies and 

 described D. nigripes as haplomonoecious. Miss Cad- 

 man, however, noted no differences, morphological or 

 physiological, between the gametes in lieficularia 

 and D. nigripes and concluded that no sex segrega- 

 tion is necessary or takes place in these species. 

 Stosch, on the other hand, implied by his statement 

 concerning crosses in D. eiinigripes that sex is geno- 

 typically determined. 



As to the time and place of karyogamy, meiosis, 

 and alternation of liaploid and diploid generations 

 in the Plasmodiophorales, a detailed account of these 

 subjects has been given in Chapter III. As is evident 

 from this description, the majority of workers have 

 assumed that the isomorphic gametes fuse in pairs, 

 after which karyogamy soon occurs. Nuclear fusion 

 in the zygote thus initiates the diploid phase which 

 includes the plasmodial stage up to the last two nu- 

 clear divisions preceding or during cleavage where 

 reduction occurs. Plasmogamy and karyogamy arc 

 accordingly not followed at once by meiosis. The 

 haploid })l!ase includes the cystosori, spores, zoo- 

 spores, and gametes, according to this viewpoint. 



However, as noted before, exceptions to this view 

 have been presented by Prowazek, Osborn, Home, 

 Webb, and Whiffen. 



In the Myxomycetes likewise there is considerable 

 disagreement and controversy concerning karyog- 

 amy, meiosis. and alternation of generations. Much 

 of the controversy about meiosis hinges upon the 

 question of whether one or two divisions occur prior 

 to spore formation in the fruiting bodies. Strasburger 

 ('84.). A. Lister ('93), Rosen ('93), Harper ('00), 

 .7ahn ( '07-36 ), Kranzlin ('07), Gilbert ("3.5), and 

 Stosch ('3.5, '37) found only one. while Wilson and 

 Cadman ('28), Cadman ('31), and Schiinemann 

 ('3.5) reported two divisions. In contrast it may be 

 noted here that most workers on the Plasmodiopho- 

 rales are in agreement that two divisions precede 

 spore formation. However, in order to draw com- 

 parisons between the two groups with respect to 

 meiosis, it is essential to outline briefly the differences 

 of opinion concerning this question in the Myxomy- 

 cetes. 



In the Exosporae, Olive ('07a) found stages re- 

 sembling syna]>sis in the young spores of Ceratin- 

 7ni)j-a and later ('07b) on observing pairing and fu- 

 sion of nuclei in the pillars, concluded that the two 

 mitoses in the spores of this genus are meiotie. 

 Olive's conclusions on pairing and fusion of gametic 

 nuclei were confirmed in general by Jahn ('07) who, 

 however, held that these processes occur earlier as 

 the Plasmodium creeps out of the wood. On the other 

 hand, he refuted Olive's contention that meiosis oc- 

 curs in the spore and claimed instead that the two 

 divisions which precede cleavage are reductional. 

 The incipient uninucleate spores are accordingly 

 haploid. Jahn ('08) reasserted his observations on 

 nuclear pairing and fusion, but maintained that only 

 one, instead of two, division occurs prior to cleavage. 

 This division is heterotypic, according to .Tahn. and 

 reduction is thus accomplished by one division. The 

 Plasmodium is formed by the fusion or coalescence of 

 numerous haploid myxamoebae, the nuclei of which 

 divide mitotically several times in the plasmodium. 

 Karyogamy is accordingly delayed until the plas- 

 modium creeps out to fructify. In 1911, however. 

 Jahn concluded that his previous observations on nu- 

 clear pairing and fusion in the mature plasmodium 

 were incorrect and that the appearances of karyog- 

 amy were the results of nuclear degeneration. His 

 observations of endosporus species led him to the be- 

 lief that nuclear fusion follows plasmogamy of amoe- 

 bae. Jahn ('1 1, '33, '36) nonetheless persistently ad- 

 hered to his early view that meiosis occurs during the 

 last division before cleavage, as is shown in text- 

 figure 1 2. Gilbert ( '3.5 ) , on the other hand, confirmed 

 Olive (m meiosis in the spore and in addition showed 

 that the haploid motile gametes fuse posteriorly in 

 pairs to initiate the plasmodium (text-figure 13). He 

 also found that karyogamy follows plasmogamy 

 within 21' hours, thus refuting Olive's observations 

 but confirming Jahn's later view. 



In the Endosporeae, .lahn (07) reported the same 

 tvjje of nuclear pairing and fusion in the young fruit- 



