18 



PLASMODIOPHORALES 



maturation stages of the plasmodium and sporogen- 

 esis, while in Cook's opinion it extends from the time 

 of gametic fusion through scliizogony and "akaryo- 

 sis" to sporogenesis, as is shown in text-figure 7. 



The data relative to sexual reproduction are even 

 more scanty in Sorodiscus. Winge assumed, as he had 

 for all members of the Plasmodiophoraceae, that 

 gametes from germinating spores copulate in pairs 

 and thus initiate the diploid phase of S. Calliirichis, 

 but he never actually observed fusion. Likewise, 

 plasmogaray and karyogamy have not been seen in 

 S. karlingii. In S. radicicolus, however. Cook ('31) 

 figured and described fusion of amoebae in pairs 

 within the host cell (PL 7. fig. 6). The two gametes 

 here figured are unequal in size, but Cook did not say 

 wliether or not this species is heterogamous. His 

 study was made on fixed material sent from South 

 Africa, and figure 6 shows the only case of pairing 

 observed in sudi material. This may possibly repre- 

 sent only a chance association of amoeba without 

 sexual significance. Obviously, additional data are 

 needed before definite conclusions can be drawn 

 about sexuality in Sorodiscus. Cook ('33), never- 

 theless, believed that fusion of gametes occurs in 

 S. radicicolus and that this species has a well-defined 

 alternation of diploid and haploid generations as is 

 shown in text-figure 8. 



In Spongospora suhterranea, Massee, Kunkel, and 

 Osborn reported that the sporogenous plasmodium is 

 formed by coalescence of numerous amoebae, but 

 they were uncertain about the origin and sex of the 

 latter. Home was of the opinion that the amoebae 

 are of opposite sex and that in this respect the plas- 

 modium is similar to that of Diciyostelium muco- 

 roides reported by Skupienski ('18). According to 

 Osborn, coalescence is followed by the akaryote 

 stage during which the nuclei disappear. New nuclei 

 are reconstructed de novo, and these subsequently 

 associate in pairs (PI. 10, fig. 28). Karyogamy soon 

 follows as the nuclear membranes break down at the 

 points of contact (fig. 29). Nuclei which do not pair 

 degenerate the manner described by .Tahn for Cera- 

 tiomi/ja. Home confirmed Osborn's report of kary- 

 ogamy before meiosis but maintained that it occurs 

 during instead of after the transitional or akaryote 

 stage. He did not observe paired and fusing nuclei 

 but based his conclusion on the discovery that the 

 chromosome number following the transitional stage 

 is twice that in amoebae and young plasmodia. Ac- 

 cording to Osborn and Home, the diploid phase of 

 S. suhterranea is quite short in duration and includes 

 only the sporogenous plasmodium, as is shown in 

 text-figure 9. Osborn's, and to some extent Home's, 

 observations and reports of karyogamy shortly be- 

 fore sporogenesis in Spongospora are strikingly 

 similar to the earlier accounts of the nuclear fusion 

 in the myxomycetes. In Ceratiomyxa, Arci/ria, and 

 Trichia,0\he ('07) , Kranzlin ('07), and .lahn ('07, 

 '08) described nuclear pairing and fusion in the plas- 

 modium shortly before resting spores are delimited, 

 but these accounts have subsequently been refuted. 



Cook ('33), on the other Iiand, reported that the 

 zoospores from germinating resting spores pair at 

 the anterior end, retract their flagella, and fuse (PI. 

 10, fig. 20-22). Plasmogamy is followed shortly by 

 nuclear pairing and fusion (fig. 22). The zygote may 

 become flagellate again, and later, apparently, in- 

 fects the host. Its nuclei divide promitotically, ac- 

 cording to Cook, and at the 6- or 8-nucleate stage the 

 zygote undergoes schizogony. Whether or not the 

 meronts later coalesce and thus form the sporogenous 

 Plasmodium is not apparent from this account. Led- 

 ingham ('35) also observed germination of resting 

 spores and formation of biflagellate zoospores, but 

 he found no evidence of gametic fusion. A few binu- 

 cleate zoospores with four flagella were present in 

 Ledingham's cultures (PI. 10, fig. 9), but he was 

 not certain whether these were the product of fusion 

 or incomplete cleavage. Thus, Cook's report of isog- 

 amy has not been substantiated. He nevertheless be- 

 lieved that the diploid generation of this species em- 

 braces the zygote, schizonts, meronts, and plasmodia, 

 while the haploid phase is limited to the cystosori, 

 resting spores, and gametes, as is shown in text-fig- 

 ure 10. The zoosporangia and zoospores found by 

 Ledingham are apparently a means of rapid vegeta- 

 tive multiplication and doubtless relate to the hap- 

 loid phase, as is indicated in this diagram. Barrett 

 found fusion stages between zoospores or gametes 

 derived from zoosporangia in S. Cotulae, but these 

 relate only to fixed and stained preparations. 



In Ligniera, Maire and Tison, and other workers, 

 assumed that plasmogamy and karyogamy take place 

 at some stage of development, because the nuclei ap- 

 pear to undergo reduction at sporogenesis. Cook 

 ('26, '33), however, reported a double fusion and 

 reduction in L. Junci. The zoospores from resting 

 spores fuse in pairs at the anterior end and give rise 

 to diploid ])lasmodia. As noted before, these cleave 

 into uninucleate segments, which develop walls and 

 become incipient zoosporangia. The first nuclear di- 

 vision in these sporangia is meiotie, and the zoo- 

 spores or gametes subsequently produced are hap- 

 loid. These fuse in pairs and form the diploid sporo- 

 genous Plasmodium in which meiosis later occurs at 

 sporogenesis. Ligniera Junci thus has two diploid 

 phases each of which is separated by a haploid ])hase, 

 according to Cook, as is illustrated in text-figure 11. 

 Cook neither observed plasmogamy and karyogamy 

 nor counted the chromosomes at meiosis, so that he 

 had no direct evidence for his assumption. It is not 

 improbable that the zoosporangia and zoospores are 

 merely means of vegetative multiplication without 

 sexual significance and relate to the haploid genera- 

 tion, provided an alternation does occur, in much t!;e 

 same manner as is indicated in text-figure 10 of S. 

 suhterranea. 



No direct evidence of gametes, gametic fusion and 

 karyogamy have been observed in Memhranosorus, 

 Poli/?ni/^-a, Octomyjra, and the doubtful genera, 7?/;;'- 

 somi/xa, Anisomij.ra, and Sorolpidium. Ledingham 

 found a few tetraflagellate binucleate zoospores in 

 Poh/mi/.ra, but he was not certain whether these were 



