Sl'.M Al.nv AM) AI.TKIIXATIO.N OF GKNK.IIATIOXS 



15 



iniu'li as solii/.ojiiiny will hv disciisscil fiirtlicr in tlit' 

 ilcMTiption lit" imli\ iiliial spii-ifs in Chapter IN', fur- 

 tluT discussion of the proc'css nt-od not l>c jircscntiil 



IllTC. 



Cytokinesis or division of tin- plasmodiuin or spo- 

 ront into resting spores takes place by cleavajie. and 

 as far as is now known may be closely associated in 

 |)oint of time with the two meiotie divisions. In Soro- 

 xpharra I'l-ronicaf, aceordinj; to Main- and Tison 

 (OS)), cleavaiie begins in tlie late ))rophases of the 

 first division (PI. (i. fig. 33). and by tiie time of the 

 cqu.itorial plate stage, spore mother cells have been 

 completely delimited (fig. 37). These cells divide 

 into two uninucleate segments (fig. 38) in which the 

 second division then takes place (fig. U). At the 

 completion of this mitosis these segments in turn di- 

 vide into the definite spore rudiments (fig. f2). In 

 tills sjiecies at least cytokinesis may follow each 

 mitosis. A similar sequence has been re))orted liy 

 Wingc for Sorodi.icus (PI. 7. fig. 26-30), although 

 the stages do not appear as sharjily defined. Figure 

 27. however, suggests that the sequence varies and 

 that the first division may be complete before cleav- 

 age begins. .Similar variations have been reported for 

 Pla.smod'iophora also. Lutman ('13) and Milovidov 

 ('31 ) found that the first meiotie division is usually 

 comiilete by the time the initial segments are delim- 

 ited. The latter may be uni-. bi-, or multinucleate, 

 and after the second meiotie division has been com- 

 l)leted (PI. 4, fig. 80. 81) they cleave into uninucle- 

 ate spore segments (fig. 82. 83). Cook ('28). Cook 

 and .Schwartz ('30), however, reported that cleavage 

 in Liqiiiera and Pla.smod'iophora does not begin until 

 both divisions have been completed. I-edinghani's 

 ))hotomicrographs suggest the same sequence of 

 events in Poli/mt/xa. In Tetrami/.ra the peripheral 

 Plasmodium first cleaves (PI. 5, fig. 8) into uninucle- 

 ate segments or sporonts (fig. 9-12). Two meiotie 

 divisions occur in these segments, and these mitoses 

 are usually over (fig. 13-17) by the time cleavage 

 into definite spores is complete. In Octomi/j-a large 

 uninucleate segments are delimited in which the two 



meiotie divisions occur, and following the completion 

 of the second divisions, which are quadri])ol.ir, the 

 segments cleave into sjiorcs. a<-eording to \\'liirt'en. 

 \'ery little is known about cytokinesis in the other 

 genera. The time relations of clc.iv;ige to the succes- 

 sive meiotie divisiiins doubtless varies in diliercnt 

 species and probably in the same species, so tli.at 

 under varying conditions it may occur during as well 

 as after meiosis. 



.Marked changes take place in the cytolilasm jirior 

 to cleavage. In /'. /yrn.v.v/cac, according to Nawaschin 

 ('99). Lutman. and .Milovid<iv, the cytoplasm be- 

 comes highly vacuolate (PI. 1. fig. 78) and thus fills 

 the host cell more or less completely. In Poli/mi/.ia, 

 however, Ledingham reported that the cytoplasm be- 

 comes less vacuolate, smaller in volume, and denser, 

 while numerous oil globules emerge and increase the 

 refringency of the plasmodium. According to Lut- 

 man, the denser cytoplasm collects around the nuclei 

 in /'. Brassicae, while the vacuoles fuse and cut the 

 jjlasmodium u]) into uninucleate segments. He con- 

 tended that the process of spore formation in the 

 Plasniodiophorales is quite difl'erent from that de- 

 scribed by Harper (00) for certain myxomycetes, 

 but his figure 3'1 (PI. -i, fig. 79) nevertheless shows 

 a well-defined cleavage furrow progressing between 

 the nuclei. Milovidov's ('31) text-figure 3 likewise 

 shows that large segments are first delimited by fur- 

 rows, and these in turn cleave into uninucleate 

 spores. Progressive cleavage by furrows is also sug- 

 gested by figure 99, Plate 4 of P. Diplantherae, fig- 

 ure 33, Plate 6 of S. J'eronicae, figures 3 and 10, 

 Plate 7 of Sorodiscus karlingii, etc. Contrary to 

 Lutnian's belief, cleavage in the Plasniodiophorales 

 appears to take ])lace by progressive furrowing as 

 in the myxomycetes. Furthermore it does not appear 

 to be simultaneous as Woronin, Nawaschin, and 

 Maire and Tison reported. 



Other cytological details such as cellular relations 

 between host and pathogen are discussed in Chapters 

 IV and VI in connection with the descriptions of in- 

 dividual species and the diseases which they cause. 



Cluipter III 



Sexuality and Alternation of Generations 



Very little is known about sexual reproduction in 

 the Plasmodiophorales, yet most workers have as- 

 sumed that it occurs. .So far, actual fusion of gametes 

 in living material has been observed only in Spoiu/o- 

 spora suhterranea. The evidence of sexuality in the 

 group as a whole is tlierefore largely indirect. It is 

 based on isolated observations of paired amoebae 

 and zoos))ores. binucleate amoebae, the a])l)earance 

 of paired .-ind fusing nuclei in the plasmodium, and 

 j)rimarily on the reported occurrence of meiosis at 

 s))orogenesis. which presup|)0ses a nuclear fusion at 

 sonic stage of development. Inasmuch as there is con- 

 siderable dilTerence of opinion about the time. ])lacc, 

 and nature of plasmogamy and karyogamy in differ- 



ent genera as well as in the same sjiecies. the data on 

 sexuality for each genus will be considered sepa- 

 rately. 



In Plasmod'wphora, Nawaschin ('99) reported 

 the union of several amoebae in the formation of 

 the jilasmodium, but lie did not regard these fusions 

 as having anv sexual significance. Later, Miss Terby 

 ('21), Milovidov ('31), and several other workers 

 expressed the same view concerning the develojiment 

 of the sporogenous iilasmodium. Prowazek ('0.5), 

 however, contended that the incipient spore seg- 

 ments or "sporogametes" fuse in pairs following 

 cleavage (PI. 1, fig. 89), after which the zygote or 

 binucleate spore begins to encyst (fig. 90, 91). One 



