24 



PLASM ODIOPHOR ALES 



Cook and Schwartz, nonethek-ss, discovered a 

 hitherto unknown stage in tlie life cycle of P. Brassi- 

 cae. The amoebae derived from flaoellate zoospores 

 penetrate root hairs, grow in size, and by regular 

 mitosis become multinucleate plasmodia which soon 

 cleave into uninucleate portions. These segments 

 round up. develop thin hyaline walls, and become in- 

 cipient zoosporangia (Hg. 37). In view of this dis- 

 covery, it seems probable that the small cleaving 

 Plasmodia which Cliupp described and figured (p. 

 •i36, fig. 104H) as stages in resting spore formation 

 in root hairs relate to the development of zoosjioran- 

 gia. The nuclei of the zoosporangia (fig. 38) divide 

 mitotically two or three times, after which the proto- 

 plasm cleaves into uninucleate segments (fig. 39— 

 42). forming thus four to eight pyriform zoospores 

 (fig. 43). These are smaller than those derived from 

 resting spores, according to Cook and Schwartz, 

 who regarded tliem as gametes. As the walls of the 

 sporangia collapse the zoospores emerge, fuse in 

 pairs, either in root hairs or after migrating into the 

 cortex, and form zygotes which grow into diploid 

 sporogenous plasmodia, as has been described in 

 Ciiapter III. 



Fedorintschik confirmed Cook and Schwartz's dis- 

 covery of zoosporangia. He reported that individual 

 amoebae in root hairs develop directly into large 

 plasmodia containing up to 100 or more nuclei. These 

 plasmodia cleave into uninucleate segments which 

 develop walls and become rudimentary zoosporan- 

 gia. The first division of their nucleus is meioiic, 

 and then follow a second and sometimes a third 

 mitosis, after which the protoplasm cleaves into four 

 to eight zoospores. A single amoebae in a root hair 

 may, according to Fedorintschik's observations, ulti- 

 mately result in the formation of 400 to 800 zoo- 

 spores. These zoospores become amoeboid and mi- 

 grate into the cortical tissues and multiply rapidly 

 by budding. After the content of the host cell is ex- 

 hausted, they fuse, presumably in pairs (?), and 

 later develop into plasmodia. Fedorintschik believed 

 this fusion constitutes the sexual phase of P. Brassi- 

 cae, and thus confirmed Cook and Schwartz's earlier 

 report of sexual reproduction in this genus. In light 

 of Ledingham's ('3.5, '39), Couch, Leitner, and 

 Whiffen's ('39) studies on Sponc/ospora, Poli/mi/.ra, 

 and Octoviifxa, however, it seems more probable that 

 these so-called gametes are only secondary zoospores 

 which reinfect the host and give rise to an additional 

 amoebae and plasmodia in much the same manner as 

 is indicated in text-figure 10 of Spongospora. 



Several workers have reported that tlie Plasmo- 

 dium of P. Brassicae may undergo schizogony and 

 give rise to a few or several meronts, whereby the 

 parasite is rapidly multiplied. Nawaschin ('99) did 

 not actually observe the ))roeess, but he believed that 

 the large number of small thalli in a host cell could 

 be explained only on the assumption that they had 

 arisen by division of a preexisting thallus. He 

 thought that the extended pseudopodia of the Plas- 

 modium were cut off as buds, a belief which was later 

 supported by I.utmau and by Henckel ('23). Subse- 



quently, Maire and Tison ('C9). Chupp, Kunkel, 

 Jones ('28), and Cook ('33) also reported schizog- 

 ony of the Plasmodium of P. Brassicae. It must be 

 noted, however, that many of the early described 

 cases of schizogony in the superficial host cells may 

 possibly relate to the development of zoosporangia. 

 As is shown in figure 36, the meronts may be uni- or 

 multinucleate, and it is not improbable that after a 

 period of growth they in turn may function as schi- 

 zonts and form secondary meronts. 



With the exception of Cook and Schwartz and 

 Fedorintschik who reported that the sporogenous 

 Plasmodium is formed by the fusion of two gametes, 

 many investigators who studied this phase of devel- 

 opment were of the opinion that the plasmodium 

 arises by the union of several vegetative amoebae or 

 small Plasmodia. Woronin was uncertain whether it 

 originates from a single amoebae or by the fusion of 

 several, although he thought the latter method more 

 plausible. Eycleshymer ('01) observed that if a 

 slide with zoospores and amoebae is kept in a moist 

 chamber, larger ])lasmodia appear, which he assumed 

 had arisen by fusion of amoebae. Honig, however, 

 maintained that the amoebae observed by Evcles- 

 hynier do not relate to P. Brassicae. Halsted ('93) 

 also believed that amoebae coalesce to form large 

 Plasmodia. Nawaschin ('99) tliought that the schi- 

 zonts and meronts remain more or less independent in 

 the host cell until shortly before sporogenesis, when 

 the}' flow together and form a large plasmodium. 

 He admitted also that single amoebae may grow in- 

 dependently into large plasmodia. Gaylord. Ericks- 

 son ('13), Esmarch ('24), Prowazek, and Terby 

 ('24) supported Nawaschin's belief on the union of 

 amoebae, but INIaire and Tison ('09) refuted this 

 contention. They pointed out that although meronts 

 and schizonts may appear to be fused, they are none- 

 theless separate and distinct. They based their view 

 primarily on the lack of synchronism in nuclear di- 

 vision in the closely associated amoebae and plas- 

 modia in the same host cell. Lutman, Chupp, and 

 Kunkel were uncertain about the union of amoebae, 

 but Lutman noted that the nuclei in a plasmodium do 

 not all divide simultaneously, which suggests that 

 they may have been derived from several amoebae 

 of different ages. Later, Jones ('28a, '28b) also re- 

 ported fusion of amoebae and ])lasmodia in cultures 

 of P. Brassicae, but there is considerable doubt about 

 the validity of the organism he had in culture. In ad- 

 dition to describing the origin of the plasmodium 

 from a zygote. Cook and Schwartz reported that in 

 the early stages of development several amoebae and 

 later small plasmodia may fuse vegetatively to form 

 the incipient sporogenous plasmodium. Since that 

 time Milovidov ('31) also reported vegetative fusion 

 of several amoebae. 



The plasmodium of /'. Brassicae is capable of slow 

 amoeboid movement, and this mobility apparentlv 

 enables it to move from cell to cell. Rochlin reported 

 that the plasmodium first sends out a hyaloplasmic 

 thread (Geissel) in the direction of movement, and 

 shortly thereafter tlie more ajranular mass begins to 



