10 



PLASMODIOP MORALES 



that the polar vesicles contract to small globular 

 areas surrounded by granules from which astral rays 

 radiate, as noted elsewhere. Thus the vesicles them- 

 selves do not become the nuclei, but tlie daughter nu- 

 clei are formed in the areas occupied by the vesicle 

 before contracting. 



The type of division described by these three 

 workers is distinctly mitotic and, except for the be- 

 havior of the nucleole, according to Home's and 

 Webb's figures, is fundamentally similar to nuclear 

 division in the higher plants. Miss Terby, as noted 

 before, held that the nucleole also undergoes the 

 same changes as in the higher plants, so that there is 

 no difference in this respect either. On the other 

 hand, the divisions figured by Home and Webb are 

 also similar to tlie promitoses illustrated in figures 1 

 to 23. The chief difference is the presence of chromo- 

 somes. It is not improbable, as Webb contended, that 

 the earlier workers overlooked the early prophases 

 and the origin of the chromosomes and that their 

 fixation and staining technique did not differentiate 

 chromosomes in the equatorial ring. As noted else- 

 where, the nuclei of the Plasmodiophorales are quite 

 small, and their structure is difficult to interpret. The 

 use of more specific and refined technique in inten- 

 sive study of the early prophases and equatorial ring 

 stages may thus possibly eliminate the present con- 

 troversy on the nature of the vegetative divisions. 



In tliis relation it is to be noted tliat typical mito- 

 sis without large nucleoli has been reported in the 

 vegetative zoosporangial stage of Ligniera, Plasmo- 

 diophora, Polf/mt/J-a, and Ociom_i/xa by Cook ('26, 

 '28), Cook and Schwartz ("30), Ledingham ('39), 

 and Miss Whiffen ('39). In these as well as other 

 genera the zoospores from germinating resting 

 spores develop into plasmodia which eventually 

 cleave into uninucleate segments — the rudiments of 

 zoosporangia. These segments develop walls, and 

 their nuclei divide twice to several times in a strictly 

 mitotic manner in preparation for zoospore forma- 

 tion. Cook and Schwartz reported that up to tlie time 

 of cleavage into zoosporangial segments the nuclei 

 in the plasmodia of Ligniera and Plasmodiophora di- 

 vide promitotically, but in Polymyxa I^edingham re- 

 ported that division in the thalli which form zoospo- 

 rangia is mitotic from the start. Miss Whiffen also 

 found that the divisions in the zoosporangia of Octo- 

 miiia are mitotic. These authors thus reported a 

 regular alternation of mitosis and promitosis. The 

 zoosporangial stage is characterized by mitosis, then 

 follows a phase of premitotic division in the early de- 

 velo))ment of the sporogenous plasmodium which is 

 terminated by the so-called transitional stage, and 

 finally two meiotic divisions. Inasmuch as the divi- 

 sions in the zoosi)orangia are mitotic and very simi- 

 lar to the two divisions at sporogenesis. Cook ('26, 

 '28, '33) and Fedorintschik ('3.5) concluded that 

 they are meiotic in Ligniera and Plasmodiophora, 

 respectively. In P. Brassicae, however. Cook and 

 Schwartz described them as merely mitotic. In an 

 attempt to explain the alternation of meiosis and pro- 

 mitosis in this species, they proposed the theory that 



promitosis is characteristic only of diploid nuclei, a 

 theory which is contradicted by their own observa- 

 tion that the first meiotic division of the diploid nu- 

 cleus in spore formation is indirect and not pro- 

 mitotic. Furthermore, if Cook's ('28, '33) previous 

 report is correct that the primary nucleus of the in- 

 cipient zoosporangia in Ligniera is diploid (and un- 

 dergoes meiosis), it should accordingly divide pro- 

 mitotically. However, he described and figured such 

 nuclei as dividing mitotically. 



The report of typical mitotic divisions during zoo- 

 spore formation, promitosis in the developmental 

 stages of the sporogenous plasmodium, and the re- 

 occurrence of mitosis during the reduction divisions 

 nevertheless raises numerous questions on the signifi- 

 cance of this alternation (if it actually does occur), 

 and it is thus obvious that future studies of karyo- 

 kinesis in the Plasmodiophorales must be closely 

 correlated with the various developmental phases. 



"Akaryote Stage" 



The period of vegetative divisions in the develop- 

 ment of the sporogenous plasmodium is reported to 

 be followed shortly by the so-called "enucleate." 

 "akaryote," "chromidial" or "transitional" stage. 

 According to most workers, this phase is charac- 

 terized by a reduction in size and disappearance of 

 the karyosome, comparatively empty, vacuole-like 

 nuclei, and the presence of numerous deeply-stain- 

 able bodies or chromidia in the cytoplasm around the 

 nuclei. Nawaschin first observed this stage in Plas- 

 modiophora in 1899. and since that time it has been 

 reported by most subsequent students for the other 

 genera of this order. In the opinion of many cytolo- 

 gists it is thus as constant and diagnostic a character 

 of the Plasmodiophorales as promitosis. 



Stages in the development of the akaryote stage 

 are shown in Plates 2 to 13, which illustrate the life 

 cycles of all the plasmodiophoraceous genera, and 

 will not be illustrated separately at this point. After 

 the vegetative divisions have been completed, the 

 karyosome decreases in size as the somatic or tro- 

 phochromatin is extruded into the cytoplasm in the 

 form of secretory chromidia, according to Prowazek 

 ('05), Maire and Tison ('09), and others. Maire and 

 Tison regarded this extrusion as a cleansing process 

 by which the generative chromatin is separated from 

 the nutritive chromatin in preparation for the sporo- 

 gonic divisions which follow. As a result of this ex- 

 trusion, the nuclei, when stained with haematoxylin. 

 appear comparatively empty and devoid of stainable 

 material and frequently have the appearance of vac- 

 uoles in a cytoplasm filled with deeply-stained chro- 

 midia. 



According to Blomfield and Schwartz, Schwartz, 

 and Osborn, extrusion of chromatin in Sorosphaera, 

 Liqniera, and Spongospora takes place along the 

 linin threads until the chromatin reticulum and 

 karyosome have disappeared. These workers be- 

 lieved that the nuclear membranes also disappear 

 during this stage. In L. Jiinci, Schwartz described 

 the process as follows: "the nuclear membrane dis- 



