PLASMODIOPHORALES 771 



the host cell. A few resting spores are formed early in the course of the 

 infection, but by far the greater number are produced somewhat later, 

 when the roots are very heavily diseased. The Plasmodium which event- 

 ually produces the resting spores is unwalled, in contrast to that forming 

 the zoosporangia. As it grows and becomes multinucleate by protomi- 

 tosis it gives rise to long radiating threadlike pseudopodia, which may 

 be retracted after a time (Fig. 57 F). The thallus then may fragment 

 into a group of secondary thalli or "meronts" (Fig. 57 G). Occasionally, 

 fusion of distinct plasmodia occurring in the same host cell was observed 

 (Fig. 57 H). It could not be determined whether these plasmodia were 

 meronts derived from the same or from different thalli. As the Plasmo- 

 dium matures, its contents become denser (Fig. 57 I), lines of cleavage 

 form around each nucleus, and the walls of the resting spores are laid 

 down (Fig. 57 J). Investigations of the nuclear condition in the maturing 

 thallus before resting-spore formation indicated that the akaryote stage 

 was recognizable. By the use of certain stains (Newton's iodine-gentian- 

 violet) it could be demonstrated, however, that the chromatin was not, 

 in fact, extruded, but that it merely failed to react to the usual stain 

 (iron-alum haematoxylon). No nuclear fusions, as reported by Webb 

 (1935) in Sorosphaera were observed. Just previously to cleavage of the 

 cytoplasm ordinary division figures (not of the promitotic type) were 

 formed, but whether meiosis took place could not be determined. 



In summing up Ledingham's results, it is evident that he regarded 

 the formation of the thin-walled zoosporangia as an essential and in- 

 tegral part of the life history of Polymyxa. He also intimated that they 

 precede the production of resting spores. Further, he showed from his 

 own studies on Polymyxa and Spongospora and from those of Cook 

 (1926) and Cook and Schwartz (1930) on Ligniera and Plasmodiophora, 

 that differences in the zoosporangial stages of these genera are well 

 marked and may, in fact, prove of greater taxonomic worth than the 

 method of aggregation of the resting spores (Palm and Burke, 1933; 

 Wernham, 1935). Although earlier Cook had discovered the sporan- 

 gial stage and mentioned it in connection with those species in 

 which it occurs, he made little taxonomic use of it, nor did he incorpo- 

 rate it into the life cycle of the Plasmodiophoraceae. 



Kole (1954) reinvestigated the morphology, cytology, and life history 



