44 . THE MYXOMYCETES 



germinating spore then liberates a naked, amoeboid swarmer pro- 

 vided with a pair of unequal flagella, anteriorly attached, as 

 shown by Ledingham (1934), Karling (1942), and others. Wo- 

 ronin (1878) in his classical studies of Plasmodiophora brassicae 

 and Cook and Schwartz (1930) described a single anterior flagel- 

 lum, \\-hich is clearly erroneous. The swarm cells are said to fuse 

 in pairs, settle on the epidermis or root hairs, and effect entrance 

 as naked, amoeboid zygotes. Actual fusion of swarm cells in liv- 

 inor material among Plasmodiophorales has been observed only in 

 Spongospora siibterranea. Once this parasitic relationship has 

 become established, the zygote increases in volume, with accom- 

 panying increase in number of nuclei, to become a naked plas- 

 modium. As the host cells divide, the plasmodium may be cut in 

 two and may appear in each daughter cell. 



All divisions in the same plasmodium are simultaneous [Cook 

 (1928)]. Finally, just before sporulation, meiosis takes place, and 

 the uninucleate spores are walled off and remain separate or ad- 

 here in groups characteristic of the genus or species. In Plasmodio- 

 phora and Ligniera the spores remain separate in the host cells; in 

 Tetramyxa they cling in tetrads; in Spongospora they remain in 

 a spongy mass; in Sorosphaera they form a hollow sphere; and 

 in Sorodiscus they aggregate into two layers. 



Flasjiwdiophora brassicae, the cause of club root of cabbage 

 and other cruciferous plants, is the best-known member of the 

 Plasmodiophorales, especially from the studies of Woronin 

 (1878), Maire and Tison (1909, 1911), Lutman (1913), Schwartz 

 (1914), Chupp (1917), Cook (1928), and Cook and Schwartz 

 (1930). Little that is new has been added, however, to the find- 

 ings by Woronin, so accurate were his observations and so pains- 

 taking his attention to details. The swarm cell loses its flagella 

 during penetration, w^hich is accomplished, according to Chupp 

 (1917), by piercing the wall of the root hair. Then the nucleus 

 divides repeatedly, and after 2 or 3 days the plasmodium is of a 

 size to cleave into uninucleate protoplasts. Each protoplast next 

 becomes enclosed in a wall and functions as a gametangium, form- 

 ing 4 to 8 swarmers [Cook and Schwartz (1930)]. These 

 swarmers fuse in pairs, initiating the plasmodium, which increases 

 in size, with repeated simultaneous divisions of its nuclei. Mean- 

 while the invaded cells become hypertrophied. All nuclear di- 



