CHYTRIDIALES 55 



related group. In an interesting discussion of the "interrela- 

 tionships of the Protista and the primitive fungi" Cavers (1915: 

 168) expresses a similar point of view but recognizes the Plas- 

 modiophorales as a separate group, preferring not to include 

 them in either the Myxomycetes or Chytridiales. The student 

 will find in his article a wealth of information concerning the 

 Protozoa which is not available in the general mycological 

 literature. 



Key to Genera of Plasmodiophoraceae 



I, Spores at maturity not united; lying free in the host cell. 



1. Plasmodiophora, p. 55 



II. Spores at maturity remaining attached to one another in some definite 

 type of aggregation. 



A. Spores clinging together in tetrads or dj-ads. 



2. Tetramyxa, p. 59 



B. Spores united in larger numbers to form more or less definite spore 



balls. 



1. Spore balls typically spherical to ellipsoidal, and hollow, 



consisting of a peripheral layer of spores enclosing a cen- 

 tral cavity. 



3. Sorosphaera, p. 60 



2. Spore balls in the form of flat two-layered plates with a 



central cavity small or lacking. 



4. Sorodiscus, p. 63 



3. Spore balls sponge-like, essentially solid, lacking a central 



cavity but traversed by prominent fissures. 



5. Spongospora, p. 64 



1. Plasmodiophora Woronin (1878). 



The genus Plasmodiophora was founded by Woronin on the 

 single species PI. hrassicae Wor., which he had shown to be the 

 cause of clubroot of cabbage and other cruciferous plants (Fig. 1). 

 Subsequently a considerable number of imperfectly known organ- 

 isms have been placed in this genus by various writers, but no 

 one of them can be retained in it with assurance. It seems best 

 for the present to regard the genus as monotypic, and to base 

 the discussion on the classical paper of Woronin and on the 

 results of certain more recent investigations on PI. hrassicae. 



The disease caused by this species is well known on cabbage, 

 and occurs less commonly on turnip, cauliflower, rutabaga, 

 Brussels sprouts, and other similar cultivated and wild hosts. 

 It results in pronounced malformation of the roots, and has 



