448 BULLETIN 387 



In a blackened root the only additional change that can be recognized 

 is the presence of hyphae. This blackness is almost a true criterion of 

 the effects of a fungus, for the bacteria seldom, if ever, produce any 

 pronounced discoloration (fig. no, c). 



It is not altogether a new phenomenon to find other organisms following 

 parasitic slime molds. For example, the earlier writers who described 

 Sorosphaera Veronica regarded it as a rust because of the mycelial threads 

 which, according to these investigators, are constantly present. Maire 

 and Tison (1909) prove with but little difficulty that the fungi are merely 

 saprophytic attendants. The case is almost identical with that which 

 Schwartz (1914) cites for species of Ligniera with which typical mycorrhiza 

 are continual associates. 



It has been shown that 'non-parasitic myxomycetes undoubtedly make 

 use of bacteria. It seems, therefore, altogether reasonable that when 

 a facultative saprophyte is grown under conditions to which Spongospora 

 subterranea was subjected by Kunkel (1915), it will assume the habits 

 of a saprophyte. As far as this discussion is concerned, the only question 

 is whether Spongospora subterranea still utilizes bacteria when in the 

 potato tuber. 



Objection may be found to each of the above experiments taken alone. 

 When considered together they cover the subject thoroly enough, and 

 coincide so fully in their results that it seems logical to draw the con- 

 clusion that Plasmodiophora Brassicae has no need for the bacteria and 

 that the latter are merely attendant saprophytic forms which incidentally 

 help to set free the spores of the parasite. Only two factors favor Pinoy's 

 theory. One is the presence of bacteria in most roots in which any con- 

 siderable swelling has taken place ; the other, the fact that there is a smaller 

 number of different species of organisms present than might have been 

 expected. Almost invariably the rod-shaped bacterium forming opalescent 

 colonies on nutrient agar was the only one isolated. The facts, however, 

 that spores can germinate in sterilized media, that infection can occur 

 on seedlings in test tubes on nutrient agar where no bacterial colonies 

 are present, and that recently infected roots never show bacteria either 

 when tested in culture or under the microscope after staining, would 

 seem to offset any evidence that heretofore has been adduced to the 

 contrary. Therefore it seems evident that Plasmodiophora Brassicae is 

 an obligate parasite, and, as such, needs no other food supply than that 

 furnished by its host. 



SUMMARY 



Neither the motility of swarm-spores nor the action of winds is an 

 important factor in the dissemination of Plasmodiophora Brassicae. 



