MYXOGASTRES 45 



visions are equational except the penultimate one [Lutman (1913), 

 Cook and Schwartz (1930), and Cook (1933)]. 



Spongospora siibterranea, the cause of powdery scab of potato 

 tubers and of lesions on the underground portions of tomato 

 stems, is the only other species of this order that is of economic 

 importance. It has been known in Europe for approximately 100 

 years and was apparently introduced from the Andean home of 

 the potato. 



The spores of S. subterranea germinate readily, each forming a 

 single uninucleate amoeba. Infection is accomplished at or near 

 the "eyes." Some workers have maintained that infection is ac- 

 complished by separate amoebae, but Kunkel (1915) presented 

 evidence that invasion comes from the action of a plasmodium 

 (zygote). Osborn (1911), on the other hand, has described uni- 

 nucleate amoebae in young potato cells. His studies show the 

 manner of enlargement of plasmodia and the formation of the 

 sponge-like mass of spores. While proof of sexuality is lacking, 

 there is evidence that nuclei fuse and that reductional division 

 occurs just before sporulation. 



Information concerning Sorosphaera can be gained from the 

 report of Blomfield and Schwartz (1910), concerning Ligniera 

 from the report of Cook (1926), and concerning Sorodiscus from 

 the report of Wernham (1935). 



Generic differences among the Plasmodiophorales are insuffi- 

 cient and too slight in taxonomic value to incline Palm and Burk 

 (1933) to retain the six usually recognized genera. On the other 

 hand. Cook (1933) in his taxonomic treatment of the group rec- 

 ognizes 6 genera and 14 species. He emphasizes the importance 

 of fusion of swarm cells by their anterior end among Plasmo- 

 diophorales rather than by their posterior end, as occurs in the 

 slime molds proper. Karling (1942) employs 8 genera in his 

 monograph of the order, which includes a complete host index 

 and bibliography. 



Maire and Tison (1909) stressed the point that the Plasmo- 

 diophorales are a distinct group, an opinion with which most my- 

 cologists are in accord, but whether they should be placed phy- 

 logenetically between the Sporozoa and the Myxogastrales or 

 considered primitive chytrids remains a controversial question. 



Myxogastres. This subclass comprises the orders Physarales, 

 Stemonitales, Liceales, and Trichiales. It includes about 400 spe- 



