770 AQUATIC PHYCOMYCETES 



parasites of Phycomycetes, can presently be given. The need applies in 

 particular to the obligate parasites of vascular plants, both aquatic and 

 terrestrial; for that reason and because such fungi do not fall within the 

 defined limits of this treatise they are not considered here. It is not 

 necessary to enumerate all the existing elements of confusion concerning 

 this group of fungi. They are discussed in the previous edition of this 

 book and by Karling (1942d). It is sufficient to emphasize that with 

 respect to a very important diagnostic feature, the type of zoospore, the 

 investigations of Ledingham (1939), Ellison (1945), and Kole (1954) 

 show beyond question that species of Plasmodiophora, Spongospora, 

 Sorosphaera, and Polymyxa, parasites of vascular plants, possess two 

 apically attached fiagella of very different lengths. Moreover, the 

 flagella, in contrast to those of all other bifiagellate zoospores, are 

 both of the whiplash type. Practically the same type of zoospore is 

 formed by the plasmodiophoraceous parasites of water molds, species 

 of Octomyxa (Couch, Leitner, and Whiffen, 1939; Pendergrass, 1948, 

 1950), Sorodiscus (Goldie-Smith, 1951), and Woronina (Goldie-Smith, 

 1954, 1956a). A heterocont zoospore is also typical of many of the 

 true Myxomycetes (Elliot, 1949), which gives weight to the theory that 

 this group has affinities with the Plasmodiophorales. 



The life-history studies of Ledingham (1939) on Polymyxa graminis 

 led him to lay considerable emphasis on the sporangial stage, so long 

 overlooked. Although not able to establish the precise connection be- 

 tween the resting and sporangial stages by direct observation, he furnish- 

 es abundant evidence that they are, indeed, phases of the same organ- 

 ism. The following taken from his account substantiates this. 



If wheat plants are grown in sterilized soil which has been inoculated 

 with finely pulverized roots containing resting spores of Polymyxa, great 

 numbers of thin-walled, septate, multinucleate thalli, which even in their 

 youngest stages are surrounded by a delicate wall, develop in the roots. 

 At maturity each of these segments becomes converted into a zoosporan- 

 gium that is provided with a discharge tube (Fig. 57 E, p. 769). This 

 tube eventually reaches the exterior, and through it numerous bifiagel- 

 late zoospores (Fig. 57 A-D), exactly like those produced at the ger- 

 mination of the resting spore, are discharged. Their further fate could 

 not be determined, although the evidence indicated that they penetrate 



