PLASMODIOPHO RALES 773 



spores. These are found as a rule in groups of eight, each group being 

 made up of two tetrads of resting spores." 



Goldie-Smith in her study of a Pythium parasitized by a Sorodiscus 

 (cokeri) reports that, "The plasmodia develop chiefly in the sporangia of 

 the host, causing little or no hypertrophy. Cruciform nuclear divisions take 

 place during vegetative growth. At maturity, a sorus of zoosporangia or 

 resting spores is formed. The zoosporangia are thin-walled, spherical or 

 ellipsoid, forming heterokont zoospores, each with a long posterior and a 

 short anterior flagellum. Exit papillae are formed only by some of the 

 sporangia on the periphery of the sorus. The zoospores formed in the 

 interior of the sorus escape by passing through narrow pores formed 

 in contiguous walls, from one sporangium to another, and thus to the 

 outside. In its developmental stages this organism closely resembles 

 Octomyxa. It is characterized, however, by its cystosori, which are 

 composed of one-layered discs of resting spores. Usually there are two 

 discs in a cystosorus, closely associated to form a spore-cake. Frequently 

 there are three or four, forming Y- and H- shaped figures, respectively, 

 in sectional view. This species is limited in its host range to Pythium, of 

 which three other species have been successfully infected." 



SYSTEMATIC ACCOUNT 



PLASMODIOPHO RALES 



Microscopic parasitic aquatic or semiaquatic fungi, frequently pro- 

 ducing hypertrophy of the host; the thallus endobiotic, holocarpic, 

 without specialized vegetative structures, naked and more or less 

 amoeboid or surrounded by a delicate wall, sometimes dividing into 

 several daughter thalli (meronts, schizonts), the membranated thallus 

 forming at maturity one or more thin-walled zoosporangia (game- 

 tangia?), the unwalled thallus becoming fragmented into a cluster or 

 aggregation of thick-walled resting spores which produce zoospores at 

 germination ; zoospores biflagellate, with one long posteriorly directed 

 flagellum and a short anteriorly directed one, both of the whiplash 

 type; sexuality, in species in which it is known, by fusion of motile 

 amoeboid or biflagellated isogamous gametes, the zygote becoming a 

 naked thallus, which at maturity forms the resting spores. 



