258 AQUATIC PHYCOMYCETES 



Rhizophydium melosirae Friedmann 

 Osterreich. Bot. Zeitschr., 99: 179, fig. 1 a-k. 1952 



Sporangia usually borne at the tip of a more or less elongate extra- 

 matrical, delicate stalk, spherical, 1 1-18.3 u. in diameter, wall thin, 

 smooth, colorless, deliquescing completely upon discharge of the zoo- 

 spores; endobiotic part a continuation of the extramatrical sporangia! 

 stalk which penetrates between the valves of the host and forms within 

 a tuft of delicate sparingly branched rhizoids; zoospores nearly spheri- 

 cal, 2.4-3.0 [i in diameter, with an eccentric, colorless globule, the 

 single flagellum 8 [i long, liberated upon the deliquescence of the spo- 

 rangium wall; resting spore (?) irregularly spherical, 12.5-15.0 jj. in 

 diameter, with a thick smooth wall, rhizoids and germination not 

 observed. 



Parasitic on Melosira various, Austria. 



The species is found continuously during the cooler months of the 

 year, but only reaches epidemic proportions as the temperature increases. 



Friedmann (he. cit.) determined that some component of light 

 (very likely the yellow) inhibited dissolution of the sporangium wall 

 and, hence, zoospore discharge. From this he conjectures that under 

 natural conditions zoospore discharge occurs only at night. 



Rhizophydium achnanthis Friedmann 

 Osterreich. Bot. Zeitschr., 99: 181, fig. 1 1-v. 1952 



Sporangium sessile or borne at the tip of a more or less elongate 

 extramatrical, delicate stalk, spherical, 3.5-4.5 \x in diameter, or obo- 

 void to transversely ovoid, 3.3-4.0 x 4.7-6.7 ;jl, wall smooth, colorless, 

 deliquescing completely upon discharge of the zoospores; endobiotic 

 part arising directly from the base of the sporangium or as a contin- 

 uation of the extramatrical sporangial stalk, composed of delicate, 

 sparingly branched rhizoids or a single unbranched one; zoospores 

 spherical, 2.5 jjl in diameter, with an eccentric colorless globule and 

 long flagellum; resting spore not observed. 



Parasitic on Achnanthes affinis, Austria. 



Another instance, as in Rhizophydium fragilariae, of the assumption 

 of the deliquescing sporangium wall habit by a plankton parasite. 



