LAGENIDIALES 921 



OLPIDIOPSIDACEAE 



Thallus endobiotic, holocarpic, without a specialized vegetative sys- 

 tem, sometimes appearing naked at first ("plasmodial") and somewhat 

 amoeboid, later definitely walled, infection tube not persisting, the walls 

 generally giving a cellulose reaction, usually forming a single reproduc- 

 tive structure, contents with refractive globules, often strongly vacuolate; 

 sporangium liberating its spores by bursting the wall or forming one or 

 more discharge tubes ; zoospores formed in the sporangium, mono- or 

 diplanetic (dimorphic), biflagellate; resting spores endobiotic, thick- 

 walled, apparently asexually formed from the thallus or produced after 

 conjugation of the receptive thallus with one or more small contributing 

 thalli, with or without periplasm and fertilization tubes, upon germi- 

 nation forming zoospores. 



Parasites of other water fungi and parasitic or saprophytic in algae. 



The family includes all endobiotic holocarpic species which form a 

 single sporangium or, rarely, a linear series of sporangia, from the 

 thallus and produce, endogenously, biflagellate zoospores. When more 

 is known about the sexual process, if any, in Pseudosphaerita and 

 Pseudolpidium it may seem better to place these genera elsewhere. 



Scherffel (1925a) called attention to the resemblance of Olpidiopsis to 

 Lagenidium, first noted by Zopf (1884). This is found not only in the 

 similarity in flagellation of the spore, the composition of the walls, and 

 the behavior and structure of the contents, but also in the tendency 

 toward diplanetism (dimorphism) of the zoospores and the likeness of 

 sexual reproduction. 



The thallus in Olpidiopsis bears at first sight little resemblance to that 

 in Lagenidium and Myzocytium. In their body form, however, Lagena 

 and Lagenidium oophilum approximate a species of Olpidiopsis, and 

 Pontisma and Sirolpidium closely approach Lagenidium. The essential 

 differences between the two groups appear to rest in the Pythium type 

 of diplanetism of a relatively small number of large zoospores in the 

 Lagenidiaceae and the Saprolegnia or Achlya type of diplanetism of a 

 large number of small zoospores in the Olpidiopsidaceae. These do not 

 seem of sufficient import when compared with the similarities to main- 

 tain the groups as distinct orders. 



