CHYTRIDIALES 121 



Thallus, at least in some phases, polycentric 



Sporangia and resting spores formed on the same endobiotic 



polycentric thallus Cladochytriaceae, p. 460 



[Sporangia and resting spores formed on separate tha'li; 

 sporangia epibiotic, thallus monocentric; resting 

 spores endobiotic, thallus polycentric Physodermataceae] 1 

 Sporangium opening by the dehiscence of an operculum 



Series Operculatae, p. 486 

 Thallus monocentric, epi- and endobiotic, or endobiotic 



Chytridiaceae, p. 486 

 Thallus polycentric, completely endobiotic, or endobiotic and 



extramatrical Megachytriaceae, p. 580 



Inoperculatae 



OLPIDIACEAE 



Thallus endobiotic, holocarpic, without a specialized vegetative sys- 

 tem, converted as a whole into a single inoperculate sporangium 

 or a resting spore; zoospores formed in the sporangium, posteriorly 

 uniflagellate, generally with a single globule; sexuality, where known, 

 by fusion of posteriorly uniflagellate planogametes, the zygote after 

 penetration forming an endobiotic resting spore which upon germina- 

 tion functions as a zoosporangium. 



A large primarily aquatic family, the members of which are found 

 as parasites and saprophytes of fresh-water algae, fresh-water and 

 marine Phycomycetes, microscopic animals, and plant spores. Several 

 species are also known as parasites of flowering plants, notably 01- 

 pidium brassicae (Woronin) Dang, (see Woronin, 1878: 557), O. viciae 

 Kusano (1912), O. trifolii Schroeter (see Kusano, 1929), O. agrostidis 

 Sampson (1932). 



Undoubted instances of a sexual act preceding resting-spore for- 

 mation have been found by Kusano in Olpidium viciae and O. trifolii 

 and by Sawada (1922) in O. bothriospermii (see Olpidium, p. 128). 



KEY TO THE GENERA OF THE OLPIDIACEAE 



Zoospore with the flagellum subapically attached, the body bearing 



an anterior ring of refractive granules .... Olpidiomorpha, p. 122 



1 See page 483. 



