120 PHYCOMYCETEAE 



Key to the Genera of Family Olpidiopsidaceae 



Zoospores escaping through discharge tubes. 

 Zoospores monomorphic (monoplanetic). 

 Zoosporangia spherical to eUipsoid, mostly with one discharge tube. 

 Resting spores produced, usually by a sexual process. Olpidiopsis 

 Resting spores unknown. Pseudolpidium 



Zoosporangia irregularly lobed or tubular, usually with more than one dis- 

 charge tube. Resting spores unknown. Petersenia 

 Zoospores dimorphic (diplanetic), encysting at the mouth of the discharge tube 

 and then escaping singly. Zoosporangia spherical or nearly so. Fertiliza- 

 tion of oogone by a conjugation tube. Oogone with one egg and peri- 

 plasm. Pythiella 

 Zoospores escaping by a large irregular break in the wall of the spherical or 

 elUpsoidal zoosporangium. Resting spores not certainly known. 



Pseudosphaerita 



Key to the Genera of Family Sirolpidiaceae 



Thallus elongate, narrowly tubular, becoming septate and then disarticulating 

 into separate zoosporangia. Sirolpidium 



Thallus elongate, broadly tubular, becoming septate into distinct zoosporangia 

 which remain attached. Pontisma 



Key to the More Important Genera of Family Lagenidiaceae 



Thallus elongated, not becoming septate, sometimes coiled, parasitic in roots of 

 grasses and other plants. Zoospores formed in a vesicle at the tip of the single 

 exit tube. Sexual reproduction by union of contents of adjacent thalli through 

 a conjugation tube to form a thick-walled resting spore. 



Lagena 

 Thallus short or elongated, not septate, sometimes branched, in the cells of 

 diatoms. Primary zoospores encysting at mouth of exit tube and emerging 

 as zoospores of the secondary type. 

 Thallus short, not branched, primary zoospores flagellate. Sexual reproduction 

 by union of adjacent thalli through a permanent conjugation tube, the empty 

 male thallus remaining attached. 



Edrogella 

 Thallus elongated and coiled, sometimes branched. Primary zoospores not 

 flagellate. Resting spores formed here and there within the elongated thallus 

 but no sexual process observed. 



Aphanomycopsis 

 Thallus more or less rounded, parasitic in the exterior cells of Phaeophyceae or 

 Rhodophyceae. Resting spores not observed. 

 Sporangia becoming extramatrical at maturity. Primary zoospores encysted 

 at the mouth of the exit tube or more often against the inner surface of the 

 zoosporangium. Upon emergence they are of the secondary type. 



Eurycha&ma 

 Sporangia remaining intramatrical with many discharge tubes. Primary 

 zoospores encysting near the mouths of these tubes and emerging as sec- 

 ondary zoospores. Eurychasmidium 

 Thallus elongated and coenocytic, mostly later divided into multinucleate seg- 

 ments by cross septa. If the latter are wanting perhaps the species should be 

 transferred to the genus Lagena. Each segment becomes a zoosporangium 



