Myxophyceae 57 



with heterocysts; reproduction by means of vegetative division, hormo- 

 gones and gonidia. 



P'amily V. Rivulariaceae. Filaments tapering from the base to the apex, 

 ending in a multicellular, colorless hair; heterocysts usually present, basal; 

 reproduction by means of vegetative division, hormogones and gonidia. 



Family I. OSCILLATORIACEAE 



Filaments frequently branched, containing one or more trichomes; 

 sheaths variable, more or less gelatinous; trichomes consisting of a simple 

 row of cells uniform along their entire length; except for the apical cells 

 which sometimes taper more or less; heterocysts absent; reproduction by 

 means of vegetative division and hormogones. 



I Sheaths not present. 



1 Trichomes straight or nearly so, never forming a regular spiral 



(i) Trichomes cylindrical, usually without sheaths, free; apex of trich- 

 ome straight or curved Oscillatoria 



(2) Trichomes cylindrical, without sheaths, united in free-swimming 

 scale-like masses Trichodesmium 



2 Trichomes forming a regular, more or less lax spiral 



(i) Trichomes multicellular Arthrospira 



(2) Trichomes unicellular Spirulina 



II Sheaths present. 



1 Filaments simple or branched; sheaths cylindrical, firm; trichomes 



single within the sheath; apex of trichome straight 

 (i) Filaments simple, more or less agglutinated by their mucous 

 sheaths Phormidium 



(2) Filaments simple, free, free-floating or forming a matted mass 



Lyngbya 



(3) Filaments often branched, forming erect tufts; false branches 



solitary Symploca 



(4) Filaments simple; sheaths usually purple or flesh-colored; apical 



cell not capitate Porphyrosiphon 



2 Filaments frequently branched; sheaths firm, lamellose, transparent 



or colored; trichomes several within the sheath 

 (i) Sheaths more or less mucous, colorless, diffluent; trichomes few 

 within the sheath; apex of trichome capitate 



Hydrocoleus 



(2) Filaments prostrate, woven "into a solid membranaceous mass, 



often slightly branched; sheaths solid, always thin, colorless; 

 plants terrestrial or aquatic Hypheothrix 



(3) Filaments prostrate at the base, above forming erect tufts; sheaths 



solid, transparent; plants terrestrial Symplocastrum 



(4) Filaments tufted, often much branched; sheaths transparent or 



scarcely colored; plants low, aquatic Inactis 



