Order: Filamentous algae without branching; cell body mostly cylindrical.
Family: Chloroplasts plate-like or ribbon-like; sexual reproduction by isogamy.
Genus: Filamentous colonies covered with mucilagenous sheath; 2, 3 or 4 daughter cells linearly connected by thin filament derived from their mother cell wall; cell body spherical; a single chloroplast parietal with a pyrenoid (Yamagishi, Introduction to the Freshwater Algae, 1999).
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P. paradoxum
x 3 μm
Kamihira Park
Ageo
Saitama, 2003
