Genus: Colonies of a fixed number of cells, flat, circular in shape; cell body polygonal in shape, with horn-like projections; cell wall garanulated, wrinkled or notched; chloroplasts plate-like or reticular; asexual reproduction by zoospore; sexual reproduction by isogametes (Illustrations of The Japanese Fresh-water Algae, 1977).
Species: Colonies of 8-32 (rarely 4, 64 or 128 cells) with large intercellular space; inner cells H or X-shaped, trapezoid to rectangular when matured; outer half of the marginal cells prolonged into two horn-like processes with a tuft of long bristles; daughter colonies formed in a vesicle released from mother cell; cell wall smooth (Photomicrographs of the fresh-water algae, 1994).

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var. gracilimum
var. gracilimum 32-cells Mizumoto p. Tokyo, 2001	var. gracilimum 32-cells Aobano-mori Park Chiba, 2003	var. gracilimum stock Tmz-52 Mizumoto p. Tokyo, 2001	var. gracilimum Koriyama Fukushima, 2001
var. gracilimum 32-cells 32 = 15+11+6 Japan, 1997-98	var. gracilimum 16-cells 16 = 11+5 Japan, 1998	var. gracilimum 8-cells 8 = 7+1 Fukushima, 1999	var. gracilimum 8-cells 8 = 7+1 Japan, 1998
var. gracilimum 4-cells Saitama, 1999	var. gracilimum 8-cells 8 = 6+2 Saitama, 2001	var. gracilimum 32-cells 32 = 15+10+6+1 Saitama, 2001	var. gracilimum 8-cells 8 = 7+1 Mizumoto Park Tokyo, 2001
var. gracilimum Kasuga Fukuoka, 2001	var. gracilimum -cells Showa-machi G.P. Showa Saitama, 2003	var. gracilimum -cells O-ike park Yabuki Fukushima, 2003	var. gracilimum -cells Karasu-ga-mori p. Nishi-nasuno Tochigi, 2004
var. gracilimum -cells Higusa-numa Hikari Chiba, 2003	var. gracilimum -cells near Ayu river Fujioka Gunma, 2004	var. gracilimum cell μm Higashi-hiroshima Hiroshima, 1998

Similar species -->> P. angertiniense

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