Key to the species of Polysiphonia

1 Pericentral cells 4 2
1 Pericentral cells 5 or more 7
2 Plants growing in brackish water or nearshore in calm places; with lateral branches arising independent of trichoblasts; trichoblasts poorly developed, infrequent P. subtilissima Montagne
2 Plants not growing in brackish water; lateral branches formed in place of trichoblasts in fixed pattern 3
3 Plants having nearly equal prostrate and erect portions 4
3 Plants having mostly erect portions 5
4 Rhizoids continuous with pericentral cells; erect axes mostly unbranched; tetrasporangia in straight series P. scopulorum Harvey
4 Rhizoids large, saccate, pit-connected to nearly every pericentral cell; epi-endophytes of Codium species P. saccorhiza (Collins & Hervey) Hollenberg
5 Rhizoids cut off from proximal end of pericentral cells; trichoblasts delicate, deciduous; tetrasporangia in slightly spiral series P. upolensis (Grunow) Hollenberg
5 Rhizoids cut off from middle part of pericentral cells; trichoblasts persistent 6
6 Plants reddish-brown, silky, branching subdichotomous, trichoblasts on every segment in 1/4 spiral arrangement; branches following spiral pattern; plants growing on various algae P. sphaerocarpa B°rgesen
6 Plants nearly black, coarse; branching irregularly pinnate; trichoblasts on every segment, but laterals formed independently of trichoblasts every 5-6 segments; on hard substrates in turf P. hancockii Dawson
7 Plants with axes to 700 Ám in diam.; laterals cigar-shaped, prominently constricted at their bases; pericentral cells 4-5(6) P. beaudettei Dawson
7 Plants with axes 175-275 Ám in diam., laterals not cigar-shaped, wide-angled; pericentral cells 5 P. pentamera Hollenberg

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