The vegetative and reproductive development of Polysiphonia lanosa (Linnaeus) Tandy (Rhodomelaceae, Rhodophyta) is reinvestigated based on material from the west coast of Ireland. This species is identified by the following unique combination of characters: a thallus consisting of 17–23 short pericentral cells; short and wide central axial cells; dichotomous branching; a complete absence of cortication; rhizoids generally remaining in contact with pericentral cells; and, its exclusive occurrence as a putative epiphyte of fucoids. Tetrasporangia are arranged in a spiral or sometimes a straight series and produced from the second pericentral cells with two presporangial cover cells. The spermatangial branches in P. lanosa are produced directly from each axial cell in the distal ends of erect branches and are not modified trichoblasts; vegetative trichoblasts are completely absent. The procarp has a four-celled carpogonial branch, as is typical for most members of the Rhodomelaceae, although species with either three- or four-celled carpogonial branches do occur in the genus Polysiphonia Greville. Although the genus Vertebrata S.F. Gray is available, is typified by, and has been used for this species, it is concluded that a molecular investigation is required to assess further the phylogenetic divergence indicated by these characters. The continued inclusion of Vertebrata in Polysiphonia is recommended until this work has been carried out.
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