Reappraisal of the type species of Polysiphonia (Rhodomelaceae, Rhodophyta)

The genus Polysiphonia Greville, nom. cons., has had a long and confused no menclatural history. At present, Polysiphonia has a wide circumscription, i ncluding at least 200 species, but it is heterogeneous in many vegetative a nd reproductive developmental features. Central to any re-evaluation of the genus is a detailed examination of the type species of Polysiphonia, P. ur ceolata (Lightfoot ex Dillwyn) Greville, which is conspecific with P. stric ta (Dillwyn) Greville. We here report on the vegetative and reproductive mo rphology of P. stricta, including P, urceolata, based on type and other mat erial from the British Isles. Thalli consist of prostrate and erect ecortic ate axes with four pericentral cells, attached by unicellular rhizoids rema ining in open connection with pericentral cells. Prostrate axes lack vegeta tive trichoblasts; trichoblasts occur seasonally on erect axes. Branch init ials are cut off from the subapical cell at intervals of four or five segme nts in dichotomous and alternating pairs rather than being formed horn each axial cell in the spiral pattern typical of most species of Polysiphonia. Spermatangial branch initials, which are trichoblast homologues, are produc ed directly from each axial cell at the tips of erect branches, not subtend ed by trichoblasts, and have two- to five-celled sterile tips when mature. The mature carpogonial branch is four-celled with a two-celled first steril e group and a one-celled second sterile group. Following presumed fertiliza tion, direct fusion apparently takes place between carpogonium and auxiliar y tell; mature cystocarps are usually urceolate. Tetrasporangia are formed from the third pericentral cell, in straight series, and have two pre-spora ngial cover cells. Previous accounts of a third, post-sporangial cover cell could not be substantiated. P. stricta and a small group of other Polysiph onia species differ in several important respects from most members of the genus, which have rhizoids cut off from pericentral cells by a cell divisio n, abundant trichoblasts, spirally arranged tetrasporangia and a post-spora ngial cover cell. The branching pattern of P. stricta highlights the diffic ulties of distinguishing between the tribes Polysiphonieae and Pterosiphoni eae.