Polysiphonia succulenta Harv.

Scientific Description

Habit and structure. Thallus grey-brown to red-brown, usually 5–12(–20) cm high with a single erect base and lower axis, branching subdichotomously (to laterally) to form a dense, fastigiate to spreading tuft. Holdfast of clumped rhizoids from lower cells of erect axes; epiphytic, usually on Posidonia australis. Structure. Lower axes (400–)500–1000 µm in diameter with segments L/D 0.2–0.5; rhizoids unicellular and cut off from the pericentral cells; branches above the basal axis 300–500 µm in diameter with segments L/D (0.5–)0.8–1(–1.5) decreasing gradually to 100–200 µm in diameter with segments L/D 0.3–0.8 close to the apices, then tapering fairly abruptly to pointed apices which are commonly slightly to markedly involute and usually have a prominent apical cell; lateral branches developing close to apices, replacing trichoblasts, with occasional cicatrigenous branchlets from lower parts. Pericentral cells 4 (often 5 within 2–5 segments from base of a lateral branch), isodiametric near apices and extending to twice (rarely 3 times) L/D below, ecorticate except for slight cortical development near the base in robust plants by elongate cells cut off laterally or from the corners of pericentral cells, never forming a complete layer; older cells not readily permeable (partially collapsing in Karo); trichoblasts or scar cells present on every segment, with a divergence of one quarter, trichoblasts relatively robust, basally several times furcate, with long, tapering ultimate branches 4–6 cells long. Rhodoplasts discoid to elongate and often in chains, irregularly arranged.

Reproduction. Gametophytes dioecious. Carposporophytes with a slight basal fusion cell and short, branched gonimoblast with clavate terminal carposporangia 25–60 µm in diameter. Cystocarps short-stalked, globular to slightly ovoid, occasionally slightly urceolate, (300–)400–700 µm in diameter; pericarp ostiolate, 2 cells thick, with the terminal ostiolar cells distinctly enlarged when mature, outer cells isodiametric, angular. Spermatangial branches usually developing as one (rarely both) basal branch of a trichoblast with the sterile branch soon lost, sometimes replacing whole trichoblast, elongate-ovoid, 150–300 µm long and (60–)100–180(–200) µm in diameter, usually with a sterile dome-shaped apical cell and often a sterile subapical cell or single sterile cell borne laterally on the subapical cell. Tetrasporangia forming spiral series of one quarter divergence in upper branches (often extending below a lateral), not or slightly distorting or swelling the branch, 80–120 µm in diameter.

Distribution.Cockburn Sound, W. Aust., to Western Port, Vic., and the N coast of Tas.

Habitat. P. succulenta is normally epiphytic on Posidonia australis in calm bays and inlets.

[After Womersley, Mar. Benthic Fl. Southern Australia IIID: 192–193 (2003)]