Heterosiphonia australis (J.Agardh) G.De Toni

Scientific Description

Habit and structure. Thallus dark red, tufted, with polysiphonous, ecorticate, erect axes 2–3 cm high and 150–250 µm in diameter, arising from prostrate axes. Attachment by rhizoids; epilithic or epiphytic. Structure. Apices of axes sympodial and distichous, developing 7–8 pericentral cells beginning 6–8 segments below apical cells, cut off in alternating sequence. Pseudolaterals 2 segments apart, each with 1–3 subdichotomies, 300–1000 µm long, cells (15–)20–65 µm in diameter and L/D 0.8–1.5(–2), tapering near the apices; basal segments may form 4–6 pericentral cells. Rhizoids developed from cells of pseudolaterals. Lateral sympodial axes arise from the basal cell of pseudolaterals. Rhodoplasts discoid in small cells, becoming chained in larger cells.

Reproduction. Gametophytes dioecious. Procarps formed on lower cells of the pseudolaterals which develop 5 pericentral cells in alternating sequence, the last formed facing the axis and developing a carpogonial branch and 2 sterile groups which divide to form the pericarp initials. Post-fertilization, the auxiliary cell, central cell of the fertile segment, lower gonimoblast cells and adjacent pericentral cells contribute to the fusion cell, with the upper much branched gonimoblast bearing ultimate rows of ovoid carposporangia 20–35 µm in diameter. Cystocarps small, urceolate, 400–600 µm in diameter, usually developing a slightly flared neck 0.5–0.8 as long as the base, sessile or shortly pedicellate; pericarp 2–3 cells thick, with slight cortication on the outer pericentral cells. Spermatangial branches occurring on pseudolaterals, with a monosiphonous (rarely polysiphonous) stalk and terminated by a row of 4–6 sterile cells, 150–400 µm long and 40–85 µm in diameter, occasionally involving branches of pseudolaterals; axial cells with 4 pericentral cells producing initials which bear an outer layer of spermatangia. Tetrasporangial stichidia occur on cells of the pseudolaterals, sessile, basally polysiphonous, cylindrical to lanceoid and apically tapering with simple or branched apical filaments, 400–800 µm long and 120–175 µm in diameter with 15–25 fertile segments; each segment with 5 pericentral cells formed alternately, each cell cutting off 2 pre-sporangial cover cells which divide transversely to form 4 cells covering each sporangium, and a tetrasporangium usually from each pericentral stalk cell, 35–65(–70) µm in diameter.

Distribution. Hopetoun, W. Aust., to Coffs Harbour, N.S.W., and E Tas.

[After Parsons & Womersley in Womersley, Mar. Benthic Fl. Southern Australia IIIC: 485 (1998)]