Heterosiphonia lawrenciana (Harv.) M.J.Parsons

Scientific Description

Habit and structure. Thallus medium to dark red-brown, erect with one to a few main axes, 10–40 cm high, axes becoming 2–3 mm in diameter for 5–30 cm (probably perennial) and denuded apart from re-growth, lesser branches alternately distichous, mostly 2–5 mm apart, terminating in flabellate tufts of rhodoplastic filaments, with the polysiphonous segments soon becoming corticated and obscured. Holdfast discoid to conical, 2–10 mm across; epilithic. Structure. Apices of axes sympodial and distichous, developing 8–9(–10) pericentral cells. Pseudolaterals (2–)3–5 segments apart, each with 2–4 lower sub dichotomies 2–4 cells apart, the lower several segments becoming polysiphonous and with upper monosiphonous filaments, lower cells 35–65 µm in diameter and L/D 0.5–1.3, tapering gradually to upper cells 15–30 µm in diameter and L/D 2–3.5. Cortication commences by cells cut off the pericentral cells and soon becomes complete, obscuring the pericentral cells; in older axes, a cortex many cells thick develops, with a conspicuous central axial cell but the pericentral cells obscured. Rhodoplasts discoid.

Reproduction. Gametophytes dioecious or monoecious. Procarps on lower segments of pseudolaterals, becoming polysiphonous, with a pericarp developed pre-fertilization, adjacent pseudolaterals usually slenderer than normal pseudolaterals. Cystocarps large, ovoid, 1–1.5 mm in diameter, with little or no neck, on polysiphonous corticated branches; gonimoblast much branched, with a basal fusion cell and ovoid carposporangia 40–50 µm in diameter; pericarp with a cortex 2–3 cells thick. Spermatangial branches as terminal branches of pseudolaterals, lanceoid to cylindrical with short monosiphonous pedicels and terminal filaments, 90–400 µm long and 35–45 µm in diameter. Tetrasporangial stichidia as ultimate branches of pseudolaterals, with a polysiphonous (becoming corticated) base and blunt apex, cylindrical, 500–1000 µm long and 200–300 µm in diameter, with 6 pericentral cells and tetrasporangia per whorl with 2 cover cells divided horizontally and somewhat irregular; sporangia 60–90 µm in diameter.

Distribution. Cottesloe, W. Aust., to Port Phillip Heads, Vic., and N Tas.

[After Parsons & Womersley in Womersley, Mar. Benthic Fl. Southern Australia IIIC: 499–501 (1998)]