Pachymenia orbicularis (Zanardini) Setch. & N.L.Gardner

Scientific Description

Habit and structure. Thallus medium to very dark red to red-brown, fading to yellow-red, usually uniformly coloured but sometimes mottled, drying cartilaginous and partly to largely adhering to paper, 20–90 cm high, foliose, complanate and usually divided to near the base with cuneate to ovate fronds 5–30 cm broad, 0.5–1 mm thick above, 1–2 mm thick below, with the margin smooth to irregularly indented or slightly spinous, occasionally with marginal proliferations usually 2–8 cm long and 0.5–2 cm broad, basally constricted; stipe usually long and tapering, occasionally short below a broad frond. Holdfast discoid, robust, 2–8 mm across; epilithic. Structure of a thick cortex 10–20 cells broad, with an outer cortex 80–120 µm thick of compact anticlinal rows of 8–15 elongate cells, outer cells 3–4 µm in diameter and L/D 3–4 (the inner ones slightly broader), branched every 2–5 cells, and an inner cortex of irregularly lobed, separated, cells 10–15 µm across, becoming slightly stellate. Medulla broad, densely and irregularly filamentous with broader and slenderer (rhizoidal) filaments, refractive ganglionic cells present or absent.

Reproduction. Sexual thalli dioecious. Carpogonial branch ampullae in the outer cortex, with few, more or less erect, secondary filaments. Auxiliary cell ampullae prominent, 60–100 µm across, with numerous secondary and tertiary filaments of ovoid cells, converging above and cutting off from their mid and upper cells short to medium chains of ovoid cells of fourth and fifth orders. Carposporophytes largely within the medulla, 150–200 µm across, compact, carposporangia ovoid 8–12 µm across. Involucre moderate to dense, 30–40 µm and several filaments thick, at first lax but becoming compacted, with the ampullary filaments separating the outer cortical filaments and leaving an ostiole. Spermatangia not observed. Tetrasporangia scattered in the outer cortex, cut off from cells 3–6 deep and lying just below the surface, 15–25 µm long and 8–12 µm in diameter, cruciately divided.

Distribution. Garden I., W. Aust., to Cape Woolamai, Vic., and around Tas.

Habitat. P. orbicularis is a deep-water alga.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIA: 209–211 (1994)]