Leptosomia rosea (Harv.) Womersley

Scientific Description

Habit and structure. Thallus rose-red, delicate, soft and mucilaginous, 15–50 cm high, foliose or usually with axes 3–9 cm broad bearing numerous marginal overlapping lobes for 1–2 orders, lobes 1–15 cm long and 2–8 cm broad, basally broader or somewhat contracted, rounded or tapering to a blunt apex, margin often with minute excrescences; young thallus 90–300 µm thick, becoming 0.5–1 mm thick near the base. Holdfast 2–5(–10) mm across, usually surrounding Amphibolis stems, with a terete to compressed stipe 0.5–2 cm long; epiphytic or possibly epilithic. Structure multiaxial, developing a cortex 1–2 cells thick, outermost cells in slight rosettes over inner cells, cells ovoid, 4–6 µm in diameter and a large-celled medulla with 1(–2) ovoid cells 50–90 µm in diameter on each side of a central space with intermixed large and small cells, with the original medullary cells becoming thick-walled (10–20 µm), 120–200 µm in diameter, and with numerous secondary pit-connections. Rhodoplasts ovoid, in chains in inner cells.

Reproduction. Gametangial thalli dioecious. Carpogonial branches 3-celled, borne on inner cortical supporting cells, together with a 2-celled auxiliary cell branch. Carposporophytes with a basal, erect, fusion cell producing 2–4 dense lobes of carposporangia each 20–30 µm in diameter; basal nutritive tissue slightly concave, erect filaments not apparent. Cystocarps scattered, 0.5–1 mm across and 250–600 µm high, immersed in the thallus but slightly protruding, with a distinct surface pericarp of anticlinal rows of 6–10 small cells and a small central ostiole 30–60 µm in diameter. Spermatangia covering the thallus, cut off directly or via initials from outer cortical cells, ovoid, 1–2 µm in diameter. Tetrasporangia scattered, intercalary, transformed from inner cortical cells and with pit-connections to outer cortical cells, subspherical to ovoid, 30–55 µm long and 20–45 µm in diameter, cruciately divided.

Distribution. Yanchep, W. Aust., to Flinders, Vic.

Habitat. L. rosea is most commonly epiphytic on the stems of Amphibolis, rarely on other algae (e.g. Acrocarpia), but is probably also epilithic. It is confined to rough-water coasts, usually in deep-water or shaded situations.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIB: 103–107 (1996)]