Habit and structure. Thallus medium to dark red, mucilaginous, (2–)5–20(–45) cm high, more-or-less pinnately branched for 3–4 orders but not strictly distichous, with scattered branchlets from the face of parent branches, with percurrent axes and main branches. Larger and mid branches compressed, basally constricted and tapering to a rounded apex, main branches (2–)3–4(–5) mm broad and 300–500 µm thick, lesser branches 1–2 mm broad, branchlets subterete and 300–600 µm broad. Holdfast discoid, 1–3 mm across, bearing 2–5 fronds; epiphytic on Posidonia or Amphibolis (or Acrocarpia) or possibly epilithic. Structure multiaxial, with a cluster of apical cells producing filaments which interconnect to form a lax medullary network on the inside of the cortex, filaments 4–6 µm in diameter and producing ovoid secretory cells 6–8 µm in diameter; older segments with denser medullary filaments, filling the segment near the thallus base. Cortex 2–3 cells thick above, 3–5 cells thick below, the inner cortex with an outer layer of ovoid to isodiametric cells 12–30 µm across, a mid layer of more elongate cells, and an inner layer of elongate cells connected to the medullary filaments, with a sparse outer cortex of small ovoid cells, 5–10 µm in diameter, cut off around the larger cells, dividing to form very slight rosettes on older branches. Hairs common. Branch constrictions solid, pseudoparenchymatous. Rhodoplasts discoid, in chains in most cells.
Reproduction. Gametangial thalli dioecious. Carpogonial and auxiliary cell branches not observed. Carposporophyte erect with a massive basal fusion cell producing slender gonimoblast filaments with all upper cells forming ovoid to clavate carposporangia 15–30 µm in diameter; basal nutritive tissue present, erect filaments disintegrating. Cystocarps scattered on lesser branches or surface of compressed branches, external, often near branch bases, globular to slightly urceolate and beaked, basally constricted, 600–900 µm in diameter, ostiolate; pericarp 35–60 µm and 4–8 cells thick, inner cells stellate. Spermatangia cut off via initials from cells of the rosettes, ovoid, 2–3 µm in diameter. Tetrasporangia in irregular to elongate, depressed sori 200–500 µm across, numerous per sorus, cut off inwardly from small cells lining the depression, subspherical, 50–75 µm in diameter, tetrahedrally divided.
Distribution. Point Peron, W. Aust., to Flinders, Vic.
Habitat. L. pyramidalis is most commonly epiphytic on Posidonia.
[After Womersley, Mar. Benthic Fl. Southern Australia IIIB: 138–140 (1996)]
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