Phacelocarpus sessilis J.Agardh

Scientific Description

Habit and structure. Thallus medium red to red-brown, (5–)10–30 cm high, with a terete stipe 2–15 mm long and 0.7–1.3 mm in diameter, much branched complanately with terete rachides of decreasing length for 4–6 orders, rachis 1–1.3 mm in diameter below, 400–800 µm in diameter in mid parts and 200–300 µm in diameter in upper branches; rachis in mid parts terete to slightly compressed, sometimes with a slight lateral protrusion between the ramuli. All branches bearing distichous, alternate, terete ramuli 1–1.5(–2) mm long, 150–250 µm in basal diameter and tapering gradually to 100–150 µm in diameter near their apices, then to a point, becoming separated on the rachis by 3–5 times their basal diameter (often closer in the lower thallus). Holdfast discoid, 2–4 mm across; epilithic or rarely epiphytic. Structure uniaxial, developing a prominent axial filament with four periaxial cells, the lateral two more strongly developed with alternate ones forming the ramuli; rhizoids relatively weakly developed around the axial filament. Cortex pseudoparenchymatous, inner cells ovoid and 20–50 µm in diameter, outer cells 3–5 µm in diameter and L/D 1–1.5; refractive cortical cells common; rhodoplasts discoid, small, few per cell.

Reproduction.All reproductive bodies borne on the rachis, developed from the periaxial filaments opposite a ramulus. Sexual thalli dioecious. Procarps with two 2-celled carpogonial branches and an auxiliary cell. Cystocarps on a medium to long stalk, compressed, ovate and strongly bilabiate, 400–700 µm broad, with a thick pericarp along the ostiolar slit, enclosing the carposporophyte with a basal fusion cell and erect gonimoblast filaments with single terminal carposporangia. Spermatangia in channels in globular, sessile or short-stalked nemathecia 400–800 µm in diameter. Tetrasporangia in sessile or sub sessile globular to ovoid nemathecia 400–600 µm in diameter, separated by tufts of sterile filaments, 30–65 µm long and 6–10 µm in diameter, zonately divided.

Distribution.Three Kings Islands, New Zealand (?). Point Peron, W. Aust., to Portland, Vic., and SE Tas.

Habitat. P. sessilis occurs in deeper water on rough-water coasts.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIA: 396–398 (1994)]