Heterothamnion platythaliae Athanas.

Scientific Description

Habit and structure. Thallus in dense spreading tufts 3–5 mm high, with numerous erect axes, each axial cell bearing (1–)2 whorl-branchlets. Attachment by rhizoids from erect axes, penetrating the host edges, probably at conceptacles; epiphytic on Platythalia angustifolia. Structure. Apical cells 12–16 µm in diameter and L/D 1–1.5, enlarging rapidly to axial cells 50–80 µm in diameter and L/D 1.5–2.7, tapering and with shorter cells near the base. Whorl-branchlets one or two per axial cell, if two situated usually at right angles and alternating on successive cells, with one of the 2 often more branched than the other and with 2–4 branches from the basal or upper cells; whorl-branchlets 200–500 µm and 6–10 cells long, basal cells slightly longer than the next outer cell, 25–40 µm in diameter and L/D 1.2–1.6(–2), tapering to terminal cells 12–22 µm in diameter and L/D 1–1.5, ends rounded; gland cells apparently absent. Lateral branches borne on the basal cell of whorl-branchlets. Cells probably uninucleate; rhodoplasts discoid.

Reproduction. Gametophytes probably monoecious. Carpogonial branches borne on the basal cells of reduced 2-celled whorl-branchlets which may continue growth. Carposporophytes terminal, surrounded by 4 whorl-branchlets, with rounded gonimolobes 100–200 µm across of ovoid carposporangia 20–25 µm in diameter. Spermatangia on small branched clusters on the adaxial side of lower cells of whorl-branchlets shortly below the carposporophytes. Tetrasporangia borne on lower cells of whorl-branchlets, sessile, subspherical, 30–40 µm in diameter, tetrahedrally divided.

Distribution.Known only from Sarge Bay, Cape Leeuwin, W. Aust., on drift Platythalia angustifolia.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIC: 162–166 (1998)]