Perithamnion muelleri (Harv.) Womersley

Scientific Description

Habit and structure. Thallus medium red-brown, 2–10(–15) cm high, ecorticate, much branched irregularly radially with long laterals from lower axes, often with laterals 4–5 cells apart on upper branches, axial cells with 5 whorl-branchlets in distinct whorls. Attachment by branches with recurved ends or by rhizoids; epiphytic or epilithic. Structure. Apical cells 6–8 µm in diameter and L/D 1–2, partly surrounded by young whorl-branchlets, enlarging to axial cells in mid thallus 120–240 µm in diameter and L/D 1–1.2, and in lower axes cells 350–500 µm in diameter and L/D 1–2.5; axial cells conspicuous throughout thallus, with gaps between whorl-branchlets 40–90 µm broad in lesser branches, increasing to 400–500 µm broad in lower axes (2–3 times the length of the whorls of whorl-branchlets). Whorl-branchlets normally 5 per axial cell, evenly spaced, becoming 200–300 µm long with 5–7 successive di- to quadrichotomous branches from the basal cells, basal cells 20–30 µm in diameter and L/D 1–1.5, decreasing to terminal cells 6–8 µm in diameter and L/D 1.5–2, often with long multicellular hairs; gland cells usually prolific, on mid to outer cells of the whorl-branchlets, arising on terminal cells and touching only the bearing cell, sessile, subspherical to ovoid, 10–15 µm in diameter. Lateral branches arising from basal cells of whorl-branchlets. Cells multinucleate; rhodoplasts discoid, forming a reticulum or ribbon like in larger cells.

Reproduction. Gametophytes dioecious. Carpogonial branches on the basal (=supporting) cell of whorl-branchlets, single per whorl and on 1–3 successive whorls. Post-fertilization the supporting cell cuts off a larger, lateral, auxiliary cell which develops terminal and lateral gonimolobes 160–220 µm across of ovoid carposporangia 20–30 µm in diameter. The axial cell, residual supporting cell, auxiliary cell and lower gonimoblast cells fuse somewhat, with broader pit-connections. No involucral filaments develop and the carposporophytes remain more or less terminal on branches. Spermatangia cut off from initials on terminal cells of whorl-branchlets. Tetrasporangia borne on mid cells of whorl-branchlets, situated above the whorl-branchlets, sessile, subspherical, 35–55 µm in diameter, decussately divided.

Distribution.Eyre, W. Aust., to Waratah Bay, Vic.

Habitat. P. muelleri occurs mainly in deep water in moderately sheltered situations.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIC: 203–205 (1998)]