Hirsutithallia laricina (Harv.) E.M.Woll.

Scientific Description

Habit and structure. Thallus erect, medium red-brown, 2–10 cm high, with a main axis bearing radially branched indeterminate laterals giving a narrowly to broadly pyramidal or spreading form, with terminal and lateral branchlets; cortication commencing from close to the apices. Holdfast 0.5–1 mm across, rhizoidal; usually epiphytic on Posidonia. Structure. Ecorticate determinate branchlets 200–300 µm long, on corticate or potentially corticate laterals, subdichotomous to laterally branched with basal cells 40–75(–90) µm in diameter and L/D 1.5–2, tapering over 6–12 cells to small terminal cells 6–12 µm in diameter and L/D 1.5–2, ends rounded and occasionally bearing hairs. Axial cells of corticated branches increasing to 200–600 µm in diameter and L/D (1–)1.5–2 (shorter near the base), becoming densely corticated by narrow, descending rhizoidal filaments arising from the basal cells of lateral branchlets, and bearing few to numerous outer anticlinal filaments 4–6 cells and 50–140 µm long and 12–22 µm in diameter, simple or becoming branched near their ends. Cells uninucleate; rhodoplasts discoid to ribbon like.

Reproduction. Gametophytes dioecious. Procarps borne on mid cells of ecorticate branchlets. Carpogonial branches borne on one of the two more-or-less opposite periaxial cells, with the carpogonium post-fertilization dividing longitudinally and each cell uniting with the auxiliary cell cut off from each supporting cell, then dividing to form the gonimolobes which have a sterile basal cell bearing an upper group of carposporangia; the two carposporophytes are rounded to pyramidal in shape or often furcate, tapering from a broader base, and may not develop simultaneously; carposporangia irregularly ovoid, 20–30(–45) µm across; involucre absent. Spermatangial clusters, 30–55 µm across, formed on cells of ecorticate branchlets, with a stalk cell cutting off several initials which may branch again, with ultimate spermatangia. Tetrasporangia sessile, subspherical, borne singly on mid to upper cells of ecorticate branchlets, 45–65 µm in diameter, tetrahedrally divided.

Distribution. Yanchep, W. Aust., to Port Phillip Heads, Vic.

Habitat. H. laricina is usually epiphytic on the seagrass Posidonia.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIC: 255–256 (1998)]