Acrothamnion preissii (Sond.) E.M.Woll.

Scientific Description

Habit and structure. Thallus rose red, with branched, prostrate and erect axes to 0.5–1.5 cm long, usually bearing whorls of 2 opposite major pinnae and (1–)2 minor pinnae between them, sometimes pinnae equally developed. Attachment by rhizoids with digitate haptera, arising from prostrate axial cells; epiphytic on various algae. Structure. Apical cells 6–8 µm in diameter and L/D 1–1.3, enlarging to 40–75 µm in diameter and L/D 1.5–3(–5) in mature axial cells. Major pinnae usually 180–280 µm and 10–12 cells long, with short basal cells and rachis cells 20–28 µm in diameter and L/D 0.8–1.2, each bearing opposite, distichous, simple pinnules, 80–100 µm and 6–9 cells long, longest near the pinna base, cells 10–15 µm in diameter and L/D 0.8–1.2, tapering near their tips; minor pinnae smaller than major ones, with one usually lost producing a dorsiventral branch; gland cells terminal on the rachis of most pinnae, formed at maturity of the rachis, transversely oval and 16–22 µm in diameter. Lateral branches irregular, arising on the basal cells of pinnae. Cells uninucleate; rhodoplasts discoid, elongate or in chains in larger cells.

Reproduction. Gametophytes dioecious. Carpogonial branches develop in series of 4–8 on the basal cells of pinnae, with 1 or 2 developing carposporophytes. Post-fertilization the supporting (basal) cell cuts off a rounded auxiliary cell with a terminal gonimolobe 200–300 µm across of ovoid carposporangia 28–35 µm in diameter, usually with later lateral gonimolobes, and with slight fusion of basal sterile cells. Pinnae on lower axial cells partly surround the carposporophyte. Spermatangia occur on branched clusters from short pinnules on lower rachis cells. Tetrasporangia occur on elongate protrusions on the upper side of basal rachis cells of mainly major pinnae, subspherical, 40–50 µm in diameter, decussately divided.

Distribution. Shark Bay, W. Aust., to Wilsons Promontory, Vic.

Habitat. Epiphytic on a variety of larger algae.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIC: 120–123 (1998)]