Amphiplexia racemosa (J.Agardh) Kraft

Scientific Description

Habit and structure. Thallus dark red-brown, 5–15 cm high, relatively stiff, with one to a few percurrent axes or main branches, bearing densely 1–2 orders of irregularly radial, elongate-clavate, terete laterals, main laterals 1–4 cm long and 2–3 mm broad near their rounded tips, lesser laterals 0.5–2 cm long and 2–3 mm broad, all basally constricted; actively growing branch systems pyramidal in form. Holdfast discoid, 2–7 mm across; epilithic or epiphytic on Amphibolis. Structure multiaxial, developing a laxly filamentous medulla (becoming dense in older branches) and a pseudoparenchymatous cortex with a single inner cortical layer of large, compact, isodiametric cells 60–80 µm across and an outer cortex 1–2 cells thick; outer cells ovoid, 8–10 µm in diameter, evenly spread in surface view and not forming rosettes. Rhodoplasts laminate, 1–3 per outer cell.

Reproduction. Sexual thalli monoecious; probably procarpic. Carpogonial branches borne on inner cortical cells, 3-celled, directed inwards with reflexed trichogynes. Auxiliary cell becoming partly surrounded by dense nutritive tissue, with several gonimoblast initials developing towards the thallus surface and forming filaments which line the inner surface of the dense pericarp of vegetative filaments; sporogenous filaments developing centripetally, forming branched tufts with rows of 1–2 obovoid carposporangia 10–15 µm in diameter. Cystocarps protuberant, slightly basally constricted, 0.8–2.2 mm in diameter, with enveloping tissue and a thick outer cortex, ostiolate. Spermatangial initials cut off from outer cortical cells, each forming 3–4 spermatangia 2–3 µm in diameter. Tetrasporangia developed from mid cortical cells, intercalary (usually bearing two outer cortical cells), basally pit-connected, ovoid, 20–30 µm long and 15–25 µm in diameter, zonately divided.

Distribution.Israelite Bay, W. Aust., to Elliston, S. Aust.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIA: 370–373 (1994)]