Amphiroa anceps (Lam.) Decne.

Scientific Description

Habit and structure. Thallus medium red-purple, often fading to grey-red in shallow-water plants, 2–10(–15) cm high, often tufted and much branched dichotomously (rarely with small proliferations) from the upper end of each intergeniculum, branching essentially complanate, intergenicula more-or-less terete near the base, compressed to flat above and 4–10(–15) mm long, (0.5–)1–3(–4) mm broad and (300–)500–1000 µm thick centrally, edges thinner and more-or-less sharp-edged, margins parallel to slightly broader above, calcification over genicula closer near the margins. Holdfast crustose, 2–10 mm across, bearing numerous fronds; epilithic. Structure multiaxial. Intergenicula consisting of numerous curved tiers of medullary cells with the filaments splaying laterally, with lateral secondary pit-connections between the medullary cells forming arcs across the intergenicula; medulla with tiers of shorter cells (10–25 µm long) separating every 2–5 tiers of longer cells (40–70 µm long), all 8–15 µm in diameter, ends rounded and separated when decalcified. Genicula with the upper and lower margins not corresponding to the ends of cell tiers, of similar structure to intergenicula but cell walls more darkly staining. Cortex compact, extending from outer cells of the medullary filaments, cortical filaments of 3–8 isodiametric or slightly elongate cells 7–12 µm in diameter, with terminal epithallial cells; usually thicker near tetrasporangial conceptacles. Apices of branches with meristematic cells surmounted with small epithallial cells. Rhodoplasts discoid.

Reproduction. Conceptacles uniporate, mostly on the surface of one side of the intergenicula. Gametangial thalli dioecious. Female conceptacles 400–800 µm in external diameter and 128–160 µm high; carposporophyte with a broad, thin fusion cell bearing marginal short chains of ovoid carposporangia 25–30 µm in diameter. Male conceptacles inconspicuous, 400–450 µm in external diameter, with spermatangia cut off from initials on the chamber floor, ovoid, 1.5–3 µm in diameter. Tetrasporangial conceptacles prominent, 250–500 µm in external diameter, with a central tuft of elongate cells and tetrasporangia lying between similar cells, elongate-ovoid, 20–50 µm in diameter, zonately divided or often as bisporangia.

Distribution. All around Australia except for Tas., but less frequent in SE S. Aust., and Vic. Widespread in the Indo-Pacific.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIB: 285–286 (1996)]