Platysiphonia victoriae (J.Agardh) Womersley & Shepley

Scientific Description

Habit and structure. Thallus red-brown with a greyish iridescence, when detached becoming rose-red and decomposing rapidly, 20–35 cm high, much branched with slender, compressed laterals of several orders, uncorticated branches (25–)40–90 µm broad with segments L/D 1–1.5(–2), becoming corticated with lower axes terete, 1–3 mm in diameter; branching irregular, largely adaxial from the centre of blades, with irregular to flat-topped and corymbose upper branch systems. Holdfast discoid, 1–5(–10) mm across with 1 to several axes; epilithic or rarely epiphytic. Structure. Apical cell dome-shaped, with axial cells cutting off first the abaxial pericentral, then the 2 lateral pericentral cells and last the adaxial pericentral cell, and the lateral cells each cutting off 2 flanking cells half as long as the parent cells and resulting in a compressed branch; no further divisions of the flanking cells occur in the vegetative branches apart from cortication of all cells as the branch matures. Lateral branches arise endogenously from the anterior ends of axial cells. Cells multinucleate; rhodoplasts discoid to elongate, linear in larger cells.

Reproduction. Gametophytes dioecious. Procarps occur on the adaxial pericentral cells near the base of young blades, cutting off first a sterile cell, then the 4-celled carpogonial branch and then the second sterile cell. The carposporophyte develops an erect basal fusion cell and much branched gonimoblast filaments with ovoid to clavate terminal carposporangia 20–30 µm in diameter. Cystocarps sessile, ovoid, 500–1000 µm in diameter; pericarp ostiolate with about 15 erect filaments arising from pericentral and flanking cells around the procarp, with each cell of the erect filaments cutting off 2 outer, transversely elongate, cells, with cortical cells on the pericarp base and the stalk. Spermatangial blades 0.5–1 mm long and 80–130 µm broad, with the lateral pericentral cells each cutting off 2–4 cortical cells which divide anticlinally to form a plate of about 16 initials, each then forming 3–4 elongate spermatangia; the transverse pericentral cells and flanking cells remain sterile, and the spermatangial blades have a long sterile apex. Tetrasporangial stichidia 400–1200 µm long and 130–180 µm broad, occurring in dense masses on lateral branches, each with 2 regular rows of tetrasporangia. The lateral pericentral cells cut off, in rapid succession, the tetrasporangium and 2 cover cells, and the flanking cells elongate horizontally on the edge of the stichidium and each divides to give a further horizontal cell and later cut off small cortical cells at their ends; following loss of the tetrasporangia, the cover cells divide further; tetrasporangia subspherical, 50–100 µm in diameter.

Distribution.Houtman Abrolhos and Israelite Bay, W. Aust., to Walkerville, Vic., and the N coast of Tas.

[After Womersley, Mar. Benthic Fl. Southern Australia IIID: 162–164 (2003)]