Platysiphonia mutabilis (Harv.) Børgesen

Scientific Description

Habit and structure. Thallus medium to dark red-brown, 4–10 cm high, much branched with slender branches corticated only near their base. Attachment by a rhizoidal holdfast; epiphytic. Structure. Apical cell hemispherical, with axial cells producing 4 pericentral cells, the abaxial one first, followed by the lateral cells then the adaxial pericentral cell, with each lateral cell cutting off 2 flanking cells giving a compressed branch 5 cells broad. Mid blades 80–140 µmbroad, segments L/D 0.6–1. Lower corticated axes 0.7–1.5 mm thick. Branches arise endogenously from the axial cells, with their basal 1–2 segments usually lacking flanking cells. Cortication is slight above by small cells cut off from pericentral cells and becomes complete and moderate only on lower parts of older branches. Cells uni- to multinucleate; rhodoplasts discoid, becoming chained.

Reproduction. Gametophytes dioecious. Procarps adaxial on lateral branches, with the adaxial pericentral cell cutting off sterile cells and a 4-celled carpogonial branch. Carposporophytes with a small basal fusion cell and branched gonimoblast filaments bearing elongate-ovoid to clavate terminal carposporangia 16–22 µmin diameter. Cystocarps sessile, ovoid, 500-700 µmin diameter. Pericarp with about 16 erect filaments, each cell producing 2 transversely elongate outer cells, slightly corticated only at the base. Spermatangia covering the lower part of laterals, the fertile part 1–1.5 mm long and 180–220 µmbroad, with spermatangia produced from the lateral pericentral cells, the transverse pericentral cells and flanking cells remaining sterile. Tetrasporangia borne in lateral blades 0.5–1.2 mm long and 100–180 µmbroad, with 2 rows of tetrasporangia derived from the lateral pericentral cells which form a cover cell on each side, with each flanking cell dividing horizontally into 2; tetrasporangia 30–45 µmin diameter.

Distribution.Yanchep, W. Aust., to Port Stephens, NSW and N Tas.

[After Womersley, Mar. Benthic Fl. Southern Australia IIID: 160–162 (2003)]