Polysiphonia mollis Harv.

Scientific Description

Habit and structure. Thallus red-brown, usually 4–20 cm high, with a single, erect, basal axis (occasionally a slight prostrate part) and profusely branched above (often denuded below in older plants) subdichotomously to laterally to form dense, fastigiate to spreading, soft tufts. Holdfast discoid, small; commonly epiphytic on Posidonia, Heterozostera, Halophila or larger algae. Structure. Basal axis 300–700 µm in diameter with segments L/D 0.3–1, attached by unicellular rhizoids cut off from the lower pericentral cells; branches above the basal axis 250–500 µm in diameter with segments L/D 1–4(–8), decreasing in lesser branches to 100–150 µm in diameter with segments L/D 0.7–3 and to 30–50 µm in diameter with segments L/D about 1 close to the apices; apices straight, apical cell and trichoblasts prominent; lateral branches arising close to apices from the basal cell of trichoblasts, with the trichoblast usually remaining lateral to the new branch rather than outside it; cicatrigenous branches often present below. Pericentral cells 4, elongate throughout the thallus except within a few segments of apices; ecorticate except for slight cortication in older parts near the base of the erect axis by filaments of relatively short cells originating from the corners of the pericentral cells and lying lengthwise over the junction; trichoblasts (or scar cells) formed on every segment, with a divergence of one quarter, relatively slender, basal cells 8–12 µm in diameter. Rhodoplasts discoid to angular; scattered or commonly in irregular chains.

Reproduction. Gametophytes dioecious. Carposporophytes with a slight basal fusion cell and short, branched, gonimoblast with shortly clavate terminal carposporangia 25–50 µm in diameter. Cystocarps short-stalked, globular to ovoid, not or slightly urceolate, 250–400 µm in diameter; pericarp ostiolate, 2 cells thick, outer cells isodiametric, angular. Spermatangial branches developing as one branch of a trichoblast, elongate-ovoid to cylindrical, (80–)120–200 µm long and 35–50 µm in diameter, without a sterile apical cell, or with an inconspicuous one. Tetrasporangia forming gently spiral series in upper branches (often extending below one to several laterals), slightly to prominently swelling and distorting the branch, subspherical to ovoid, 50–70 µm in diameter.

Distribution.Rottnest I., W. Aust., around southern Australia and Tas to Port Phillip, Vic. Indian Ocean.

[After Womersley, Mar. Benthic Fl. Southern Australia IIID: 183–184 (2003)]