Dictyomenia sonderi Harv.

Scientific Description

Habit and structure. Thallus medium to dark red-brown, erect, 10–40 cm high, complanately and marginally branched with regularly to irregularly pinnate lateral branches, linear and (3–)4–6(–7) mm broad, with regular marginal fringes of branched determinate laterals 1–2(–3) mm long. Main branches with a well defined midrib and prominent alternate lateral veins, midrib thickened below and remaining as a subterete stalk 1–1.5(–2) mm thick where the lateral membrane is lost. Branch blades corticated from close to apices. Holdfast fibrous, branched, 1–2 cm across; epilithic. Structure. Apical cell of branches dome-shaped, 7–15 µm in diameter, with the axial cells branching on alternate sides every 2 cells to originate determinate laterals and cutting off 6 pericentral cells, 2 larger lateral ones and 2 smaller ones on each side. The determinate laterals remain active for some time, cutting off the 6 pericentral cells and the lateral pericentral cells forming chains of 2–3, resulting in (3–)5–6 large inner cells between the lateral veins; the determinate laterals fuse congenitally to form the 4–6 mm broad main branches, with the apices of the determinate laterals forming the marginal fringe; trichoblasts occur as dense tufts on the end branches of the determinate laterals. Cortication generally 1(–2) cells thick over the blades, heavier over the midrib. Cells probably uninucleate; rhodoplasts discoid.

Reproduction. Reproductive organs borne on branched clusters arising over the lateral veins on the branch surface. Gametophytes dioecious. Procarps not observed. Carposporophytes with a basal fusion cell and branched gonimoblast with clavate terminal carposporangia 35–90 µm in diameter. Cystocarps subspherical to slightly ovoid, 700–1500 µm in diameter; pericarp ostiolate, corticated. Spermatangial organs with 1–2 sterile basal cells, no sterile apical cells, ovoid to slightly tapering, 60–90 µm in diameter and 100–150 µm long. Stichidia in much-branched clusters 1–2 mm high, branches 100–180 µm in diameter with segments L/D 0.4–0.8, tetrasporangia single per segment, in short series maturing acropetally, 130–200 µm in diameter.

Distribution.N of Dongara, W. Aust., to Backstairs Passage, S. Aust; doubtfully N Aust.

[After Womersley, Mar. Benthic Fl. Southern Australia IIID: 347–349 (2003)]