Halydictyon arachnoideum (Harv.) Harv.

Scientific Description

Habit and structure. Thallus forming an irregularly expanding net 1–12 cm high and broad, with more-or-less compressed lobes or branches 2–10 mm broad. Usually epiphytic on other algae, attached by small adherent branchlets or long rhizoidal branches. Structure. Growth from plate-like areas of small meristematic cells at the end of branches, the cells regularly positioned and producing laterals which join to adjacent filaments at their cross walls thus forming 4 cells (rarely 3 or 5) around square interstices. Young cells 20–60 µm in diameter and L/D 1–2, expanding to 80–200 µm in diameter and L/D (2–)4–8(–10), with the largest cells in the lower thallus 120–240(–400) µm in diameter and L/D (3–)4–10. Cells multinucleate; rhodoplasts discoid, chained in larger cells.

Reproduction.Female plants unknown. Spermatangial organs ovate, compressed, 100–200 µm broad and 140–400 µm long with a single sterile basal cell and sterile marginal cells 20–25 µm in diameter and L/D 1–2(–3) surrounding spermatangial cells. Axial cells 4–7, each (except the terminal one) cutting off a lateral pericentral cell which produces a sterile marginal cell and a transverse cell forming a plate of initials each cell of which cuts off 3–4 spermatangia. Tetrasporangial stichidia compressed, 120–180 µm broad, when first fertile 200–300 µm long, later extending to 800–1000 µm long. Axial cells cutting off 4 pericentral cells, the transverse ones remaining undivided and the lateral cells each cutting off a marginal cell, two lateral cells and a tetrasporangium which becomes 40–50 µm in diameter; cover cells uncertain, doubtfully present.

Distribution.Cottesloe, W. Aust., to Deal I., Bass Strait and Coles Bay, Tas.

[After Womersley, Mar. Benthic Fl. Southern Australia IIID: 494–495 (2003)]