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Pachydictyon polycladum (Kützing) Womersley 1967: 216.

Phylum Phaeophyta – Order Dictyotales – Family Dictyotaceae – Tribe Dictyoteae


Dictyota polyclada Kützing 1859: 10, pl. 23 fig. II.

Dictyota furcellata sensu Harvey 1858: pl. 38.

Pachydictyon furcellatum (Harvey) J. Agardh 1894a: 83.

Thallus (Fig. 71C) medium brown, 5–20 cm long with axes usually 0.5–1 mm broad and bearing lateral tufts (Fig. 72A) of slender, divaricate, dichotomous, compressed, branchlets 150–300 µm broad, without proliferations. Apical cells hemispherical, protruding. Holdfast rhizoidal, with lower branches also becoming entangled with host; epiphytic on seagrasses. Cortical cells 10–16 (–25) µm across, L/B (2–) 3–4, usually in blocks and transverse rows corresponding to underlying medullary cells; in older parts of axes, the cortex becomes 2–4 cells thick (Fig. 72C) by mostly periclinal divisions of original cortical cells, around the whole of the axes. Medulla (Fig. 72C) one cell thick throughout, often with intracellular wall thickenings. Hair tufts usually just below points of branching (Fig. 72A), often also scattered, hairs 8–16 µm in diameter.

Reproduction: Tetrasporangia (Fig. 72A,B) 100–160 in diameter, single, scattered along finer branchlets. Oogonial sori small, ovate, scattered along the branches, 120–300 (–400) µm long and 100–180 (–270) µm across, with surrounding cortical cells sometimes extended upwards; oogonia (Fig. 72D,E) 4–14 per sorus, broadly ovoid, (40–) 60–90 µm long and 30–60 µm in diameter. Antheridial sori unknown.

Type from W. Aust. (Preiss); in Herb. Sonder, MEL, 17003.

Selected specimens: Champion Bay, W. Aust. ("Alg. Muell.", MEL 17194). Rocky Bay, Rottnest I., W. Aust., 0–1 m deep on Amphibolis antarctica (Gordon-Mills, 6.xii.1984; ADU, A56666). Elliston, S. Aust., uppermost sublittoral on A. antarctica (Womersley, 9.i.1976; ADU, A46760-"Marine Algae of southern Australia" No. 172). Marion Bay, S. Aust., drift on Posidonia (Womersley, 8.iv.1950; ADU, A16099). Outside Tapley Shoal, Gulf St Vincent, S. Aust., 15 m deep (Shepherd, 2.ii.1969; ADU, A33459). Off Semaphore, S. Aust., 10 m deep (Gray, 9.i.1979; ADU, A50230). Emu Bay, Kangaroo I., S. Aust., upper sublittoral on Amphibolis (Womersley, 15.i.1965; ADU, A28943). Sorrento, Vic., 11 m deep on channel edge in bay (Womersley, 7.iv.1959; ADU, A22765).

Distribution map based
on current data relating to
specimens held in the
State Herbarium of SA

Distribution: From Champion Bay, W. Aust. to Port Phillip, Vic., usually on the seagrasses Posidonia and Amphibolis, occasionally on Heterozostera, in sheltered localities.

Taxonomic notes: Pachydictyon polycladum is characterised by its very slender, divaricate, branches, with only the larger branches or axes developing a cortex more than one cell thick. It is invariably found in sheltered waters.


AGARDH, J.G. (1894a). Analecta Algologica. Cont. I. Acta Univ. lund. 29, 1–144, Plates 1, 2.

HARVEY, W.H. (1858). Phycologia Australica. Vol. I, Plates 1–60. (Reeve: London.)

KÜTZING, F.T. (1859). Tabulae Phycologicae. Vol. 9. (Nordhausen.)

WOMERSLEY, H.B.S. (1967). A critical survey of the marine algae of southern Australia. II. Phaeophyta. Aust. J. Bot. 15, 189–270.

The Marine Benthic Flora of Southern Australia Part II complete list of references.

Author: H.B.S. Womersley

Publication: Womersley, H.B.S. (14 December, 1987)
The Marine Benthic Flora of Southern Australia
Part II
©Board of the Botanic Gardens and State Herbarium, Government of South Australia

Illustrations in Womersley Part II, 1997: FIGS 71C, 72 A–E.

Figure 71 image

Figure 71   enlarge

Fig. 71. A,B. Dilophus gunnianus. A. Common form (ADU, A56796). B. Type of Dilophus foliosus (Herb. Agardh, LD, 49641)=D. gunnianus. C. Pachydictyon polycladum (ADU, A46760). D. Pachydictyon paniculatum (ADU, A46861).

Figure 72 image

Figure 72   enlarge

Fig. 72. A–E. Pachydictyon polycladum (ADU, A16099). A. Lateral branch system with tetrasporangia and hair tufts below branchings. B. Transverse section of branch with an undivided sporangium. C. Transverse section of older axis with multilayered cortex. D. Upper branches of plant with oogonia. E. Transverse section of branch with oogonial sorus. F–J. Pachydictyon paniculatum (F,G, ADU, A29663; H,J, ADU, A43709; I, ADU, A32634). F. Lateral branch system with tetrasporangia and hair tufts. G. Transverse section of branch with tetrasporangia. H. Transverse section of older axis with multilayered cortex. I. Upper branches of plant with antheridial sori. J. Transverse section of branch With antheridial sorus. K,L. Lobospira bicuspidata (ADU, A42264). K. Apex of axis with apical (a.m.) and lateral (1.m.) meristems and two laterals, the right-hand one becoming bicuspid. L. Transverse section of a lateral with undivided sporangia.

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