Electronic Flora of South Australia Genus Fact Sheet

Genus PACHYDICTYON J. Agardh 1894a: 81

Phylum Phaeophyta – Order Dictyotales – Family Dictyotaceae – Tribe Dictyoteae

Thallus dichotomous, usually 5–20 cm long, developing axes which bear alternate, lateral, branch systems, branches compressed, 0.5–2 (–3) mm broad, axils rounded, basally attached by rhizoids or fibres, usually epiphytic. Growth by a single, protruding, apical cell, segmenting to form a monostromatic, large-celled medulla with the small-celled cortex monostromatic when young and becoming polystromatic (around the axes) on older, lower axes. Phaeoplasts numerous per cell. Phaeophycean hairs present as scattered tufts, often related to the positions of branching.

Reproduction: Unilocular tetrasporangia scattered over the branches, external. Sexual sori scattered.

Life history probably diplohaplontic and isomorphic.

Lectotype: P. _furcellatum J. Agardh [=P. polycladum (Kützing) Womersley].

Taxonomic notes: A genus of four species, two from southern Australia, P. coriaceum (Holmes) Okamura from the N. Pacific, and P. aegerrime Allender & Kraft (1983, p.116) from Lord Howe I.

Pachydictyon differs from Dictyota in that the cortex becomes multi-layered by periclinal divisions only, all around the lower branches or axes in the type and P. paniculatum, though young plants may not show this. However, both P. coriaceum and P. aegerrime show a distromatic cortex only at the margins, and the medulla of the former also may be two cells thick near the margins; these two species are less distinctive as Pachydictyon than the Australian species.

References:

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

ALLENDER, B.M. & KRAFT, G.T. (1983). The marine algae of Lord Howe Island (New South Wales): The Dictyotales and Cutleriales (Phaeophyta). Brunonia 6, 73–130.

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

KEY TO SPECIES OF PACHYDICTYON

1. Terminal branches 150–300 µm broad, divaricate; lower axes 0.5–1 mm broad; hair tufts just below points of branching, often also scattered; sporangia single, scattered along the branches, 100–160 pm in diameter; usually epiphytic on seagrasses	P. polycladum
1. Terminal branches (300–) 400–1200 (–1500) pm broad, usually fastigiate; lower axes 1–3 mm broad; hair tufts scattered; sporangia single or clustered on branches, 80–130 µm in diameter; epiphytic on larger algae	P. paniculatum

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