Electronic Flora of South Australia Genus Fact Sheet

Genus PACHYMENIA J. Agardh 1876: 143

Phylum Rhodophyta – Class Florideophyceae – Order Gigartinales – Family Halymeniaceae

Thallus foliose, split or lobed, relatively thick, mucilaginous to cartilaginous, expanding gradually or suddenly from a short to cuneate stipe, with a discoid holdfast. Structure multiaxial with broad to tapering apices, developing a thick cortex with outer anticlinal filaments of ovoid to elongate cells, usually 8–25 cells long and branched every 2–5 cells, and an inner cortex of isodiametric cells, often slightly stellate and separated (apart from pit-connections), in some species pseudoparenchymatous. Medulla dense, of irregular filaments of two thicknesses, with or without refractive ganglionic cells.

Reproduction: Sexual thalli dioecious. Carpogonial branch ampullae originating in the inner cortex, with several secondary and tertiary filaments (sometimes to 5 orders) of short cells, converging above, and a 2-celled carpogonial branch. Auxiliary cell ampullae in the inner cortex, profusely branched with many filaments, up to 5 orders, broad below, slightly converging above and extending through the outer cortex. Carposporophytes compact, usually with a basal fusion cell; involucre prominent, derived from the ampullary filaments which cut off numerous further short chains of cells, forming a loose surround several filaments thick to the carposporophyte, becoming more compact as the latter matures; ostiole present or absent. Spermatangia derived from surface cortical cells.

Tetrasporangia scattered, cut off from mid cells of the outer cortex and lying just below the surface, cruciately divided.

Life history triphasic with isomorphic gametophytes and tetrasporophytes.

Lectotype species: P. carnosa (J. Agardh) J. Agardh 1876: 145.

Taxonomic notes: A genus of 5 or 6 species, confined to South Africa, southern Australia and New Zealand. The very robust fronds are characterised by the thick outer cortex of anticlinal filaments branched every 2–5 cells, the medullary filaments of two thicknesses, and the much-branched auxiliary cell ampullae of small ovoid cells, with further small-celled filaments cut off to form the prominent involucre. Simons & Hewitt (1976) consider elongate cells ("rays") radiating from the basal parts of the auxiliary cell, below the carposporophyte, are also characteristic of Pachymenia.

References:

AGARDH, J.G. (1876). Species Genera et Ordines Algarum. Vol. 3, Part 1 - Epicrisis systematis Floridearum, pp. i-vii, 1–724. (Weigel: Leipzig.)

SIMONS, R.H. & HEWITT, F.E. (1976). Marine algae from southern Africa. 2. Morphology and taxonomy of five foliaceous Florideophyceae (Rhodophyta). Invest! Rep. Sea Fish. Brch S. Afr. 110, 1–46.

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

Author: H.B.S. Womersley & J.A. Lewis

Publication: Womersley, H.B.S. (14 January, 1994)
The Marine Benthic Flora of Southern Australia
Rhodophyta. Part IIIA, Bangiophyceae and Florideophyceae (to Gigartinales)
Reproduced with permission from The Marine Benthic Flora of Southern Australia Part IIIA 1994, by H.B.S. Womersley. Australian Biological Resources Study, Canberra. Copyright Commonwealth of Australia.


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