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Electronic Flora of South Australia Genus Fact Sheet
Phylum Rhodophyta – Family Rhodomelaceae – Tribe Laurencieae
Thallus erect, moderately to much branched radially or distichously for 2–5 orders, soft to firm, usually drying cartilaginous, branches with truncated apices with a depressed apical pit; holdfast discoid or stoloniferous, epilithic or epiphytic. Structure. Apical depression with a central, short, apical filament bearing well-developed trichoblasts and 4 pericentral cells per axial cell, axial cells short, these and the pericentral cells soon obscured and not recognisable in sections, cortex extensive with relatively large, ovoid, usually compact, irregular cortical cells and a small-celled epidermis with secondary pit-connections between the cells lengthwise to the branch and usually with corps en cerise; inner cortical cells often producing rhizoids and in some species with lenticular wall thickenings. Lateral branches arising from basal cells of trichoblasts. Cells uni- or multinucleate; rhodoplasts discoid to elongate, often chained in larger inner cells.
Reproduction: Gametophytes dioecious. Procarps formed on the fourth pericentral cells on the suprabasal cell of trichoblasts, consisting of a lateral sterile group of 4–6 cells, a 4-celled carpogonial branch and a basal sterile group of 2–3 cells. Carposporophytes with a basal fusion cell and branched gonimoblast bearing clavate terminal carposporangia, usually replaced from cells below when lost. Cystocarps usually lateral on branches, usually sessile, basally constricted or broad-based and in some species conical; pericarp arising pre-fertilization, ostiolate, corticated and 3–6 cells thick. Spermatangia borne in lateral whorls on cells of modified trichoblasts in the apical cup-like depressions, each axial cell with 4 pericentral cells and ultimate spermatangia, the filaments terminated by an enlarged globular cell.
Tetrasporangia borne, usually abaxially, on outer ends of laterally elongate pericentral cells or pit-connected to outer cortical cells, usually in "parallel" (down the branch) arrangement, tetrahedrally divided.
Type species: L. obtusa (Hudson) Lamouroux 1813: 42.
Taxonomic notes: A genus of many species, common on most coasts, with some 12 species on southern Australian coasts. The genus in southern Australia was monographed by Saito & Womersley (1974), but details of number of pericentral cells, presence of corps en cerise, structure of procarps, spermatangial trichoblasts and origin of tetrasporangia still await description for nearly all species.
Old records frequently include L. obtusa, but the species involved remain uncertain.
References:
LAMOUROUX, J.V.F. (1813). Essai sur les genres de la famille des thalassiophytes non articulées. Ann. Mus. Hist. Nat., Paris 20, 21–47, 115–139, 267–293, Plates 7–13.
SAITO, Y. & WOMERSLEY, H.B.S. (1974). The southern Australian species of Laurencia (Ceramiales: Rhodophyta). Aust. J. Bot. 22, 815–874.
The Marine Benthic Flora of Southern Australia Part IIID complete list of references.
Publication:
Womersley, H.B.S. (24 February, 2003)
The Marine Benthic Flora of Southern Australia
Rhodophyta. Part IIID. Ceramiales – Delesseriaceae, Sarcomeniaceae, Rhodomelaceae
Reproduced with permission from The Marine Benthic Flora of Southern Australia Part IIID 2003, by H.B.S. Womersley. Australian Biological Resources Study, Canberra. Copyright Commonwealth of Australia.
KEY TO SPECIES OF LAURENCIA
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1. Thallus irregularly radially branched, branches terete | 2 |
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1. Thallus complanately branched, branches slightly to strongly compressed | 9 |
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2. Epidermal cells near apices with slightly projecting, convex, outer walls; thallus densely branched with many short laterals | 3 |
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2. Epidermal cells near apices without distinctly convex outer walls; thallus loosely to densely branched | 4 |
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3. Thallus usually with percurrent axes mostly | L. majuscula |
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3. Thallus without distinct percurrent axes, more-or-less entangled, branches slender, | L. aldingensis |
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4. Thallus usually less than 8 cm high, growing on Posidonia, with a single discoid holdfast and axis, branches subdichotomous; lenticular thickenings in cortical cells usually abundant | L. forsteri |
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4. Thallus usually over 8 cm high, rarely on Posidonia, with one to several axes irregularly branched, lenticular thickenings absent or occasional | 5 |
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5. Thallus soft when young, not cartilaginous; branches often clavate, | 6 |
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5. Thallus firm, usually cartilaginous when dried, irregularly branched with smal branchlets, not markedly basally constricted | 7 |
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6. Laterals distinctly basally constricted with a narrow stalk; young branches appearing articulate, older branches terete with a thickened cortex; laterals often clustered at upper ends of segments | L. clavata |
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6. Laterals not or only slightly basally constricted, often subopposite but rarely clustered | L. shepherdii |
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7. Thallus relatively slender, axes usually | L. arbuscula |
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7. Thallus moderately to very robust, axes often over 2 mm in diameter, lesser ramuli usually about 1 mm in diameter; stichidia simple or branched, transformed from branchlet ends, usually | 8 |
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8. Thallus robust, with percurrent axes bearing irregularly short laterals, cortical cells without lenticular thickenings | L. tasmanica |
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8. Thallus slender to moderately robust, with or without percurrent axes, branching often distant and without short laterals, cortical cells often with lenticular | L. filiformis |
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9. Branches subterete to slightly compressed, main and lateral branches alternately distichous; stichidia (and spermatangial ramuli) in botryoidal clusters | L. botryoides |
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9. Branches distinctly compressed, branching distichous or irregular; tetrasporangia in ramuli, not botryoidal | 10 |
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10. Branching irregularly alternately distichous, rarely pinnate, with laterals usually distant; branches slightly to moderately compressed, usually over 750 µm thick; mature thallus usually | L. elata |
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10. Branching regularly alternately pinnate, laterals closely arranged; branches strongly compressed, usually less than 700 µm thick; mature thallus usually | 11 |
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11. Thallus usually | L. brongniartii |
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11. Thallus usually | L. distichophylla |
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Email Contact: State Herbarium of South Australia |
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