Electronic Flora of South Australia Genus Fact Sheet

Genus PLOCAMIUM Lamouroux 1813: 137, nom. cons.

Phylum Rhodophyta – Class Florideophyceae – Order Gigartinales – Family Plocamiaceae

Thallus erect, complanately branched, with the opposite margins bearing alternating series of 2, 3, or more (to 6) compressed lateral branches or ramuli, the lower 1 (–3) normally unbranched with entire or serrate margins, occasionally recurved, the uppermost branched and often continuing growth with the same pattern; growth of axes and main branches sympodial, axes straight or somewhat flexuous; in P. leptophyllum small adventitious ramuli occur on the opposite margin to the alternating series. Attachment by stoloniferous branchlets with small discoid holdfasts; epilithic or epiphytic. Structure uniaxial with a central row of elongate cells surrounded by a pseudoparenchymatous cortex with large inner cells decreasing to a small-celled outer cortex.

Reproduction: Sexual thalli dioecious; procarpic. Carpogonial branches in the upper ramuli, adaxial, or in small axillary branchlets, situated on mid cortical cells, 3-celled with the supporting cell functioning as the auxiliary cell. Cystocarps sessile or pedicellate in the axils of ramuli, carposporophyte with a small basal fusion cell and gonimolobes developing into clusters of ovoid carposporangia, with a cellular pericarp. Spermatangia cut off from surface cortical cells of ramuli or on axillary tufts of branchlets.

Tetrasporangia in simple or branched, elongate or clavate, stichidia in the axils of upper ramuli, zonately divided.

Life history triphasic with isomorphic gametophytes and tetrasporophytes.

Type species: P. vulgare Lamouroux 1813: 137, type cons. [= P.                  cartilagineum
(Linnaeus) Dixon 1967: 58.]

Taxonomic notes: A genus of 20 (or more?) species, characterised by the very distinctive branching system. The southern Australian species were monographed by Womersley (1971a).

References:

DIXON, P.S. (1967). The typification of Fucus cartilagineus L. and F. corneus Huds. Blumea 15, 55–62.

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 (1–7).

WOMERSLEY, H.B.S. (1971a). The genus Plocamium (Rhodophyta) in southern Australia. Trans. R. Soc. S. Aust. 95, 9–27.

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

Author: H.B.S. Womersley

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.

KEY TO SPECIES OF PLOCAMIUM

1. Ramuli mostly in alternate pairs

2

1. Ramuli mostly in alternate series of 3–4 (–5)

6

2. Axes mostly under 2 mm broad; cystocarps sessile, spermatangia covering young ramuli

3

2. Axes mostly over 2 mm broad; cystocarps appearing pedicellate and axillary, spermatangia on terete, axillary branch clusters (or on reduced, axillary branchlets in P. dilatatum)

4

3. Axes slender, mostly less than 1 mm broad; lower ramulus slender and usually entire; stichidia single or in basally branched clusters

P. angustum

3. Axes mostly over 1 mm broad; lower ramulus serrate; stichidia in clusters, branched in their upper part

P. costatum

4. Upper ramulus often remaining dormant and digitate, 2–2.5 mm broad and usually 3–5 mm long; lower ramulus simple and usually entire; axes mostly 3–4 mm broad

P. patagiatum

4. Upper ramulus commonly developing into a lateral branch, not digitate if undeveloped; lower ramulus usually serrate, simple or divided; axes mostly 2–3 (–5) mm broad

5

5. Thallus relatively delicate, often large (to 50 cm high); lower ramulus entire or serrate, in many plants becoming multifid; stichidia mostly 75–100 µm in diameter

P. mertensii

5. Thallus robust, to 25 cm high; lower ramulus serrate and undivided; stichidia robust and clavate, about 250 µm in diameter

P. dilatatum

6. Ramuli mostly in series of 3; thallus robust, axes (1–) 1.5–2 (–3) mm broad and becoming thickened below; stichidia normally in axillary clusters, basally branched only; cystocarps sessile and verrucose

P. preissianum

6. Ramuli in series of 3–4 or more, rarely with some in pairs; thallus slender, axes less than 1.5 mm broad; stichidia axillary but on margins of ramulus and axis, becoming branched in their upper parts; cystocarps sessile but smooth

7

7. Ramuli in alternating series of 3–4, occasionally more or in pairs; axes mostly 1–1 5 mm broad, tapering above; adventitious ramuli, and hooked branch ends or hooked lower ramuli, absent

P. cartilagineum

7. Ramuli in alternating series of (3–) 4–5 with short adventitious ramuli developing from the axis opposite each series and often between the members of a series; axis about 0.5 mm broad, often flexuous; ends of branches or lowest ramuli commonly recurved to a hook

P. leptophyllum


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