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Electronic Flora of South Australia Family Fact Sheet
Phylum Phaeophyta
Thallus moderate to large, usually 10 cm to 3 m long, epilithic or rarely epiphytic, or occasionally free-floating or loose-lying, differentiated into a discoid or hapteroid holdfast, rarely stoloniferous, a small stipe, and a moderately to much and variously branched frond. Structure polystichous, internally differentiated into a solid (usually) or hollow medulla of elongate cells or filaments and an outer cortex of several layers of small, isodiametric cells, with a surface meristoderm and usually with paraphyses in cryptostomata. Vesicles present in many genera. Growth from an apical cell(s) sunk within an apical depression formed by rapid development of lateral derivatives; apical cells single and three- or four-sided in transverse section, or several and four-sided, or a group of 4 apical cells each segmenting outwardly. Phaeoplasts numerous per cell, discoid, without a pyrenoid (except in eggs of some taxa-Evans 1968).
Reproduction: Reproduction oogamous, with oogonia and antheridia borne, together with hairs, within conceptacles scattered over the branches or on more or less defined receptacles. Thalli monoecious or dioecious, conceptacles unisexual or bisexual; oogonia sessile on the conceptacle wall, containing 1, 2, 4 or 8 eggs following meiosis and formation of 8 nuclei; antheridia sessile or on branched paraphyses, with 64 sperms per antheridium, the posterior flagellum longer than the anterior one.
This is a distinctive order, with no gametophyte present in the life history and the sporophyte bearing conceptacles containing oogonia and antheridia within which meiosis occurs. The sunken apical cells are also distinctive of the order. However, the life history has been interpreted (Jensen 1974) as heteromorphic, with a highly reduced endosporic gametophyte within the unilocular sporangia (oogonia or antheridia). Growth in most genera occurs by segmentation of a single three- or four-sided apical cell, or from a compact group of four in Hormosiraceae or from a line of a few in Seirococcaceae. Fucus (Fucaceae) from early reports has a single, four-sided, apical cell, but Moss (1967) claimed that this apical cell rarely segments though it controls adjacent cell divisions. McCully (1966) in F. vesiculosus and F. edentatus however, found a group of 4–8 actively dividing initials in rapidly growing plants.
This order contains six families, five of which are well represented on southern Australian coasts. The Fucales have their greatest richness here, but nearly all taxa (except Hormosira) are sublittoral, in contrast to the several intertidal taxa on temperate northern hemisphere coasts.
Life history diplontic, with meiosis at gametogenesis.
References:
EVANS, L.V. (1968). Chloroplast morphology and fine structure in British fucoids. New PhytoL 67, 173–178.
JENSEN, J.B. (1974). Morphological studies in Cystoseiraceae and Sargassaceae (Phaeophyceae) with special reference to apical organisation. Univ. Calif Publ. Bot. 68, 1–61, Plates 1–16.
MOSS, B. (1967). The apical meristem of Fucus. New Phyla 66, 67–74, Plate 5.
The Marine Benthic Flora of Southern Australia Part II complete list of references.
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 FAMILIES OF FUCALES
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1. Growth by a group of four apical cells, each three-sided in transverse section; thallus of branched, moniliform, chains of spherical to elongate vesicles, with conceptacles in the wall; oogonia with 4 eggs | HORMOSIRACEAE |
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1. Growth by a single (rarely a short row or cluster) apical cell; thallus usually much branched and compressed (at least in part); oogonia with one or four eggs (in Australian taxa) | 2 |
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2. Apical cell(s) four-sided in transverse section; one or four eggs per oogonium | 3 |
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2. Apical cell three-sided in transverse section; one egg per oogonium | 4 |
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3. Thallus usually | FUCACEAE |
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3. Thallus usually 50 cm-3 m long, irregularly laterally branched, with receptacles lateral to the main branches or laterals; holdfast discoid but with marginal haptera; apical cells often | SEIROCOCCACEAE |
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4. Receptacles developed from vegetative branches, not axillary; lower laterals usually not differentiated from upper fertile parts (except in some species of Cystoseira), terete to flattened | CYSTOSEIRACEAE |
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4. Receptacles distinct from vegetative branches, developed in axillary positions, simple or branched; lower laterals usually thin and leafy (in Sargassum) | SARGASSACEAE |
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