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ORDER SCYTOSIPHONALES Feldmann

Phylum Phaeophyta

Thallus (gametophyte) terete or compressed, foliose, or convolute to subglobose, or clathrate, solid or hollow, simple or branched, mostly 2–30 cm high. Growth diffuse or subapical. Structure polystichous, with an inner medulla of larger, clearer cells and an outer cortex of small cells each with a single phaeoplast with a conspicuous pyrenoid; physodes common; phaeophycean hairs usually frequent, single or in groups in pits, or on the surface. Alternate thallus crustose or discoid, haplostichous and pseudoparenchymatous, with a radiating basal layer producing erect, compact filaments several cells long, developing apically.

Reproduction: Gametophytes with sori of plurilocular organs producing zooids or, under certain conditions, iso- or anisogametes. Crustose stage diploid or haploid, bearing sori of unilocular sporangia.

Life history potentially diplohaplontic and heteromorphic but often direct or from the erect gametophyte (mainly in winter) through a filamentous or discoid stage (mainly in summer).

Taxonomic notes: An order with two families, the Scytosiphonaceae, with some 8 genera, four of which occur on southern Australian coasts (and Endarachne and Rosenvingia on eastern Australian coasts), and the Chnoosporaceae with Chnoospora from tropical-subtropical waters.

The Scytosiphonales was separated from the Dictyosiphonales by Feldmann (1949) on account of the single phaeoplast per cell with a large pyrenoid, and the presence of only plurilocular reproductive organs on the conspicuous (potentially) gametophytic thallus. The zooids from these reproductive organs usually develop asexually, but under certain conditions of day length and temperature they function as gametes. The presence of a heteromorphic cycle, with a crustose sporophyte (corresponding to subgenus Stragularia of Ralfsia), was first established by Nakamura (1965) and Tatewaki (1966) for Scytosiphon, and the reproductive complexities have been discussed by Wynne & Loiseaux (1976, p. 444), Clayton (1980), Wynne (1987, p. 70), Pederson (1981, p. 206) and others.

References:

CLAYTON, M.N. (1980). Sexual reproduction-a rare occurrence in the life history of the complanate form of Scytosiphon (Scytosiphonaceae, Phaeophyta) from southern Australia. Br. phycol. J 15, 105–118.

FELDMANN, J. (1949). L'ordre des Scytosiphonales. Mem. Soc. Hist. Nat. Afr. Nord 2, 103–115.

NAKAMURA, Y. (1965). Development of zoospores in Ralfsia-like thallus, with special reference to the life cycle of the Scytosiphonales. Bot. Mag. Tokyo 78, 109–110.

TATEWAKI, M. (1966). Formation of a crustaceous sporophyte with unilocular sporangia in Scytosiphon lomentaria. Phycologia 6, 62–66.

WYNNE, M.J. & LOISEAUX, S. (1976). Recent advances in life history studies of the Phaeophyta. Phycologia 15, 435–452.

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
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