Electronic Flora of South Australia Genus Fact Sheet

Genus HOMOEOSTRICHUS J. Agardh 1894a: 14

Phylum Phaeophyta – Order Dictyotales – Family Dictyotaceae – Tribe Zonarieae

Thallus more or less erect, 5–15 (–20) cm long, linear or flabellate at least at the branch ends, branched by apical splitting, stupose below with a matted, rhizoidal holdfast. Growth by a marginal row of apical cells on each branch, segmenting posteriorly in actively growing apices and usually becoming wedge shaped, segmenting on two faces in relatively inactive apices. Fronds linear or broadest near their apices and tapering below, corn planate, often with faint, concentric, growth zones. Structure 5–7 cells thick throughout, cells of uniform size and in regular rows in transverse section (20–25 µm across), elongate in longitudinal section, with the cortical cells the same size as medullary cells (rarely paired) in transverse section and 2–4 (–6) overlying each medullary cell lengthwise; phaeoplasts dense in cortical cells, less so in medullary cells. Hair tufts (phaeophycean) absent. Paraphyses only in sporangial sori or in scattered tufts or occasionally in more or less concentric zones, without a basal meristem but with apical and subapical divisions, determinate, cells with phaeoplasts.

Reproduction: Sporangia in scattered sori, usually with paraphyses, producing four spores. Sexual reproduction unknown.

Life history unknown, possibly direct from the sporophyte.

Lectotype species: H. sinclairii (Hooker & Harvey) J. Agardh (replacing H. multifidus J. Agardh — see below).

Taxonomic notes: Homoeostrichus was established by J. Agardh (1894a, p. 14) for species of Zonaria where the cortical cells are mostly not paired, and the generic type description also stated that the surface sori were prominent, with obovate fertile cells (sporangia) and erect, subclavate, articulate, free, paraphyses. J. Agardh included four species in two sections; H. multifidus from South Africa was in the first section, with partly decumbent fronds, and the second section, with erect fronds, included the Australian species H. sinclairii, H. stuposus (=H. sinclairii) and H. canaliculatus. The only species for which J. Agardh described sporangia was H. sinclairii.

Papenfuss (1944, p. 340) considered Homoeostrichus not generically distinct from Zonaria and in showing that harvevana is the correct specific name for the South African species, he stated that "the first species listed under Homoeostrichus and presumably the type of the genus ... is Z. multifida". Later Papenfuss (1977, p. 274) stated he had "designated H. multifidus as lectotype of the genus."

When J. Agardh described Homoeostrichus, no description of H. multifidus referred to sporangial sori, and his generic description must have been based on H. sinclairii in this respect. Further, it is now clear that the sporangial sori* of H. multifidus (Z. harvevana) do not contain paraphyses, in this way excluding it from the generic description.

The vegetative features of J. Agardh's generic description cover H. multifidus and H. sinclairii equally well. It thus seems clear that Papenfuss' selection of H. multifidus as the lectotype species was inappropriate, and H. sinclairii is here selected as lectotype in that it agrees better with the generic type description.

With H. sinclairii as lectotype species, the genus Homoeostrichus is scarcely distinct from Zonaria on vegetative features since the difference in pairing of the cortical cells is one of degree-in Zonaria they are mostly paired, in Homoeostrichus relatively few are paired. However, the realisation that the sporangia of Homoeostrichus sinclairii contain only four spores in contrast to the eight in species of Zonaria provides a more satisfactory separation and justifies maintaining Homoeostrichus as a distinct genus.

H. olsenii, described below, agrees well generically with H. sinclairii, but H. canaliculatus is retained in the genus on vegetative features only (sori are unknown). H. flabellatus Okamura (1931, p. 57, pl. 279 figs 11–13) from Japan is almost certainly not a species of Homoeostrichus.

A B

References:

AGARDH, J.G. (1894a). Analecta Algologica. Cont. I. Acta Univ. lund. 29, 1–144, Plates 1, 2.

OKAMURA, K. (1931). Icones of Japanese Algae. Vol. 6, pt. 6, pp. 47–62, Plates 276–280.

PAPENFUSS, G.F. (1944). Notes on algal nomenclature. III. Miscellaneous species of Chlorophyceae, Phaeophyceae and Rhodophyceae. Farlowia 1, 337–346.

PAPENFUSS, G.F. (1977). Review of the genera of Dictyotales (Phaeophycophyta). Bull. Jap. Soc. Phycol. 25, Suppl. Mem. Iss. Yamada, 271–287.

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
©Board of the Botanic Gardens and State Herbarium, Government of South Australia

KEY TO SPECIES OF HOMOEOSTRICHUS

1. Thallus branches linear, mostly 1–1.5 mm broad, drying grey-brown with the surface slightly farinaceous

H. canaliculatus

1. Thallus branches not linear, either broadly flabellate or with flabellate ends and slender, denuded, lower branches; light brown to grey-brown

2

2. Thallus light brown, much branched, with slender lower parts and flabellate branch ends 2–6 (–10) mm broad; mostly six cells thick

H. sinclairii

2. Thallus grey-brown, sometimes slightly farinaceous, flabellate and 5–10 (–15) mm broad above; five cells thick throughout

H. olsenii


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