Electronic Flora of South Australia Genus Fact Sheet

Genus DICTYOTA Lamouroux 1809a: 331, nom. cons.

Phylum Phaeophyta – Order Dictyotales – Family Dictyotaceae – Tribe Dictyoteae

Thallus dichotomous or becoming irregularly laterally branched, complanate or largely so, 2–40 cm long with branches 1–10 mm broad, axils usually rounded, basally attached usually by rhizoids. Growth by a single apical cell, slightly to distinctly protruding, segmenting by transverse and then longitudinal walls to form, throughout the thallus, a monostromatic, large-celled medulla with a monostromatic small-celled cortex. Phaeoplasts numerous in each cortical cell, discoid. Phaeophycean hairs present as scattered tufts.

Reproduction: Unilocular tetrasporangia scattered or grouped, on both surfaces of the thallus, external to the cortex, producing four large, non-motile meiospores. Sexual thalli usually rare, dioecious, with either scattered sort of oogonia or of antheridia (surrounded by sterile paraphyses).

Life history diplohaplontic and isomorphic.

Lectotype species: D. dichotoma (Hudson) Lamouroux.

Taxonomic notes: A genus of numerous species, many of which are separated only with uncertainty from others, mainly on habit and dimensions. There are few good specific concepts in Dictyota and the degree of morphological variation in many species is considerable. Detailed comparisons are needed between the Australian species described below and similar species from other countries.

This account of southern Australian species should therefore be taken as tentative.

References:

LAMOUROUX, J.V.F. (1809a). Observations sur la Physiologie des Algues marines, et description de cinq nouveaux genres de cette famille. Nouv. Bull. Sci. Soc. Philom. Paris 1, 330–333, Plate 6 fig. 2.

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 DICTYOTA

1. Thallus with occasional irregular proliferations and with basal, terete, multicellular attachment fibres; sporangia (and antheridial, probably also oogonial, sort) in more or less square, block-like patches across the branches, on alternating concave surfaces

D. prolifera

1. Thallus only rarely with proliferations and without basal attachment fibres but with a holdfast of unicellular rhizoids; sporangia scattered or in elongate sort over the flat thallus

2

2. Thallus with elongate sort of sporangia; branches (2–) 5–25 (–30) mm broad

3

2. Thallus with scattered (singly or in small groups) sporangia; branches 0.3–6 (–8) mm

4

3. Thallus medium to dark brown, when mature 20–40 cm long and 1–2.5 (–3) cm broad and usually with unbranched upper parts often 10–15 cm long, with hair tufts and sort often more or less in lines across the branches

D. naevosa

3. Thallus olive-brown to medium brown, usually 10–30 cm long and 3–8 (–12) mm broad, branching fairly close to the apices, with scattered hair tufts and sort

D. diemensis

4. Thallus regularly dichotomous, branches usually 3–8 mm broad with rounded apices (occasionally tapering to narrow apiculate apices), or 1–3 (–4) mm broad with elongate, bare, lower parts (in var. intricata); sporangia densely scattered on broader forms, less so on narrower forms

D. dichotoma

4. Thallus regularly dichotomous and relatively slender [0.3–1 (–1.5) mm broad]. or 1–3 mm broad and either with alternate laterals or proliferous below; sporangia usually sparse

5

5. Branches 0.3–1(4.5) mm broad, dichotomous, not proliferous below; apical cell hemispherical, protruding

D. furcellata

5. Branches usually (1–) 1.5–3 mm broad, dichotomous with proliferations developing from mid and lower parts of the thallus, or with alternate, tapering laterals and with no or few lateral proliferations; apical cell hemispherical to transversely lenticular

6

6. Thallus 1–2 (–3) mm broad, dichotomous with lateral proliferations developing from mid and lower parts of older thalli; sporangia 50–100 µm in diameter

D. alternifida

6. Thallus (1–) 2–3 (–4) mm broad, with alternate laterals and usually without proliferations; sporangia (80–) 100–160 mm in diameter

D. fenestrata


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