Heliodendron Gill.K. Br. & Bayly, PhytoKeys 205: 321. 2022.

Figs 226, 227, 228, 235

Archidendropsis subg. Basaltica I.C. Nielsen, Bull. Mus. Natl. Hist. Nat., B, Adansonia Sér . 4, 5(3): 325. 1984. Type: Archidendropsis basaltica (F. Muell.) I.C. Nielsen [≡ Acacia basaltica F. Muell. (≡ Heliodendron basalticum (F. Muell.) Gill.K. Br. & Bayly)]

Type.

Heliodendron basalticum (F. Muell.) Gill.K. Br. & Bayly [≡ Acacia basaltica F. Muell.]

Description.

Trees or shrubs to 37 m. Stipules either resembling small thorns to 1.2 mm long that are caducous, or persistent circular-ovate glands 1-3 mm in diameter. Leaves bipinnate; extrafloral nectaries at junction of pinnae circular or triangular to rhombic, circular glands sometimes at junction of leaflet petiolules; pinnae 1-2 pairs; leaflets 1.5-11 pairs per pinna, opposite, subsessile or long (3.5-7 mm) petiolulate, elliptic to elliptic-lanceolate or oblong. Inflorescence capitula, either simple or arranged into a panicle to 35 cm long. Flowers uniform, bisexual, 4- or 5-merous, yellow to cream, hairy, sessile; calyx gamosepalous, tubular to subcampanulate, symmetrical; corolla gamopetalous, tubular to narrowly campanulate; stamens numerous, united basally into a tube that equals or slightly exceeds the corolla tube; pollen in 16-celled polyads, tectum with isometric channels; ovary solitary and shortly stipitate. Fruits brown, valves chartaceous, oblong, flat and dehiscing along both sutures. Seeds lacking a pleurogram, flat, circular to ovate or obliquely ovate, with a narrow 0.2-1 mm peripheral, membranous wing.

Chromosome number.

Unknown.

Included species and geographic distribution.

Three species restricted to the state of Queensland in Australia (Fig. 235).

Ecology.

One species [ H. xanthoxylon (C.T. White & W.D. Francis) Gill.K. Br. & Bayly] is found in rainforest remnants on granitic soils whereas the other two species [ H. basalticum and H. thozetianum (F. Muell.) Gill.K. Br. & Bayly] are found in drier habitats on sandy to sandy-clay loam soils, either in eucalypt woodland, acacia shrubland or semi-deciduous vine thickets.

Etymology.

From helios (Greek = sun) in reference to the endemic distribution in the Australian state of Queensland, widely known as the "sunshine state", and to the capitate, sun-like inflorescences of yellow flowers, and dendron (Greek = tree) to the tree habit.

Human uses.

Heliodendron has been used as timber for general building and the manufacture of window frames, cabinets, barrels and boat building (Swain 1928).

Notes.

Heliodendron was newly described for the morphologically distinct Archidendropsis subg. Basaltica after phylogenetic studies found the two subgenera of Archidendropsis to be non-monophyletic (Brown et al. 2022). The rainforest H. xanthoxylon has larger leaflets and fewer leaflets per pinna than the two drier habitat species, but they all share the diagnostic features of the genus (inflorescences in capitula, seeds flat, lacking a pleurogram and with a narrow peripheral membranous wing, calyx and corolla with hairs, pods narrowly oblong, brown, opening along both sutures, pollen polyad diameter of 55-68 μm and pollen tectum with isometric channels) and there would be little benefit in separating H. xanthoxylon as a monospecific genus.

Taxonomic references.

Brown et al. (2022); Cowan (1998); Nielsen (1981a).