2665

12(3): Medicinal and poisonous plants 3

Homalanthus A.Juss.

Euphorb. gen.: 50, t. 16, f. 53 (1824; 'Omalanthus')

EUPHORBIACEAE

x = unknown; Homalanthus populneus: n = 76, 2n = 36

Homalanthus comprises about 22 species and occurs from Taiwan and southern Thailand, throughout the Malesian region, to the Pacific Islands, New Caledonia and Australia. In Malesia most of the 13 indigenous species are endemic.

Several parts of Homalanthus plants are used in traditional medicine. The wood is used as an anodyne, leaves to treat fever and after childbirth, and as a vermifuge in cattle, leaf sap as a mild laxative, terminal buds as abortifacient, a decoction of the bark during and after childbirth, and fruits for treating wounds.
The sap is a fish poison. A decoction of bark and leaves is used as a black dye; the leaves are suitable for wrapping food. The wood is sometimes used for house construction, and as firewood. Homalanthus may be suitable for afforestation.
The pulverized wood of Homalanthus nutans (J.G. Forster) Guillemin steeped in hot water produces a brew that is drunk in Samoa by patients with yellow fever.

The whitish sap from Homalanthus plants is irritant to the eyes. It is poisonous and should be administered in low doses.
Ethanol extracts of leaves, stem and roots of Homalanthus nervosus showed a broad-spectrum antibacterial activity, but no antifungal effect.
A wood extract of Homalanthus nutans from Samoa exhibited strong in-vitro activity against human immunodeficiency virus (HIV-1), with the phorbol derivative prostratin as the active compound. This compound does not appear to be a tumour promotor as is the case with several related compounds.

Monoecious, small to medium-sized trees, with latex in all parts; bark nearly smooth; branches pseudoverticillate. Leaves alternate, simple and entire, often whitish below, mostly glabrous, glands present, pinnately veined, petiolate and not too distinctly peltate; stipules large, with distinct parallel venation, caducous. Inflorescence a thyrse with female flowers at base and cymules of male flowers higher up, bracteate. Flowers unisexual, zygomorphic, distinctly pedicellate, with calyx consisting of (1—)2(—3) free, kidney-shaped lobes, petals absent; male flowers yellowish, with articulate pedicel and 4—30 free stamens; female flowers with non-articulate pedicel and superior, 2—3-celled ovary, style very short to distinct, stigma usually recurved, undivided or divided. Fruit a thinly woody capsule, opening primarily loculicidally, (1—)2—3-seeded. Seeds usually irregularly foveolate and often flattened, smooth, covered apically or over the whole length by a pale to whitish, membranous arillode.
The foliar glands attract ants, which may protect the plant from grazing and insect damage. The flowers seem to be pollinated by various insects, whereas the often reddish seeds with a pale arillode are probably dispersed by animals such as birds.
Homalanthus has an isolated position within the tribe Hippomaneae. The genus was originally described as Omalanthus, but recently the later orthographic variant Homalanthus has been conserved in the International Code of Botanical Nomenclature.

Homalanthus is characteristically found in secondary forest, where it occasionally dominates the vegetation. It is usually most common in montane forest and may fill there the ecological niche as a pioneer, in the same way as Macaranga in the lowland.

Homalanthus often occurs in secondary habitats, is locally common and does not seem to be liable to genetic erosion. Most species are relatively homogeneous, but the more widespread ones, such as Homalanthus novoguineensis and particularly Homalanthus populneus, are variable.

More research on the phytochemistry and pharmacological properties of Homalanthus seems justified because it is used throughout its area of distribution as a medicinal plant, albeit to a limited extent. Homalanthus may also be useful for afforestation and for pulp and paper production.

[234]Esser, H.-J., 1997. A revision of Omalanthus (Euphorbiaceae) in Malesia. Blumea 42(2): 421—466.
[304]Gustafson, K.R. et al., 1992. A nonpromoting phorbol from the samoan medicinal plant Homalanthus nutans inhibits cell killing by HIV-1. Journal of Medicinal Chemistry 35(11): 1978—1986.

P.C. van Welzen

Homalanthus fastuosus
Homalanthus longistylus
Homalanthus nervosus
Homalanthus novoguineensis
Homalanthus populneus

van Welzen, P.C., 2003. Homalanthus A.Juss.. In: Lemmens, R.H.M.J. and Bunyapraphatsara, N. (Editors): Plant Resources of South-East Asia No 12(3): Medicinal and poisonous plants 3. PROSEA Foundation, Bogor, Indonesia. Database record: prota4u.org/prosea

The following species in this genus are important in this commodity group and are treated separatedly in this database:
Homalanthus fastuosus
Homalanthus longistylus
Homalanthus nervosus
Homalanthus novoguineensis
Homalanthus populneus