Geissois is a genus of trees and shrubs in the family Cunoniaceae, order Oxalidales. The genus comprises approximately 19 species concentrated in New Caledonia, with additional representatives in Fiji, Vanuatu, and the Solomon Islands.
Plants in this genus bear opposite, palmately compound leaves with 3–9 leaflets. Margins are typically entire, though Geissois hirsuta and juvenile plants of several species have serrate margins. Intrapetiolar stipules are a consistent feature across the genus.
The inflorescences are generally simple racemes with a characteristic bottle-brush appearance, formed by dense clusters of flowers with four red sepals, no petals, and numerous long red stamens. Geissois hirsuta is unusual in producing trident-branched inflorescences. The fruit is a capsule containing flat, winged seeds.
A notable ecological feature of the genus is its relationship with ultramafic (serpentine) soils: most species are nickel hyperaccumulators, capable of concentrating exceptionally high levels of nickel in their tissues. Geissois polyphylla is recorded as an aluminium hyperaccumulator. These traits make the genus of interest to researchers in phytoremediation and plant metal tolerance.
The genus was formerly broader; two Australian species have since been transferred to the segregate genus Karrabina, making Geissois in its current circumscription an essentially Melanesian group.
Distribution
Geissois is centred on New Caledonia, which holds the great majority of its roughly 19 species. The genus also occurs in Fiji, Vanuatu, and the Solomon Islands. Most species grow on ultramafic (serpentine) substrates, which are widespread across New Caledonia.
Ecology
Most Geissois species grow on ultramafic rocks and are nickel hyperaccumulators, concentrating high levels of nickel in their tissues — a rare adaptation shared by only a small fraction of vascular plants worldwide. Geissois polyphylla is additionally recorded as an aluminium hyperaccumulator. This physiological tolerance allows the genus to colonise serpentine soils that exclude most other woody plants.
Taxonomy Notes
Geissois belongs to the family Cunoniaceae in the order Oxalidales. Two species previously included in the genus — both Australian — have been transferred to Karrabina, restricting Geissois to its Melanesian range. GBIF recognises 18 accepted species-level taxa. The genus was described by Labillardière.
