Plagianthus is a small genus of flowering plants in the family Malvaceae (subfamily Malvoideae), order Malvales, endemic to New Zealand and the Chatham Islands. The genus comprises just two currently recognised species with a naturally occurring hybrid: Plagianthus divaricatus, a divaricate shrub found at the edges of salt marshes, and Plagianthus regius (ribbonwood), a tree whose juvenile form may also be divaricate. A natural hybrid between the two, Plagianthus × cymosus, occurs on the Chatham Islands and on both the North and South Islands of New Zealand.
The taxonomic history of Plagianthus was long debated: related genera including Hoheria (New Zealand) and several Australian genera such as Asterotrichion, Lawrencia, and Gynatrix were at various times placed within it. Modern molecular studies have resolved the genus firmly within Malvaceae subfamily Malvoideae; these formerly included relatives are now grouped informally as the "Plagianthus alliance" but maintained as distinct genera.
Etymology
The genus name Plagianthus derives from Greek, meaning "slanted flowers", a reference to the asymmetrical appearance of the blooms.
Distribution
Plagianthus is endemic to New Zealand and the Chatham Islands. P. divaricatus grows in coastal salt-marsh habitats, while P. regius occupies a broader range of lowland forest and scrubland across both the North and South Islands and the Chatham Islands.
Taxonomy Notes
The familial placement of Plagianthus was disputed for many years, with several related genera (including Hoheria, Asterotrichion, Lawrencia, and Gynatrix) historically placed within it. Modern genetic studies confirm the genus belongs in Malvaceae subfamily Malvoideae. The formerly included genera are now treated as distinct but are informally grouped in a "Plagianthus alliance".
