Oldenlandia is a genus of flowering plants belonging to the family Rubiaceae (the coffee or bedstraw family), placed in the order Gentianales. With roughly 240 accepted species, it is pantropical in distribution, occurring across tropical regions of Africa, Asia, the Americas, and Pacific islands.
The genus is morphologically diverse but typically comprises small annual or perennial herbs, often with opposite leaves and small, inconspicuous four-petaled flowers. The type species is Oldenlandia corymbosa, a widespread annual weed of disturbed habitats.
The circumscription of Oldenlandia has been contentious. Historically, many of its species were absorbed into a broadly defined Hedyotis, but phylogenetic work has since narrowed Hedyotis to a monophyletic group of approximately 115 species that excludes Oldenlandia. As currently defined, Oldenlandia is itself polyphyletic and is expected to be revised: the core group around the type species (informally called Oldenlandia sensu stricto) is sister to a lineage of Kohautia that will eventually be described as a separate genus.
Several species hold significance in traditional medicine across tropical Asia and Africa. A number of species—particularly island endemics—are threatened with extinction, and at least one species and one variety are recorded as completely extinct.
Etymology
The genus was established by Linnaeus in 1753 in Species Plantarum and named in honor of Henrik Bernard Oldenland (c. 1663–1697), a Danish botanist who worked in southern Africa and collected plants at the Cape of Good Hope.
Distribution
Oldenlandia is pantropical, with species occurring across tropical Africa, Asia, Australia, the Pacific islands, and the Americas. Many species are island endemics, making them vulnerable to extinction.
Taxonomy Notes
Oldenlandia has a long and unsettled taxonomic history. Some botanists submerged the genus into a broadly circumscribed Hedyotis; molecular phylogenetics has since shown that Hedyotis (restricted to ~115 species) and Oldenlandia are separate groups. Oldenlandia as currently defined is polyphyletic and will eventually be split: the core around the type species (O. corymbosa) is sister to part of Kohautia and will form a revised, monophyletic genus. The former species Oldenlandia diffusa has been transferred to Scleromitrion diffusum.
Cultural Uses
Several species of Oldenlandia are used in ethnomedicine across tropical Asia and Africa. Oldenlandia diffusa (now Scleromitrion diffusum) has been widely used in traditional Chinese medicine as an anti-inflammatory and in cancer-supportive care.
