The genus Oldenlandia belongs to a complicated group of genera that is mainly Paleotropical and includes Hedyotis, Kohautia, Pentodon, Cordylostigma, Astiella, and others. The generic limits of Oldenlandia are not yet understood, and the genus has been circumscribed very differentlly in different floras. The circumscription here provisionally follows Verdcourt (1976). The species included here in Oldenlandia are also found in Africa, and several are weedy and adventive in other continents; in particular O. lancifolia and O. corymbosa are found in tropical areas worldwide.
Oldenlandia is quite similar to Cordylostigma, but Cordylostigma is distinguished by its single, ovoid or egg-shaped stigma and usually pink, red, purple, or blue flowers that are not distylous, vs. two linear stigmas and white to pink or occasionally purple flowers that usually are distylous in Oldenlandia. Often species of Oldenlandia have mostly axillary inflorescences that are unbranched or branched only to 1 order while Cordylostigma has terminal inflorescences that are branched to several orders; however some species of Oldenlandia, such as O. affinis, also develop lax terminal inflorescences. Oldenlandia flosculosa Hiern has been reported in sched. from Madagascar (Razakamalala et al. 1372), but that specimen is here re-identified as Cordylostigma virgata based on its single ovoid stigma and capsules borne on short to well developed pedicels; Oldenlandia flosculosa is so far only known from coastal Tanzania, and has mostly subsessile capsules about half as large as those of C. virgata.
Also similar is Hedyotis trichoglossa, which is keyed below but treated here in Hedyotis (where it has a validly published name). Groeninckx (2006) suggested that this species may be best included in Oldenlandia but the information available did not allow a firm statement of its relationships.
Oldenlandia is also very similar to Pentodon, but Pentodon is separated by its consistently 5 calyx and corolla lobes, vs. 4 in Oldenlandia except variably 4-8 in O. capensis.
!CMT, V 2011