CMT VIII 2011: Tarenna comprises several species of shrubs and small trees found in various habitats of Madagascar. This genus is characterized by its interpetiolar stipules; its generally tough-textured leaves; its cymose, terminal, bracteate inflorescences; its homostylous flowers with 4-5 calyx and corolla lobes; its salverform, generally white or yellow corollas with the tube usually (though not always) shorter than the lobes and the lobes contorted in bud; its exserted clavate stigmas; its bilocular ovaries with 2 to several ovules per locule; and its indehiscent fruits with the exocarp leathery to fleshy and generally 2 to several rather lenticular seeds with entire (i.e., not ruminate) endosperm. A characteristic of Tarenna is wide variation in number of ovules and seeds per locule, and per fruit. Tarenna's homostylous flowers also have variation in the position of the anthers and stigmas, which are usually exserted but not always.
Coptosperma is very similar to Tarenna in general aspect and most of its characters. De Block et al. (2001) outlined the separation of these two genera in detail, and noted that the previously used name Enterospermum is actually a synonym of Coptosperma. Coptosperma can be recognized by its fruits with a single developed locule and a single generally rounded seed with the endosperm ruminate, at least near the hilum; additionally species of Coptosperma generally have several to many flowers borne in branched cymes. Tarenna has fruits with 2 to several generally somewhat flattened seeds with entire endosperm; additionally Tarenna often has one to a few flowers that are often borne on well developed pedicels or peduncles. Coptosperma apparently includes many more species in Madagascar than Tarenna. Also similar to both of these genera is Robbrechtia, which differs from both Coptosperma and Tarenna by its large salverform corollas, and can also be recognized by its combination of quite large stipules that are shortly united around the stem and relatively long calyx limbs. Paracephaelis is also similar; it has fruits similar to those of Tarenna but inflorescences similar to those of Coptosperma, and usually has soft-textured pilosulous leaves. Also similar to Tarenna in general aspect and some characters is Ixora; however Ixora can be separated by its petioles that are articulated at the stem attachment and its drupaceous fruits with two planoconvex pyrenes.
An excellent and extremely useful synoptic taxonomy of Tarenna in Madagascar was presented by De Block (2005).