Psychotria is a large ("hyper-diverse") pantropical genus, with more than 2000 described species. It is systematically complex and not yet delimited in detail (Andersson, 2002), and in its current circumscription is heterogeneous due to the taxonomic inclusion of species that belong to other lineages, and also it is paraphyletic due to the separation into segregate genera of some of the species of individual Psychotria clades. These segregate genera are diagnosd by distinctive, unusual characters that have been considered to have been derived only once, however recent molecular and morphological review suggests that in several cases these characters have been derived in parallel several times. Much study remains to understand the systematics of this complex and the taxonomy of the individual species, and the project is complicated by the large number of species involved, the lack of specimen curation or good knowledge of most of them, and the undescribed status of many additional species; all of these factors produce not only a large data set for study, but make good systematic sampling complicated due to difficulty of selecting a broadly representative sample and of identification of materials.
In Madagascar the species of Psychotria and related genera were studied taxonomically by Bremekamp (1958, 1960, 1963), who described the vast majority of the species and most of the genera in this treatment, and also provided revised circumscriptions of several previously described genera. Bremekamp recognized seven separate genera in what is generally considered Psychotria s. str. (Andersson, 2002): Apomuria Bremek., Cremocarpon Bremek., Mapouria Aubl., Psychotria L., Pyragra Bremek., Psathura Comm. ex Juss., and Trigonopyren Bremek. Psychotria can generally be recognized within Rubiaceae by its tissues with raphides and a gray to brown or reddish brown drying color; (4)5-merous flowers; ovary with a single basal ovule in each of its 2(-5)locules; drupaceous fruits with two (to five) planoconvex pyrenes; starchy endosperm; and distinctive stipules. As in all large genera of Rubiaceae the stipules vary within Psychotria, but in general these are characteristically interpetiolar to fused around the stem, triangular to very often bilobed, and deciduous exposing a ring of persistent ferrugineous colleters or pilosulous trichomes. Generally the deciduous stipules plus the persistent colleters separate Psychotria from very similar species of the Palicourea group, which usually have stipules that are persistent or are partly deciduous through fragmentation with the portion of the stipule that remains becoming indurated or marcescent and any trichomes present drying whitened or yellowed. Based on stipule form all of the genera that Bremekamp separated belong to Psychotria except perhaps some species of Trigonopyren. Most species of Trigonopyren have stipules deciduous via fragmentation with the basal portion becoming somewhat hardened, though not yellowed as in most Palicourea Group genera, and with persistent, matted, long pilose-hirsute pubescence that dries brown to ferrugineous; these stipules are also similar to those of some species of Carapichea, a Neotropical genus of the Palicourea Group. Also not reported yet but expected in Madagascar is another genus of the Palicourea Group, Margaritopsis, which is pantropical and known from the Americas, Africa, southeastern Asia, and the Pacific Islands (Barrabé et al., 2012). Margaritopsis is generally characterized by costate stem internodes; a green drying color; stipules that are deciduous in fragments, with the lobes usually glandular at the tip when the stipule is young and the old persistent stipule portions on older nodes becoming hardened and yellow; terminal, congested to laxly cymose inflorescences with reduced bracts; small flowers with white corollas that are densely pubescent in the throat; and red fruits with pyrenes that are plane or costate adaxially. Several Madagascar species that were classified by Bremekamp in his Psychotria and perhaps Trigonopyren have similarities to Margaritopsis, but these species are poorly known at present and difficult to completely evaluate for generic status.
Bremekamp separated Psychotria and the closely related genera in Madagascar based primarily on pyrene and seed characters, which he considered evolutionarily stable, and also classified a number of species for which the fruits were unknown apparently based on what he saw to be their similarity to other species for which the fruits were known. However more recent work has documented variation, sometimes extensive, in pyrene characters within many Rubiaceae genera. Thus now several of Bremekamp's genera appear to be heterogeneous groups of species, which have parallel evolution of the particular pyrene characters Bremekamp noted. Additionally, some of the pyrenes characters that Bremekamp used to diagnose his genera, in particular the bisulcate adaxial furrow of his Psychotria, seem to form late in the development of the fruits, at least in some species, and thus are easily overlooked when immature fruits are surveyed. For more detail about the various individual genera, see their Rubiaceae Project web pages (in the upper right corner of this screen, click on "Choose Project", and select "Rubiaceae"). The key below outlines Bremekamp's segregate genera of Psychotria and the characters that diagnose them.
Bremekamp classified most of the Madagascar species of this group in Psychotria and Mapouria. His circumscription of these two genera is different from that of other authors. The genus Mapouria was originally described for plants of South America, and this name is today considered a synonym of Psychotria (Andersson, 2002); within Psychotria s. str., Mapouria is a name that applies to the Neotropical clade of Psychotria, which is distinct from the Paleotropical species but at present is not separated taxonomically. Bremekamp was aware that this is a Neotropical name that was a synonym of Psychotria s. str., and also that Psychotria was complex with a problematic genus circumscription and latitude to change the type of the genus by formal nomenclatural conservation, which Bremekamp recommended. Bremekamp recognized two large species groups among the Madagascar Psychotrias, and used Mapouria for the group that corresponds to what is today considered Psychotria s. str., and used the name Psychotria for the species that dry green and have stipules that are sometimes persistent and often lack the ring of ferrugineous colleters (i.e., some of which could belong to the Palicourea Group). In this usage of these two names Bremekamp followed Schumann in the Pflanzenfamilien, although Schumann was apparently not aware that the type of Psychotria matches the type of Mapouria. Subsequently Davis et al. (2008) reviewed Bremekamp's nomenclatural and taxonomic work and found no support for separation of his Psychotria and Mapouria in Madagascar, and they nomenclaturally transferred Bremekamp's Madagascar Mapouria species into Psychotria.
Psychotria in Madagascar is commonly confused with Chassalia, a genus of the Palicourea Group that can be recognized by its stipules that are deciduous via fragmentation leaving a yellowed hard basal portion and its lack of a ring of ferrugineous colleters. Additionally many species of Chassalia can be recognized by their rather well developed curved white corollas that are sometimes winged in bud, their ovoid purple-black fruit with the pyrenes smooth and rounded abaxially (i.e., dorsally) and deeply hollowed adaxially (i.e., ventrally), and/or their pedicels that become thickened and colorful in fruit. However a few species of Psychotria also have such thickened pedicels (e.g., Psychotria rubropedicellata).
One of the most commonly collected Psychotria species in Madagascar is Psychotria ankafinensis, which is found in humid middle-elevation forests and has rather small, generally elliptic leaves that lack domatia; short, pedunculate, several-flowered cymes; small yellow to white flowers; and small white to blue fruits. Another of the most commonly collected species is Psychotria imerinensis, which is found widely in humid to subhumid forests at middle to upper elevations, and has generally medium-sized, stiff-textured obovate, apically rounded leaves with domatia; bilobed stipules with narrowly triangular to aristate lobes; pedunculate, several-flowered, corymbiform inflorescences; small yellow flowers; and medium-sized red fruits. Also commonly collected is Psychotria parkeri, which is also found in humid mid-elevation forests and has stiff-textured, quite shiny, generally obovate leaves with a few small domatia; stipules that are united around the stem into a tube with a single triangular lobe on each interpetiolar side; pedunculate corymbiform inflorescences; subsessile flowers with a yellow corolla; and medium-sized red fruits.