Acrosynanthus includes perhaps 7 species of shrubs and small trees, and is characterized by its xeromorphic form; ample resin produced at the stem apices; relatively small, stiffly coriaceous leaves with usually revolute margins; persistent, tubular, generally small stipules; terminal cymose inflorescences; generally small 4--6-merous flowers; deeply lobed calyx limbs; funnelform to salverform, apparently usually white corollas with the lobes imbricated in bud; and small, loculicidal, woody fruits with a few ellipsoid, unwinged seeds. The leaves of some species described as having or appear to have a whitened lower surface (e.g., Acrosynanthus jamaicensis). The inflorescences range from subsessile with solitary flowers, to subsessile and congested-cymose with several flowers, to lax with developed peduncles and axes (e.g., Acrosynanthus jamaicensis).The persistent calyx lobes are often markedly fleshy.  The label of Rova et al. 2230 describes the flowers as smelling like honey. Several similar Antillean genera consistently have the flowers well documented and the fruits infrequently collected, but for some reason the fruits of Acrosynanthus are well represented while the flowers are rarely collected. Acrosynanthus is one of several morphologically enigmatic Rubiaceae genera found on serpentine and limestone substrates in the Antilles. The largest leaves of the genus appear to be those of Acrosynanthus jamaicensis (2.5-4.5 x 0.3-0.8 cm) and Acrosynanthus latifolius (2.5-6.5 x0.5-3 cm). None of the species seem to be characterizable as commonly encountered or distributed outside a local area.  

The flowers often characterized as 6-merous (Torres-Montúfar et al., 2020. Borhidi et al. 2017 in part), but various species are usually 4-5-merous (e.g., Acrosynanthus jamaicensis, Acrosynanthus minor) as documented by Borhidi et al. in their species descriptions. Borhidi et al. noted variation in calyx and corolla lobe number among and within the Cuban species, but stated that the stamens are always 6. However, it seems unlikely that the stamens, which are alternate to the petals, do not vary along with corolla lobe number. This stamen number may have been taken directly from the genus protologue, which Urban bsed on two specimens representing one species, Acrosynanthus revolutus, that sometimes has 6-merous flowers. The flowers of this species (not just the corollas) were characterized by Borhidi et al. as (4)5(6)-merous (2017: 35), which seems to indicate that stamen number has this range also. Several years after he named the genus, Urban detailed several additional Acrosynanthus species and characterized the flowers in more detail, as well as describing the fruits he had not observed previously. The corollas were characterized as white by Torres-Montúfar et al. (2020), but otherwise flower color is not noted on the specimens checked nor detailed by Borhidi et al.   

All of the species of Acrosynanthus are considered uncomon to rare. Acrosynanthus has its center of diversity in Cuba, where most of the species are endemic, and it is also represented by one species in Jamaica. Borhidi et al. (2017) recognized 6 species in Cuba, several of them incompletely known. The Cuban species were separated largely by leaf size and shape and details of pubescence, with a couple also separated by calyx form; they also synonymized two species that Standley recognized (1918) based on leaf shape. Only one physical specimen was available for this current review, and the taxonomy of the Cuban plants is catalogued but not evaluated here.  

In its relatively small and thickly coriaceous leaves with revolute, thickened margins, growth pattern, heavily resinous vegetative growth, and stipule form and size, Acrosynanthus is remarkably similar to Phialanthus; Phialanthus differs, however, in its consistently 4-merous flowers and indehiscent fruits. 

Torres-Montúfar et al. (2020) clarified that Acrosynanthus belongs to Rondeletieae, and within this tribe they found it on a clade characterized by wingless seeds and comprising several Antillean and Central American genera. Acrosynanthus was basal on this clade, and Tainus was basal to the rest of the clade. They separated Tainus based on their results as a monotypic genus, which they distinguished morphologically in the protologue from Acrosynanthus by its stamens that are inserted in the upper part of the corolla tube and 5-merous flowers.

Author: C.M. Taylor.
The content of this web page was last revised on 25 August 2021.
Taylor web page: http://www.mobot.org/MOBOT/Research/curators/taylor.shtml

• Borhidi, A. L., M. Fernández Zequeira & R. Oviedo Prieto. 2017. Rubiác. Cuba 1–494.
• Adams, C. D. 1972. Fl. Pl. Jamaica 1–848. University of the West Indies, Mona.
• Torres-Montúfar, A., T. Borsch, S. Fuentes-Bazán, J. Gutiérrez & H. Ochoterena-Booth. 2020. It is not a disaster: molecular and morphologically based phylogenetic analysis of Rondeletieae and the Rondeletia complex (Cinchonoideae, Rubiaceae). Pl. Syst. Evol. 306(26): 1–25.