Platycarpum includes at least a doezn species of shrubs and trees found in northeastern through western Amazonian South America. This genus is characterized by its usualy densely pubescent habit; stems often with with ample resin in a layer just below the outer surface; opposite or usually whorled, robust, petiolate leaves without domatia; petioles that usually have 1 to several large round glands at the base; well developed stipules that are calyptrate in bud and then caducous; terminal, usually robust inflorescences with cymose axes; rather large, showy, 5-merous flowers; well developed calyx lobes; funnelform, white to red corollas with the lobes imbricated in bud; stiff-textured, usually rather large, distinctive capsules that are semi-inferior to superior and strongly laterally flattened; and relatively large, ellipsoid, flattened seeds. The distinctive corolla form and markings and partially superior ovary are evident in the photo of Ríos 4254. The species are characteristically found on nuttrient-poor sand substrates. Platycarpum was studied in detail by Steyermark (1952) as a genus, and morphologically by Rogers (1984). Thie center of diversity of this genus is in the Rio Negro basin, with most species found overall within the Amazon basin except Platycarpum eglandulosum, which is disjunct in Guyana. A number of the species appear to be rather localized, although some live in areas that are poorly explored botanically so their commonness and range are not known with certainly. Platycarpum orinocense is the most widespread and commonly collected species.

The stem resins of Platycarpum were described by Dávila (2016) as variously red, translucent, or absent. The leaves are generally quite regularly elliptic to obovate with obtuse to rounded apices. The leaves are also usually densely pubescent with more than one layer of trichomes, and frequently covered wtih very silky sericeous pubescence. The calyptrate, caducous, densely pubescent stipules are characteristic for the genus, but these are usually not present on fertile specimens. The pubescence of the plants, and especiallty of the lower leaf surface, is varied and often comprises trichomes of several different types and sizes. The secondary inflorescence axes are variously paired or whorled. The calyx lobes of Platycarpum are relatively well developed and pale, but not expanded into calycophylls. The semi-inferior ovary is subglobose and densely pubescent on the top part, above the insertion of the corolla. The corollas are showy and widely flared, and rather large (1-2.5 cm long).These are strikingly marked inside with red lines and spots. These corollas are also densely pubescent internally, and ihave yellow bearded pubescence. The capsules are flattened parallel to the septum. As the fruit develops it enlarges in the upper part to become partially to almost ocmpletely superior, with the scar of the deciduous calyx displaced to near the middle or base. The capsules are ditinctive in form, with a suborbicular to ellipsoid outline but indentations on the top and bottom between the loculares. These then dehisce along the lateral margins, and the valves become reflexed from their margins then eventually spread completely open to release the relatively large seeds. These are flattened with thin margins, but not quite winged. 

Platycarpum was studied taxonomically in detail by Steyermark (1952, 1963), Bremekamp (1957), Rogers (1984), and Dávila (2016). Each of these studies added several species. The two recent studies by Rogers (1984) and Dávila (Dávila 2016; Dávila & Kinoshita, 2016; Dávila et al., 2018) are comprehensive for the genus, and differ in their species taxonomy. Platycarpum is not reviewed here taxonomically again in detail. The species taxonomy here follows Rogers and also incorporates the portion of Dávila's work that was finalized and formally published. Both of these authors used details of pubescence to separate species, but the work of Dávila used more specific details that seem useful and did not separate species based on the number of leaves per node or calyx lobes, which she reported as variable in most species. 

The classification of Platycarpum as a genus has varied and has been problematic due to several of its unusual features, notably the semi-inferior ovary, odd calyptrate stipules, and glands subtending the petioles. The family classification of this genus was problematic for some time, and this genus was placed variously by itself and with Platycarpum in its own family, and with both of these placed in Bignoniaceae. Bremekamp (1957, 1966) found the unusual (autoapomorphic) features to make inclusion in the Rubiaceae problematic and placed it in a separate family, Henriqueziaceae, allied with Bignoniaceae, while Rogers (1984) and Cronquist (1981) found these to be idiosyncratic individual features that do not indicate its relationships while the other characters of this genus showed it belongs to Rubiaceae. More recently, molecular data have found Platycarpum nested well within Rubiaceae (Cortés-B. & Motley, 2015). The separation between Henriquezia and Platycarpum also was not always completely clear, and some species described in one genus now are included in the other. The molecular analysis of Cortés-B. & Motley also found these two genera supported as separate. 

Author: C.M. Taylor The content of this web page was last revised on 29 December 2023.
Taylor web page: http://www.mobot.org/MOBOT/Research/curators/taylor.shtml

Humid, evergreen to seasonal, lowland to montane forest and scrub, generally on nutrient-poor and often sand substrates, 40-1800 m, Guayana region of northeastern South America to the western Amazon basin (Brazil, Colombia, Guyana, Peru, Venezuela).

• Rogers, G. K. 1984. Gleasonia, Henriquezia, and Platycarpum (Rubiaceae). Fl. Neotrop. Monogr. 39: 1–135.
• Dávila, N. 2016. Phylogeny Taxonomy Henriquezieae 1–123.
• Steyermark, J. A. 1963. Rubiaceae In: B. Maguire & collaborators, The botany of the Guayana Highland—Part V. Mem. New York Bot. Gard. 10(5): 186–278.
• Cortés-Ballén, R. & T. J. Motley. 2015. Phylogeny of the Henriquezieae-Posoquerieae-Sipaneeae, a Guayanan-centered clade of Rubiaceae: implications for morphological evolution. Phytotaxa 206(1): 90–117.
• Bremekamp, C. E. B. 1957. On the position of Platycarpum Humb. et Bonpl., Henriquezia Spruce ex Bth. and Gleasonia Standl. Acta Bot. Neerl. 6(3): 351–377.
• Bremekamp, C. E. B. 1966. Remarks on the position, the delimitation and the subdivision of the Rubiaceae. Acta Bot. Neerl. 15: 1–33.
• Steyermark, J. A. 1952. The genus Platycarpum (Rubiaceae). Amer. J. Bot. 39: 418–423.
• Cronquist, A. J. 1981. Integr. System Classific. i–xviii, 1–1262. Columbia University Press, New York.
• Dávila, N. & L. S. Kinoshita. 2016. A new species of Platycarpum (Rubiaceae, Henriquezieae) from Peruvian Amazon. 260(3): 276–282.
• Dávila, N., L. S. Kinoshita & C. M. Taylor. 2018. Platycarpum vriesendorpiae sp. nov., a second new species of Tribe Henriquezieae (Rubiaceae) from nutrient-poor soils in the Peruvian Amazon. Nordic J. Bot. 2018(e01654): 1–6.