Retiniphyllum includes about 22 Neotropical species of shrubs and small trees. The genus is characterized by its usually resinous stems and inflorescences, opposite medium-sized leaves, stipules that are interpetiolar or fused around the stem, spiciform or racemiform inflorescences that are terminal to pseudoaxillary or rarely axillary, homostylous 5-merous flowers each enclosed by an involucel or calyculus, salverform pink to red or greenish white corollas with convolute lobes, ovaries with (4)5(8) locules and the ovules 2 per locule, and ellipsoid drupaceous fruits with 5 reniform pyrenes. The distinctive calyculus or involucel is a structure formed of a pair of small bracts that are fused and enclose the base of the flower; similar structures are found in several Paleotropical genera (e.g., Coffea, Tricalysia) but this feature is not common in the Neotropical Rubiaceae. The fruits are fleshy or juicy, and become red then finally dark purple. The pyrenes are 1-locular, and usually contain a single seed due to abortion of one of the ovules; the pyrenes are 2-seeded in Retiniphyllum francoanum and Retiniphyllum secundiflorum. These pyrenes are sometimes winged. Retiniphyllum is closely associated with white sand and sandstone habitats across the Amazon basin, in the Guayana Highlands, and on scattered sandstone outcrops in western Amazonia. Retiniphyllum schomburgkii is the most commonly collected species, followed by Retiniphyllum concolor.

The stipules are variously persistent to deciduous, and sometimes are spathaceous. The stipules of this genus are unusual in a few species in being dimorphic on the same plant (e.g., Retiniphyllum pauciflorum, Retiniphyllum scabrum). The inflorescences are axillary on paired peduncles and produced well below the stem apex in Retiniphyllum secundiflorum and Retiniphyllum francoanum, in contrast to the single terminal inflorescence of the other species. As noted by Cortés-Ballén (2003), some species have the flowers sessile directly on the primary axis but several others species have short to well developed pedicels or peduncles. The distinction between these flower arrangements can be subtle at times, though. In some of the sessile-flowered species, the flowers are partially embedded into the fleshy primary axis, but in others the flowers appear to be shortly stipitate at the base at least on some dried specimens. In the species with pedicellate flowers, the pedicels may be very short, down to 0.5 mm long; usuaally at least some flowers on an inflorescence do have better developed pedicels in those species but sometimes confirming pedicel development requires careful observation. About half the species of Retiniphyllum have sessile flowers, and half have pedicellate flowers.

The involucels (calyculi) vary from lobed to truncate; Steyermark (1965: 230, fig. 34) illustrated the involucels and calyx limbs of several species of Retiniphyllum. The flowers are articulated at the base of the ovary, above the involucel. The corollas are often zygomorphic, due to an asymmetric bending of the tube and/or lobes. As in many genera of Neotropical Rubiaceae, Retiniphyllum has a variety of pollination modes: shorter greenish white flowers pollinated by bees, pink flowers perhaps pollinated by butterflies, and red flowers pollinated by hummingbirds (Cortés-Ballén, 2003). The long styles and relatively long anthers are both exserted from the flower, and the anthers are generally spreading to reflexed. The anthers have sterile appendages on both the top and the bottom.

Retiniphyllum was studied phylogenetically and monographed by Cortes-Ballén (2003), who has not yet published her taxonomic treatment but we continue to hope it will come out sometime soon. Her treatment supercedes the previous work by Steyermark (1965), and has clarified many problems in this genus. Cortés-Ballén did publish part of her taxonomic treatment as a floristic overview of Retiniphyllum in northeastern South America (Taylor et al., 2004: 792-800) along with the description of a new species (Cortés-Ballén, 2014), but more than half of her work still need formal publication. Cortes-Ballén et al. (2009) also presented part of the phylogenetic portion of her study, which supported separation of Retiniphyllum into its own monotypic tribe, Retiniphylleae; the analysis of Bremer & Eriksson (2009) reached the same conclusion. Cortés-Ballén (2003) further analyzed the relationships among the species of Retiniphyllum, and concluded that no infrageneric taxa are supported. The taxonomic treatment here is based on Cortés-Ballén (2003) and Cortés-Ballén in Taylor et al. (2004), and is presented in support of the numerous excellent determinations she put on specimens from many herbaria.

Author: C.M. Taylor.
The content of this web page was last revised on 6 March 2017.
Taylor web page: http://www.mobot.org/MOBOT/Research/curators/taylor.shtml

• Cortés-Ballén, R. 2003. Systematics and Biogeography of Retiniphyllum (Rubiaceae). 1–408. In R. Cortés-Ballén Syst. Biogeog. Retiniphyllum. Ph.D. Dissertation, City Univ. New York, New York.
• Taylor, C. M., J. A. Steyermark, P. G. Delprete, A. Vincentini, R. Cortés-Ballén, D. C. Zappi, C. H. Persson, C. B. Costa & E. Anunciação. 2004. Rubiaceae. 8: 497–847. In J. A. Steyermark, P. E. Berry & B. K. Holst (eds.) Fl. Venez. Guayana. Missouri Botanical Garden Press, St. Louis.
• Cortés-Ballén, R., P. G. Delprete & T. J. Motley. 2009. Phylogenetic placement of the Tribe Retiniphylleae among the subfamily Ixoroideae (Rubiaceae). Ann. Missouri Bot. Gard. 96(1): 61–67.
• Bremer, B. & T. Eriksson. 2009. Time tree of Rubiaceae: Phylogeny and dating the family, subfamilies, and tribes. Int. J. Pl. Sci. 170(6): 766–793.
• Steyermark, J. A. 1965. Rubiaceae. In: B. Maguire & collaborators, The botany of the Guayana Highland—Part VI. Mem. New York Bot. Gard. 12(3): 178–285.
• Cortés-Ballén, R. 2014. Retiniphyllum francoanum (Rubiaceae), a new species from the Colombian Guayana. Novon 23: 27–30.