Aphanocarpus is a genus with one species of odd, pachycaulous shrubs found in wet scrub vegetation in the tepuis of the Guayana Highlands, in northeastern South America. These plants are characterized by their stout, little-branched stems; tissues with raphides; narrow, stiff-textured leaves that are verticillate and clustered at the ends of the stems due to the stems not elongating, and remain attached to the stem after they fall, perhaps for insulation; truncate persistent stipules that are interpetiolar and fused to the bases of the leaves, and also setose or fimbriate; terminal and pseudoaxillary inflorescences with small bracteate heads borne on well developed peduncules; small 5-merous flowers; funnelform white to blue or purple corollas with valvate lobes; and small woody fruits that are apparently indehiscent and contain one angled seed in each locule. The leaves have a layer of appressed silky pubescence that given them a silvery appearance. The inflorescences are frequently displaced to a pseudoaxillary position as the stems continue to grow. so the straight unbranched stems apparently have sympodial growth. The flowers are small, with the corollas about 3-4 mm long, and the calyx lobes are about as long as the corolla tube.

Steyermark noted (1965) that the fruits of this species were originally described, in Pagamea, as 2-locular, but he had only found them to be 1-locular. Piesschaert (2001) then studied these fruits in some detail, and noted that the fruits are sometimes 1-locular but sometimes 2-locular, and that Steyermark (1974: 1016, fig. 157) himself illustrated that variation in his figure in the Flora de Venezuela Rubiaceae. Taylor et al. (2004) also noted that it was not clear if Steyermark was studying flowers or fruit, and that often in Rubiaceae only one locule of a fruit develops. The nature of the fruit has not been investigated in detail, as to whether itis an indehiscent capsule or drupaceous, with the seeds enclosed in and dispersed with a layer of endocarp. The habit of these plants is similar to that of various other species of páramo and open, high-elevation scrub habitats, but unusual in Rubiaceae. Nothing is known of the pollination or dispersal of these plants. The flowers are remarkably similar in form, size, and arrangement in the inflorescence to those of Aphanocarpus, so perhaps some polinator is shared between them.  

Only one species of Aphanocarpus is known. Steyermark recognized, which is a localized endemic in the unusual montane formations of the Guayana Highlands. Several distinct forms within this species have been separated, but those were not distinguished by Taylor et al. (2004). Aphanocarpus was included by Steyermark and others in the Tribe Psychotrieae, but this is problematic.The genera of Psychotrieae in Steyermark's time have since been separated into two tribes and several were found to be morphologically incongruent with either of those tribes, including Aphanocarpus (Taylor, 1996). This genus has not been studied with molecular data nor in detail morphologically, and its unusual or reduced characteristics so far known do not clarify its relationships within the family. Piesschaert et al. (2001) noted several of its features that are discordant in the subfamily Rubioideae, but were not able to suggest a better classification. 

Aphanocarpus is similar to Coryphothamnus and Pagameopsis, which are also found on tepuis in the same region. Coryphothamnus can be separated by its 4-merous flowers, partially superior ovary in flower, pilose disk, and dehiscent fruits that are apparently fully superior and have several seeds in each locule. Pagameopsis can be separated by its opposite leaves, and branched cymose inflorescences with groups of sessile flowers fused by their ovaries. Aphanocarpus also shares features with Coccochondra, another poorly known genus from the same region; Coccochondra has elongated stems with developed internodes, triangular stipules, and axillary cymose inflorescences. 

Aphanocarpus is also similar to Shaferocharis, of serpentine areas in eastern Cuba. Neither of these genera is well known, and they seem to differ mainly in stipule form, with the stipules of Shaferocharis 3-lobed, and habitat; the relationships between these may deserve study.  

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

• 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.
• Taylor, C. M. 1996. Overview of the Psychotrieae (Rubiaceae) in the Neotropics. Opera Bot. Belg. 7: 261–270.
• Piesschaert, F. 2001. Carpology and Pollen Morphology of the Psychotrieae (Rubiaceae-Rubioideae). Ph.D. Dissertation, Catholic University of Leuven, Belgium.
• Piesschaert, F., S. Jansen, I. Jaimes, E. Robbrecht & E. Smets. 2001 [2002]. Morphology, anatomy, and taxonomic position of Pagameopsis (Rubiaceae–Rubioideae). Brittonia 53(4): 490–504.