Pentodon includes perhaps two species of weedy herbs, which are widespread but not actually well studied. Pentodon is native to Africa and perhaps Madagascar, and one species is adventive in the tropical and subtropical Americas. Pentodon is characterized by its low, rather succulent habit; subsessile elliptic leaves; truncate fimbriate stipules; terminal and pseudoaxillary, 5-merous, distylous flowers borne singly or in  smallcymes or fascicles; white to pink or pale blue corollas about 3-5 mm long; and small papery capsules 2-3 mm long. The Rubiaceae phylogenetic analysis of Bremer & Eriksson (2009) found Pentodon as the sister group of Dentella, and these both placed in the Spermacoceae. The common widespread species, Pentodon pentandrus, is found throughout the range of the genus, and is morphologically quite variable and apparently grows as an annual at least sometimes. The other species, Pentodon laurentioides, has been separated as an endemic species of Somalia. A good analytic figure of the large-flowered form (forms discussed below) of Pentodon pentandrus was presented by Verdcourt (1976: 264, fig. 38), and one of the small-flowered form by Correll & Correll (1982: 1409, fig. 615). 

Pentodon is frequently collected across Africa, and shows extensive variation there in plant size, leaf size, degree of development of the inflorescences, and size and color of the corollas. This genus was studied in det4ail by Bremekamp (1952), who presented an analysis with a number of astute observations and the synonymization of several names. He noted that that some populations apparently have lost their disyly and are isomerous, and that the apparently isomerous flowers are also smaller. Then Verdcourt (1976: 263-266) presented another detailed review of this genus, in the context of floristic studies in Africa, and found some problems with Bremekamp's treatment. The plants of Madagascar are not markedly variable morphologically, and may be adventive rather than originally native three. The Neotropical plants are regarded as definitely adventive, and these have relatively small leaves and habit withih the range of forms found in Africa, but they agree with various African plants from East Africa (Verdcourt, 1976) and southern Africa (e.g., type of Pentodon decumbens) and have been included in Pentodon pentandrus by recent authors (Bremekamp, 1952; Lorence et al. 2012).

Bremekamp separated two varieties of Pentodon pentandrus, the typical variety with isostylous flowers that are similar to a long-styled form and relatively smaller corollas and capsules, and (confusingly) var. minor with distylous flowers and larger corolla tubes and capsules (Verdcourt, 1976: p. 26,4, fig. 38). Bremekamp included most of the plants of Pentodon pentandrus, including adventitious populations, in var. pentandrus and reported var. minor as found ony in eastern and southern Africa and the islands in the nearby Indian Ocean. No studies of the reproductive biology of this species seem to have been done, but a possible change from distyly to isostyly with resulting enhanced reproductive compatability in the population may be an adaptation that has facilitiated its adventitious dispersal, similarly to a similar change in floral biology in some other adventitious weeds. Verdcourt (1976: 265-266) noted however that the situation may be more complicated, with both of Bremekamp's varieties growing together and their capsule sizes continuous and not useful to separate the varieties. Many recent collections of Pentodon pentandrus from Africa and Madagascar are actually problematic to assign to a variety, but the variation among these plants is striking.These two varieties are catalogued here as accepted taxa for the African and Malagasy plants that were identified to variety level by previous authors, but the adventive plants of Pentodon pentandrus in America and Asia and more recent collections from Africa and Madagascar that were not identified by Verdcourt and Bremekamp are all treated here only under the species name pending further study.

The American plants are adventive, and some have bluish white corollas ca. 3-4 mm long that agree with Pentodon pentandrus var. minor and may be distylous (e.g., Proctor 21027, Marie Galante in Lesser Antilles; Galdelha Neto 3088, northeastern Brazil). Most American plants agree more with var. pentandrus, with white corollas that are as small as the African plants of this form, with corollas 1.5-2  mm long (e.g., Davidse & Brant 33132, Belize; Robleto 896, Nicaragua; Godfrey 67432, Florida), and these appear to be possibly isostylous but too few fully developed flowering collections have been seen from this region to confirm this  Whether these adventive plants correspond to the two varieties separated by Bremekamp and are best considered varieties of ome species, or if these small-flowerd plants are a distinct lienage and perhaps self-pollinating, or even possibly these represent chasmogamous and cleistogamous forms will need further study. 

Pentodon is often overlooked as a genus, at least in the Americas. Here it is similar to and frequently confused with Oldenlandia and Hedyotis, which have 4-merous flowers. Pentodon also has sometimes been confused with Campanulaceae and Scrophulariaceae, which lack interpetiolar stipules. The spelling "pentander" has sometimes been used in place of "pentandrus", but this is an incorrect spelling. This variant seems to be a confusion over the gender of the genus name and the appropriate declension of this epithet to agree with it. 

Author: C.M. Taylor
The content of this web page was last revised on 15 December 2023.
Taylor web page: http://www.mobot.org/MOBOT/Research/curators/taylor.shtml

Freshwater and saltwater swampy sites and beaches from lowlands to montane regions, tropical and subtropical Africa and Madagascar and widely aventitious in the warm Americas; in the Americas, 1 species adventive at 0-100 m in the Caribbean region and eastern Brazil.

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• Verdcourt, B. 1976. Rubiaceae (part 1). 1–414. In W. B. Turrill & R. M. Polhill (ed.) Fl. Trop. E. Africa. A. A. Balkema, Rotterdam.
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• Correll, D. S. & H. B. Correll. 1982. Fl. Bahama Archip. (1)–(50), 1–1692. J. Cramer, Vaduz.
• 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.