Declieuxia includes 28 species of perennial herbs and low shrubs found in savanna vegetation in the Neotropics. Declieuxia is characterized by its generally low habit with well developed, woody, underground parts; opposite to verticillate, generally small leaves; triangular interpetiolar stipules that are persistent; terminal and/or axillary cymose inflorescences with developed bracts; small 4-merous flowers that are distylous and subsessile to sessile; white to purple corollas with valvate lobes; and distinctive black to purple fruits. The small fruits are drupaceous in morphology but mostly dry and basically schizocarpous, with two 1-seeded segments that are circular and flattened. These fruits and their segments are flattened perpendicularly to the septum. The stipules are interpetiolar, persistent to deciduous, and generally triangular to subulate or sometimes reduced to a gland (Declieuxia aspalathoides). The cymose inflorescences are composed of mostly scorpioid (i.e., asymmetrically developed) dichasia, and are distinctive; their arrangement was analyzed in detail by Kirkbride (1976: 3-5, fig. 1). The corolla tubes are only 1-7 mm long (to 10 mm in Declieuxia verticillata). Declieuxia was studied in excellent detail by Kirkbride (1976), with only one additional species discovered subsequently; the taxonomy on this web page follows this work. Declieuxia has not been studied as a whole since the seminal monograph by Kirkbride, but is included in some recent floristic treatments (e.g., Taylor et al., 2004; Jung-Mendaçolli & dos Prazeres, 2007; Delprete, 2004, 2010). Declieuxia fruticosa is the most commonly collected species, and is found throughout the range of the genus and by far the most widely distributed species.

Kirkbride (1976) keyed many of the species of Declieuxia based on vegetative characters, and overall the flowers and fruits of most species are quite similar and the differences in inflorescence morphology are sometimes subtle. Inflorescence form also appears to change markedly as flowering proceeds, because flowering in some species starts when the axes are short and then elongate to produce an infructescence of quite different size and degree of branching. Some Declieuxia species show distinctive morphological features, especially for Rubiaceae. Declieuxia deltoidea is notable for its short but proportionally broad, sessile, cordate leaves that are clasping on the stem, and Declieuxia cordigera and Declieuxia passerina have a similar form in miniature. Some plants of Declieuxia deltoidea even have the leaves of a pair fused marginally into an unusual cupular or involucral structure. Also striking are Declieuxia lancifolia, with small sesssile leves erect and clasped closely to cover the stems, and Declieuxia oenanthoides with well developed photosynthetic stems and only a few scattered small leaves. In his analyses of inflorescence arrangement Kirkbrid found one of the notable differences to be the form and size of the bracts that subtend the individual dichasia of flowers that are separated on the scorpioid axes; he called these "dichasial bracts", a useful term not often taken up by other authors.

Declieuxia has its center of diversity in the planalto region of central-eastern Brazil, and many of the species there show significant morphological variation even across a limited geographic range. Kirkbride (1976) documented a number of clearly marked infraspecific taxa of several Declieuxia species, with most of these treated as varieties and found in this planalto region. Declieuxia coerulea differs from the other species in its calyx lobes unequal in pairs and ovaries with a basal placenta that bears 1 or 2 ovules, vs. a solitary basal ovule in the other species. This genus is unusual and notable in Rubiaceae for having documentation of inter-species hybridization (Kirkbride, 1976).

Declieuxia was included for a long time in the tribe Psychotrieae, but the molecular analysis of Bremer & Manen (2000) found it grouped with a very different set of genera that they classified in an expanded, wholly Neotropical tribe Tribe Coussareeae. In this circumscription this tribe is centered in South America, and quite heterogenous in habit, habitat, flower size and form, ovary arrangement, and fruit form. Within the tribe Declieuxia is similar in habit, habitat, and geographic distribution to Hindsia, which differs in its capsular multi-seeded fruits.

Several species of Declieuxia are very similar to Psyllocarpus, which is often sympatric but can be separated by its capsular fruits, seeds with a well developed strophiole, and truncate stipules with 3-8 short setose appendages and often a well developed sheath that is fused to the petioles or leaf bases on each side. Some species of Declieuxia from eastern Brazil are also similar to some species of Perama found in the same region; Perama has cirumscissile capsules. 

Authors: C.M. Taylor & J.H. Kirkbride, Jr.
The content of this web page was last revised on 16 April 2020.
Taylor web page: http://www.mobot.org/MOBOT/Research/curators/taylor.shtml

• Kirkbride, Jr., J. H. 1976. A revision of the genus Declieuxia (Rubiaceae). Mem. New York Bot. Gard. 28(4): 1–88.
• 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.
• Bremer, B. & J. -F. Manen. 2000. Phylogeny and classification of the subfamily Rubioideae (Rubiaceae). Pl. Syst. Evol. 225: 43–72.
• Delprete, P. G. 2010. Rubiaceae. Fl. Estados Goiás e Tocantins 40(1–3): 1–1610.
• Jung-Mendaçolli, S. L. & J.N. dos Prazeres. 2007. Declieuxia H.B.K. 5: 308–311. In M. G. L. Wanderly, G. J. Shepherd & A. M. Giulietti (eds.) Fl. Fanerog. Estado São Paulo. Editora Hucitec, São Paulo.
• Delprete, P. G., L. B. Smith & R.M. Klein. 2004. Rubiaceae. 1: 1–344. In P. R. Reitz (ed.) Fl. Il. Catarin. Herbário "Barbosa Rodrigues", Itajaí, Brasil.
• Kirkbride, Jr., J. H. 1976. Confirmation of hybridization between Declieuxia fruticosa and D. passerina (Rubiaceae). Brittonia 28: 341–347.