This species is characterized by its robust habit; rather large thin-textured leaves; stipules with the lobes obtuse to rounded; pedunculate pyramidal inflorescences with red to yellow axes and reduced bracts; tubular-funnelform, medium-sized yellow to orange corollas that are externally pubescent with short, stout, colored trichomes; and rather small, ovoid, laterally flattened fruits. This species is widespread in primary and secondary vegetation, and is morphologically variable. It is part of a complex of species and/or forms centered in the Amazon basin of South America, which includes Palicourea grandifolia, Palicourea rigida, Palicourea buntingii, Palicourea semirasa, Palicourea premontana, Palicourea flavifolia, and Palicourea sellowiana.

The circumscription and infra-species systematics of Palicourea guianenses are not entirely clear, and it is morphologically variable both across its range and in many local areas. Steyermark reviewed this group in northeastern South America, and separated several species, subspecies and varieties based on characters such as leaf and corolla sizes and density and distribution of pubescence. Taylor (various works) subsequently circumscribed Palicourea guianensis broadly, to include plants from Mexico and the Antilles to Bolivia and a wide range of variation; she synonymized Palicourea grandifolia, which was recognized by Steyermark, and Palicourea sellowiana, which had been synonymized in annotations by Standley and generally by Brazilian workers. Limited molecular sampling (e.g., Sedio et al., 2013) found the broadly circumscribed Palicourea guianensis of both Steyermark and Taylor to be "polyphyletic", with one sample grouped with Palicourea rigida and the other grouped with some other sympatric Palicourea species of wet lowland Amazon forest.

Here Palicourea guianensis is circumscribed with a broad geographic range and notable morphological variation, but more narrowly than before with Palicourea grandifolia and Palicourea sellowiana now separated. Palicourea guianensis is here regarded as a "meta-species", a species that lacks the autapomorpies that separate the closely related species, and that could potentially be ancestral to (or a source population to) some other currently extant species. Further study beyond morphological analysis of herbarium specimens is necessary to understand the systematics and relationships of the populations included here. The molecular results that found the two samples of Palicourea guianensis placed separately are not problematic for this species circumscription, because speciation in plants is not hypothesized (by biologically-based models) to instantly separate one population into two genetically distinct groups, nor to block all introgression between individual populations of the species and their locally sympatric related species. This revised species-level classification is outlined in the key below, as a general identification guide and, it is hoped, an outline for more and field-based studies.

See the pages for the other species for comments about them and their relationships to Palicourea guianensis. These other species are generally well marked except for Palicourea buntingii, which was probably distinctive when Steyermark studied the Venezuelan plants but is difficult to separate today. His knowledge of the Rubiaceae of that region was deep and insightful, however, so this species is provisionally separated here.

Within Palicourea guianensis, a number of characters vary strikingly but continuously across and often within regions: leaf size, number of secondary veins, stipule size, peduncle length, number and degree of branching of secondary inflorescence axes, pedicel length, lobing and length of calyx limbs, corolla size, color of the inflorescence axes and corollas, fruit size, and pubescence of all structures. Some localized or clinal patterns are evident in some of these characters, but the entire range of variation in others can be found in a given region. The characters Steyermark used to distinguish his subspecies, varieties, and forms of Palicourea guianensis no longer fully separate groups, because newer specimens show all combinations and intermediate conditions of these features. Only his Palicourea guianensis subsp. guianensis had a biogeographic as well as morphological characterization, but even that morphological characterization is no longer diagnostic.

One quantitative character used to separate taxa by Steyermark is notable because it is not usually variable in Palicoureeae genera: number of ovary locules and styles. All plants of Palicourea guianensis have 2 locules, 2 styles, and 2 pyrenes across most of its range, but some plants have 3, 4, or 5 locules in northeastern South America. Plants with different locule numbers have been separated as varieties by some authors, but this number varies in many local populations and occasionally on an individual plant. In particular, Steyermark (1972: 728) separated plants of Palicourea guianensis with 3- and 4-locular ovaries from plants with 2-locular ovaries as distinct subspecies, but some plants are now documented with 2- and 3-celled ovaries in the same area of the northwestern Amazon basin and link these plants to the more widespread, 2-locular form. Steyermark also noted that Palicourea grandifolia has 4-to 7-locular ovaries, and that species is sympatric with the Palicourea guianensis populations with 3- and 4-celled ovaries. This suggests there may be some unfinished lineage sorting or perhaps some introgression between these species.

Palicourea guianensis as circumscribed here is distylous throughout most of its South American range, but appears to be monomorphic (or isostylous) in the Antilles (Taylor, 1989). It also appears to be either monomorphic or markedly skewed in morph ratio in Central America and Colombia west of the Andes. This species is also uncharacteristically uncommon in much of the Antilles, which could be due to relatively recent colonization there, differing ecology there, or genetic differentiation there: again, more and field-based studies are needed.

In addition to the species keyed below, Palicourea guianensis is similar to Palicourea lasiantha, with purple inflorescences and flowers and larger fruits. Also similar is Palicourea grandiflora, with smaller inflorescences, longer corollas that are glabrous externally, and markedly larger fruits. Also similar are Palicourea anisoloba and Palicourea blanchetiana, with short stipules and racemiform inflorescences, and Palicourea macrobotrys, with stipules with a truncate sheath and acute lobes.

Morphological and Geographic Patterns in Palicourea guianensis

Palicourea guianensis in French Guiana is generally robust, with relatively large leaves and well developed inflorescences with numerous pedicellate flowers. The ovaries range from 2- to 4-locular. Both subsp. guianensis, with 3- or 4-locular ovaries, and subsp. occidentalis of Steyermark, with 2-locular ovaries, are represented here; Steyermark only reported subsp. guianensis from here but more recent collections documente more variation (e.g., Croat & Ferry 102807). Plants from Guyana, Surinam, northeastern Brazil (Pará, Amapá, Roraima, Amazonas), and lowland eastern Venezuela are similar to the French Guiana plants, except plants from Brazil vary to have somewhat smaller leaves and inflorescences and most of these plants have 2-locular ovaries; the farther away from French Guiana they grow, the less common are 3- and 4-locular ovaries until they are no longer represented in the populations. Plants from central to western Amazonia, in Peru, Mato Grosso, and Bolivia are similar to those from central Brazil, with somewhat smaller leaves and inflorescences, often somewhat shorter corollas, and 2-locular ovaries.

Plants from the Andean mid-slopes to lowlands of central to western Venezuela and eastern Colombia agree with Steyermark's Palicourea buntingii, and differ a bit in general aspect from Palicourea guianensis of eastern lowland Venezuela. Plants that agree with Palicourea buntingii generally have somewhat smaller leaves, and inflorescences with the branched portion laxer with fewer less-branched axes and so the flowers fewer; in contrast, plants of Palicourea guianensis from lowland eastern Venezuela have leaves that are larger on average and inflorescences with more numerous axes that are branched to more orders, and thus have more flowers and fruits that are more closely grouped. Plants from this region in Venezuela and Colombia appear to be separated geographically from [other] populations of Palicourea guianensis, and ihere separated provisionally as Palicourea buntingii. These are not completely distinct morphologically from populations in eastern Colombia that match Palicourea guianensis (e.g., Lawrance 352), and some plants that agree with Palicourea buntingii are found in Santander in the Magdalena Valley (e.g., Gentry et al. 20097), and in Antioquia (e.g., Alverson et al. 67) where they show continuous variation with more robust plants.

Palicourea guianensis is quite commonly collected across northern Colombia and in western Colombia. Plants here vary from moderately robust plants, which generally agree with Palicourea buntingii or are more slender than Venezuelan plants, to quite robust plants that agree with those from Amazonian eastern Peru. Plants from Central America, the Antilles, and western Ecuador are similar to those of western and nothern Colombia, and show a similar range of variation from slender to robust forms. Some plants from this region have leaves as large and inflorescences as well developed as plants from the Guianas (e.g., Croat et al. 96031), while others are very slender for this species (e.g., Dodson & Gentry 9767), and this variation appears to be related to local habitat: more robust plants are found in wetter sites. Steyermark noted that plants from the Antilles often have regularly developed, pilosulous domatia in the abaxial axils of the secondary veins, and he separated those as subsp. occidentalis. This character is variably developed, however, in Central American and northwestern South American plants that he did not study in detail. Thus, this character seems to have some geographic pattern, but does not clearly diagnose a distinct group within Palicourea guianensis.

In Amazonian Ecuador, both robust forms similar to those of eastern Venezuela (e.g., Gudiño al. 1605, Croat 49726) and smaller, "buntingii" forms are found (e.g., Neill 10723). The very robust forms are more common at lower elevations, but the more slender form is well represented there also. Palicourea guianensis has not been documented frequently in eastern Ecuador, Peru, Bolivia, and Colombia, and in Colombia it is found in Vaupés, Meta, Caquetá, and Putumayo but not yet Amazonas. This suggests that its habitat requirements may be quite specific, although it is found so commonly in the Guianas, eastern Brazil, and Central America that this does not seem likely. Perhaps more likely is that this species has expanded its range asymmetrically, and that some regions have been colonized from well separated source populations. Palicourea buntingii thus may represent a population derived from plants further west that now are found close to plants from eastern South America. Much further study will be needed to elucidate Palicourea guianensis's systematics, including field work and characterization of regional populations. For now, no smaller taxa can be confidently separated so this species is circumscribed broadly as noted above.