Tropicos | Name - !!Pentagonia Benth.

!!Pentagonia Benth.

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Project Name Data (Last Modified On 12/8/2022)

Acceptance : Accepted

Note : Tribe Dialypetalantheae

Project Data (Last Modified On 12/8/2022)

Notes:

Pentagonia is an intriguing, unusual genus that includes perhaps 45 species that are morphologically distinctive in several ways. The genus is characterized by its shrub and small tree habit, leaves that are distinctive in venation and epidermis and also often in size, interpetiolar triangular stipules that are generally quite large, axillary bracteate inflorescences of various arrangements, relatively large, fleshy, 5-merous, homostylous flowers with a developed calyx limb, funnelform white to red corollas with the lobes valvate in bud, and coriaceous baccate fruits with numerous angled seeds. Plants of Pentagonia are frequently monocaulous, or at least very sparsely branched. The leaves are petiolate or sometimes subsesssile, and in the latter species detritus is sometimes accumulated in the leaf bases and adventitious roots develop from the stem to infiltrate the detritus. The leaf blades are distinctive in their venation pattern and epidermis, as described in more detail below. The leaf blades of some Pentagonia species are remarkably large, to 1 m long or more, and in some species these are pinnatifid or, in a few species, variably to regularly pinnate (Hammel, 2015). The inflorescences are several-flowered and cymose, and their axes and pedicels are developed in some species but often these are reduced and the inflorescence is subcapitate. The bracts are often enlarged and colored. As in many Neotropical Rubiaceae genera, the inflorescences and flowers vary widely among the species in arrangement and color, from greenish yellow to white, yellow, pink, and bright red. The fruits are generally subglobose and frequently roughened on the surface, and appear to be yellowish brown to coffee-brown or orange at maturity. Species of Pentagonia are easily recognizable as to their genus but their species characterizations and distinctions are not always clear. The genus has centers of diversity in southern Central America and northwestern Colombia, and it is also well represented in the western Amazon basin. A number of the species seem to have quite limited ranges. Pentagonia spathicalyx and Pentagonia macrophylla are perhaps the most frequently collected species.

Within Rubiaceae, the leaves of Pentagonia and several other genera of Tribe Dialypetalantheae are distinctive in having a finely striate epidermis, which appears to be unique in Rubiaceae. The striations are formed by parallel fibers, as detailed by Rova & Andersson (1995), and these fibers can be seen when the leaf blade is torn. Pentagonia is unique among the genera with a striate epidermis in its distinctive venation pattern, with generally only the costa and distinctive secondary veins visible on the surface and the striations extending between the secondary veins (vs. in the other genera, the tertiary and quaternary venation also visible and the striations arranged within their areoles). These secondary veins extend to near the margins and branch only dichotomously, usually once or sometimes several times. The leaves are unusual in Rubiaceae in some species in having pinnatifid margins, with each segment including one secondary vein. A few species of Pentagonia are unusual (if not unique) in Rubiaceae in having partially to fully pinnate leaves, which appears to be a condition derived from the pinnatifid form. Leaf size and form can vary on an individual plant, for example with juvenile leaves entire and leaves on reproductive stems pinnatifid (Cornejo & Rova, 2019). The Pentagonia species with these pinnatifid and pinnate leaves are found in southern Central America and western Colombia. The stipules of Pentagonia are also distinctive in their convolute arrangement in bud along with their usually quite large size (to 25 cm long).

Plants of Pentagonia are striking in their size and form, and they are often difficult to collect for herbarium specimens. The specimens frequently are so large they have to be mounted on several sheets, or sometimes smaller specimens are selected in the fied because they are easier to accession but these are not adequate for documentation of the plant. Many of the characters of Pentagonia species are evident on living plants but difficult to discern on specimens, so unclear if not noted on labels. For this reason as well as our generally low field knowledge of Neotropical plants, the species taxonomy of Pentagonia is somewhat provisional and field observations are badly needed. Several morphological characters have been treated, somewhat mechanically, as species distinctions but have not been surveyed in the field for variability (Taylor in Lorence et al., 2012: 182). Such characters include degree of branching of the plant, presence and details of pubescence on the leaf undersurface, development of the petiole, development of basal auricles (i.e., leaf lobes) on the leaf blade, development of pinnatifid lobing on the leaves, size of the leaf blades and calyx limb, and bract and corolla color. These color characters are too infrequently noted on labels, however, and documentation of the living plants by photos is not often enough accompanied by reference specimens. Andersson & Rova (2004) noted that growth habit may vary widely within a species, from monocaul in plants of open disturbed vegetation to extensively branched in shaded understory. They also noted that there is probably some bias in collections in terms of representation of average leaf size, with collectors probably preferring the smaller leaves that fit in a press and often not noting on labels the full leaf size.

The calyx of Pentagonia is variable, and its form is here considered to be a species characteristic. The calyx limbs are usually regularly lobed, with the lobes generally developed, obovate, and imbricated in bud. Some Pentagonia species have a spathaceous calyx limb, which is fused into a calyptrate cap in bud and then splits along one or two (or more) sides as the flower bud protrudes. Pentagonia rubriflora is intermediate in form of the calyx, with a limb that is fused in bud with five longitudinal thickened segments, which resemble lobes and are separated by longitudinal lines of thinner tissue that bear sparser pubescence. The calyx limb of Pentagonia rubriflora appears to be open at the very tip in bud, and this top part is also twisted to 360°. As the corolla emerges, its calyx limb splits irregularly in to 2-3 parts along some of the thinner-tissue longitudinal lines.The calyx limbs of some species have only short lobes or these develop tardily, e.g., Pentagonia costaricensis, Pentagonia osaensis. In some other species the calx limb is relatively large, tubular to lobed, and bright red and sometimes confused with the actual corolla, e.g., Pentagonia grandiflora.

The corollas appear to vary in size within species, but these are fleshy and their preserved size also varies on dried specimens. The interpretation of floral species characters can also be complicated by misinterpretation of the floral biology of Pentagonia, or at least of Pentagonia macrophylla and similar species. McDade (1986) studied this species and found a distinctive biology in which the flowers are protandrous and the plant produces only one sexual phase per day, with all its flowers open on that day synchronized. The first, staminate phase has well developed anthers held erect on their filaments, a short stigma, and a small style. In the second, pistillate phase, the flowers has anthers that are positioned to the side of the corolla and a notably larger stigma and style (McDade, 1986: fig. 1), and all the flowers on an individual plant are then synchronized to this phase. Without knowledge of this floral biology, comparison of these two floral phases could easily suggest that the two floral forms belong to two species. She referred to this floral biology as "sequential phenotypic unisexuality"; her term was incorrectly cited by Taylor in Lorence (2012) only as "unisexual", but the flowers of Pentagonia are bisexual or hermaphroditic. Other species of Pentagonia appear to have flowers of similar form (e.g., Hammel 2015), including having narrow constriction of the corolla tube at the stamen insertion, but further observations are needed to clarify the floral biology throughout this genus. This is only one of many ecological aspects of these species that needs further study. For example, the mature fruits have not been characterized in any detail.

The species taxonomy of Pentagonia has varied over the last several decades, and among authors. In particular, since regional treatments were published a number of species have been described from southern Central America, Colombia, western Ecuador, and Peru. Perhaps the most problematic species taxonomically has been Pentagonia macrophylla (Rova et al., 2017, Taylor 2010), which is simultaneously distinctive in life and similar to some other species as a dried specimen. This species was circumscribed quite broadly by Andersson & Rova (2004), and more narrowly by Taylor in Lorence (2012) and Taylor & Hammel in the Costa Rica Manual (2014). Recently the identity of the type of Pentagonia macrophylla was revisited by Rova et al. (2017), who however did not address the circumscription of the species. Here, Pentagonia macrophylla is circumscribed following Taylor in Lorence et al. (2012). Several of the species, especially in the Amazon basin, are distinguished apparently only by calyx characters, so sterile Pentagonia plants are difficult to identify to species.

The relationships of Pentagonia were unclear for some time, and it was variously classified in different Rubiaceae tribes that were very broadly circumscribed and are now known to have been polyphyletic. The first focused study of this genus was the elegant work of Rova & Andersson (1995). They detailed its morphology and evaluated its relationships. They found similar fruits, flowers, generally stipule form, and other fibrous networks in the leaf blades in Hippotis and Tammsia, and demonstrated that these genera are closely related and grouped them in the tribe Hippotideae. Subsequently, Kainulainen et al. (2010; as Condamineeae) found these three genera nested within their newly circumscribed Tribe Dialypetalantheae; in their Dialypetalantheae, the various unusual characteristics of Pentagonia are less anomalous because most of the genera of this tribe have one or more highly unusual features for Rubiaceae. The name Pentagonia is formally conserved for the Rubiaceae genus, against the earlier name Pentagonia Heist. ex Fabr., which applies to the Solanaceae genus now treated as Nicandra Adans. The genus name Pentagonia was later published again for a genus of Campanulaceae, but that name is illegitimate and that genus is now treated as Triodanis Raf. Several Pentagonia species have epithets that are similar in referring to their large leaves and flowers, and these names are frequently confused in annotations. This spelling confusion has contributed to taxonomic and biogeographic confusion in this genus.

Author: C.M. Taylor; helpful comments from J. Rova and B. Hammel on this content are gratefully acknowledged but any mistakes are solely attributable to the author.
The content of this web page was last revised on 8 December 2022.
Taylor web page: http://www.mobot.org/MOOT/Research/curators/taylor.shtml

Distribution: Wet lowland forest at generally 0-1500 m and occasionally up to montane forest at 2000 m, in Central America from Guatemala south, and through western South America from Colombia to southern Peru and western Brazil.

References:

Taxa Included Here:

Species with pinnate leaves: Pentagonia gymnopoda, Pentagonia osapinnata, Pentagonia imparipinnata

Species with pinnatifid (or varously lobed) leaves: Pentagonia gymnopoda, Pentagonia lobata, Pentagonia gambagam, Pentagonia pinnatifida, Pentagonia tinajita, Pentagonia lancilloba, Pentagonia baumannii

Species with subsessile leaves (consistently or sometimes): Pentagonia tinajita, Pentagonia lanciloba, Pentagonia pinnatifida, Pentagonia wendlandii, Pentagonia subsessilis, Pentagonia hirsuta, Pentagonia bonifaziana, Pentagonia carniflora, Pentagonia magnifica, Pentagonia tapacula, Pentagonia microcarpa, Pentagonia australis, Pentagonia williamsii, Pentagonia subauriculata, Pentagonia gigantifolia, Pentagonia pachiteana, Pentagonia gomez-lauritoi, Pentagonia baumannii

Species with leaves abaxially pilosulous to velutinous: Pentagonia hirsuta, Pentagonia osaensis, Pentagonia macrophylla, Pentagonia bonifaziana, Pentagonia dwyeriana, Pentagonia sanblasensis, Pentagonia gigantifolia, Pentagonia williamsii, Pentagonia rubriflora, Pentagonia spathicalyx, Pentagonia villosula

Species with leaves remarkably large: Pentagonia wendlandii, Pentagonia subsessilis, Pentagoinia gomez-lauritoi, Petntagonia bonifaziana, Pentagonia tapacula, Pentagonia grandiflora, Pentagonia clementinensis, Pentagonia subauriculata, Pentagonia gigantifolia, Pentagonia williamsii

Species with consistently relatively small leaves with secondary veins usually extending unbranched to margins: Pentagonia amazonica, Pentagonia involucrata, Pentagonia pachiteana

Species with spathaceous calyx (tardily sometimes with irregular splitting and lobes): Pentagonia spathicalyx, Pentagonia carniflora, Pentagonia gigantifolia, Pentagonia williamsii, Pentagonia subauriculata, Pentagonia wurdackii, Pentagonia villosula, Pentagonia lobata, Pentagonia rubriflora

Species with flowers and usually fruits subtended by involucral bracts: Pentagonia involucrata, Pentagonia clementinensis

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Shrubs and small trees, unarmed, terrestrial, without raphides in the tissues, frequently monocaulous. Leaves opposite, sessile to petiolate, entire to pinnatifid or pinnate or sometimes sinuate, often large (1+ m long), with higher-order venation not or sometimes very weakly visible and leaf surface finely striate, without domatia; stipules interpetiolar, triangular, erect and convolute in bud, sometimes large (10+ cm long), caducous or sometimes persisting. Inflorescence axillary, several-flowered and capitate to congested or laxly cymose or sometimes reduced to a solitary flower, sessile to pedunculate, bracts developed and sometimes large and occasionally colorful. Flowers sessile to pedicellate, bisexual, homostylous, protandrous and at least sometimes sequentially unisexual, often rather large, apparently diurnal; hypanthium turbinate to ellipsoid or cylindrical; calyx limb developed, sometimes enarged, 5-6-lobed with lobes sometimes imbricated or limb occasionally spathaceous opening by a longitudinal slit, without calycophylls; corolla tubular to funnelform, sometimes weakly zygomorphic, generally fleshy to coriaceous with fibers that are evident when tissues are broken, white to yellow, green, pink, or red, internally with pubescent ring, lobes 5-6, triangular, valvate in bud, without appendage; stamens 5-6, inserted in lower part of corolla tube from markedly thickened fiilament bases, filaments pubescent at base and sometimes perhaps of different lengths, anthers narrowly oblong, basifixed or dorsifixed near base, opening by longitudinal slits, included and at least sometimes all grouped on one side of corolla; ovary 2-locular, ovules numerous on axile and parietal placentas, stigmas 2, oblong to subglobose, included. Fruit baccate, indehiscent or perhaps sometimes splitting from base, subglobose to ellipsoid, leathery to woody, medium-sized (1-4 cm diam.), at maturity brown to yellowed or orange, smooth or usually pustulose to warty, with calyx limb persistent; seeds angled, embedded in pulp, smooth to rugulose, medium-sized (3-6 mm long).