Revision of Chassalia (Rubiaceae-Rubioideae-Palicoureeae) in Borneo, with 14 new species

The genus Chassalia (Gentianales: Rubiaceae-Palicoureeae) in Borneo is revised based on a morphological survey of herbarium specimens using classical taxonomic methods. The tribal placement and probable paraphyly of Chassalia as currently delimited is reviewed. Previously, four described species of Chassalia were known from Borneo, with only one endemic species, Chassalia psychotriformis I.M.Turner nom. nov. (≡ Cephaelis psychotrioides Valeton). A key is given to the 18 species of the genus recognised from Borneo in this study. In total, 14 new species are described, all of them endemic to Borneo. These are: Chassalia atropurpurea T.Y.Yu sp. nov., Chassalia beamanii T.Y.Yu sp. nov., Chassalia calamus T.Y.Yu sp. nov., Chassalia chewii T.Y.Yu sp. nov., Chassalia involucrata T.Y.Yu sp. nov., Chassalia kinabaluensis T.Y.Yu sp. nov., Chassalia lancifolia T.Y.Yu sp. nov., Chassalia lancifolioides T.Y.Yu sp. nov., Chassalia longipes T.Y.Yu sp. nov., Chassalia macrocarpa T.Y.Yu sp. nov., Chassalia muluensis T.Y.Yu sp. nov., Chassalia muscicola T.Y.Yu sp. nov., Chassalia northiana T.Y.Yu sp. nov. and Chassalia ramosa T.Y.Yu sp. nov. Circumscriptions and discussions are given for all Bornean species recognised. A morphological glossary for Asian species of Chassalia is provided. Three separate species groups are recognised, the Chassalia curvifl ora and C. javanica groups, represented by a single species each in Borneo, and a new informal group, the ‘Involucrate group’, which is proposed and circumscribed to encompass the majority (16) of the Bornean species. Proposals for further work on Asian Chassalia are given.

To date, the genus Chassalia contains about 120 accepted species, the majority of which are from Africa and Madagascar . Most species of this genus are erect shrubs and shrublets, sometimes monopodial, or small trees, but several species in West and Central Africa are stem-twining climbers and a few species are epiphytic (Cheek et al. 2004). The morphological characters that are used traditionally to diff erentiate Chassalia from related genera in the Palicoureeae include the presence of corky (indurated) stipules, fl owers usually sessile in capitula, corolla tube usually long, sometimes curved or winged (Bremekamp 1962;Verdcourt 1976). Capitulum. a = peduncle; b = bract subtending fi rst internode of rachis; c = rachis; d = fi rst internode of rachis; e = bract subtending second internode of rachis; f = second internode of rachis; g = fi rst branch; h = fi rst internode of fi rst branch; i = bract subtending second internode of fi rst branch; j = second internode of fi rst branch; k = second branch; l = capitulum; m = stipule; n = apical (chaff y) part of stipule; o = basal (membranous) part of stipules; p = petiole; q = leaf-blade; r = midrib; s = secondary nerve; t = internode; u = acumen; v = outer (capitular) bracts; w = inner (capitular) bract. Drawn by T.Y. Yu. style; c = corolla lobe refl exed; d = corolla tube; e = calyx lobes; f = calyx tube; g = hypanthium; h = apical connective appendage of anther; i = anther cell; j = fi lament, usually very short or absent; k = basal appendage of anther; l = band of hairs; m = stamen; n = disc, usually cylindrical; o = pericarp; p = endocarp; q = seed; r = endocarpal outgrowth; s = groove between endocarp margin and endocarpal outgrowth; t = longitudinal crests of pyrene ventral surface; u = groove between two longitudinal crests; v = pedicel, usually absent in fl ower and extending as fruits mature; w = furrow where the two pyrenes meet; x = veins on the dorsal surface of pericarp; y = bony spine, usually at distal side of raphal opening; z = raphal opening, usually round. Drawn by T.Y. Yu. described mainly based on Kew herbarium specimens but supplemented by loans from the institutes indicated. All the newly described species appear to be Borneo endemic species: none was found on other islands or on mainland Asia. A key to all Bornean species of Chassalia is presented below. Treatments of all 18 species are presented in alphabetical order, with descriptions of all new species to science.
In Borneo, species of Chassalia, including the new species described here, can be found from sea level to approximately 2000 m a.s.l. They occur in forest habitats in peat-swamp and well-drained habitats and on both ultramafi c and non-mafi c substrates.
Diff erent species of Chassalia live in a wide range of diff erent habitats; most of the larger-leaved species prefer lowland forest while some of the smaller-leaved species are restricted to summits with montane forest. Some of the previously known biodiversity hotspots in Borneo are also centres of diversity for species of Chassalia, such as Mount Kinabalu, Mount Matang, Brunei and Gunung Mulu.

Diagnosis
Diff ers from other big-leaved species in having leaves clustered at apex of stem, in infructescences mostly terminal, in fruits signifi cantly obovoid (fruits of most other Asian species of Chassalia are ovoid or round).

Etymology
Named after American botanist, John Homer Beaman (1929-2015, who collected this species and most other specimens of Chassalia from around Mount Kinabalu.

Distribution, habitat
Endemic to Borneo. Only known from the Ranau District of Sabah. Lowland forest, alt. 650-750 m.

Remarks
The lectotype specimen of Chassalia blumeana [L 0057738] was collected in Java. The infructescence of the lectotype specimen is a 3-branched compound cyme with branches and rachis 2-4 mm long. The branches and rachis remain short during fruit maturation. The exact distribution of this species in Borneo is unknown. The specimen most similar to the lectotype found in Borneo from the Kew herbarium is: BRUNEI • Bang Dangung, Bila pinggan group; 12 Jul. 1993; Jay H. Bernstein JHB 405; K!.
Chassalia blumeana diff ers from other species by the base of the infl orescence peduncle not having bracts, and by having the branches and rachis of the infructescence less than 5 mm long. It diff ers from Chassalia ramosa T.Y.Yu sp. nov. by having just 1 fl owering branch, rather than two or more per stem.
Chassalia blumeana may be the taxonomically most diffi cult species among all Asian species of Chassalia. Most of the specimens of Chassalia from Borneo were previously identifi ed as Cephaelis psychotrioides (= Chassalia psychotriformis) and Cephaelis stipulacea (synonym of Chassalia blumeana). These specimens contain a wide range of morphological variation. The lectotype specimen of Chassalia blumeana from Java shows that this species seems to have 1 peduncle and 3 capitula on highly reduced partial peduncles less than 0.5 cm long. This makes the infl orescence look as though it is one big capitulum. The description in the protologue indicates that this species is distributed from West to Central Java (Blume 1826(Blume -1827. Additional information is provided by the description and illustration of Valeton (1913a). These highlight the short partial peduncles (5-10 mm long) which are slightly longer then the partial peduncle of the lectotype specimen. Valeton (1913a) recorded it as distributed from Java and in Borneo from northwest Indonesian Borneo to southwest Malaysian Borneo (Kuching). In this survey, specimens which appear to be a reasonable match for the type specimen of C. blumeana have been found in both Indonesian Borneo and Malaysian Borneo (Kuching), as well as Brunei. However, several specimens from west Sarawak to Sabah seem to share similarity with the typical C. blumeana but also vary somewhat from this species. Therefore, the delimitation of C. blumeana with similar specimens from Borneo still needs further study.

Diagnosis
Diff ers from other species in having a strong and thick, but hollow, stem 5-12 mm diameter (vs stem solid, ca 3-5 mm diameter); leaves large 17-33 × 5.5-12 cm (in most species except C. atropurpurea sp. nov. and C. involucrata sp. nov. not reaching these dimensions); infl orescence usually very short with peduncle very short or absent, (< 0.1-)0.5-3.5 cm (in other species usually exceeding 3.5 cm long); secondary peduncle usually very short to absent making the infl orescence highly compact.

Etymology
The epithet, treated as a noun in apposition, is derived from the Latin ʻcalamusʼ, a reed or more generally a hollow-stemmed plant. The name refers to the hollow stem, unusual in Bornean species of Chassalia.

Diagnosis
Diff ers from all other Kinabalu species of Chassalia in having elliptic to ovate leaves, in the fi rst internode of fl owering branches usually very short (1.5-4 cm long), in having 3 peduncles, the main rachis with 3 capitula, and in the bracts of the capitulum being so big that usually they cover the fl ower buds.

Etymology
Named after Chew Wee Lek who was the collector of most specimens of this new species.

Distribution, habitat
India to the Philippines. Lowland evergreen forest.

Remarks
Chassalia curvifl ora is taken here as a single widespread (India to the Philippines) and variable species. Revision of this complex was beyond the scope of the present study, and only a single entity is keyed out above for Borneo.
Chassalia involucrata T.Y.Yu sp. nov. urn:lsid:ipni.org:names:77215406-1 Fig. 12-13 Diagnosis Diff ers from other species of Chassalia in Borneo in having large leaves up to 48 cm long and 15 cm wide (they are much smaller in other species), in the capitulum having outer and inner bracts up to 1.5 cm wide (broader than in any other species), in the infructescences usually persisting for a whole season until new infl orescences are produced (not soon disappearing after fruit maturity). Most similar to, but diff ering from, C. calamus sp. nov. in having bracts usually longer than the fl ower bud (not shorter), in having a longer peduncle, in having bigger leaves (to 48 × 12 cm, not to 33 × 12 cm) and more secondary veins (up to 26 on each side of the midrib, not < 18), and in having longer (4-14 cm, not < 5 cm) petioles.

Etymology
The epithet alludes to the involucrate bracts of rachis and capitulum, which are larger and more conspicuous than in other species of the genus in Borneo.
Open fl owers not seen. Fruits narrowly ellipsoid, 5-6 × 4 mm, with pedicels 5-6 mm long. Pyrene dorsal surface rough, with a central longitudinal groove 1.5-2 mm wide, pyrene ventral surface with a central groove 1.5-2 mm wide and a ridge at centre of the groove 0.5 mm wide and as high as the plane of the endocarpal outgrowth, the 2 grooves between endocarpal outgrowth and pyrene margin 0.1 mm wide or absent. Seed not seen.

Remarks
Diff ers from other Asian species of Chassalia by usually being epiphytic, by having narrowly ellipsoid fruits only 5-6 mm long, pyrenes with dorsal surfaces bearing a longitudinal groove but without a crest, and ventral surfaces with only 1 ridge, reaching as high as the plane of the endocarpal outgrowth.

Diagnosis
Similar to Chassalia atropurpurea sp. nov., diff ers in having green-brown, not dark purple stems, pedicels, midribs and infl orescence axes; in having a shorter peduncle, 3.5 cm long when fl owering (not 3.5-6 cm long); diff ers also in having relatively long pedicels, 2-4 mm long (not subsessile or sessile).

Etymology
The epithet alludes to Mount Kinabalu to which the new species is endemic.

Etymology
Based on the unpublished name given to some specimens of 'Cephaelis lancifolius' by H.N. Ridley.

Remarks
No PGS was found on the pyrene of this species like other members of the 'Involucrate group'. However, in this species, the pyrene splits very easily at the base of the dorsal crest close to the raphal opening. The raphal opening of this species is very small.

Diagnosis
Diff ers from Chassalia lancifolia sp. nov. in having more coriaceous leaves, leaf-blades lanceolate but not oblanceolate; in having a longer peduncle 1-6.5 cm × 1.5-2.5 mm, while C. lancifolia sp. nov. has a long fi rst internode that resembles a peduncle, but a sessile infl orescence (true peduncle more or less absent); in having the fruit not signifi cantly ridged (0.2-0.3 mm high) compared to C. lancifolia sp. nov.; in the fruit ventral side not as deeply concave as C. lancifolia sp. nov.

Etymology
The epithet reflects the leaf and infl orescence characters that are similar to Chassalia lancifolia sp. nov.

Remarks
The pyrenes of both this species and Chassalia lancifolia sp. nov. can be very easily separated into two parts between the raphal opening and the base of dorsal groove between the two crests.

Diagnosis
Diff ers from all other species of Chassalia in having a peduncle up to 10 cm long and infl orescence usually longer than the longest leaf; in the infl orescence usually having 3 branches with each branch bearing a single capitulum.

Etymology
The chosen epithet refl ects the relatively long peduncle, rachis and branches of infl orescence.

Diagnosis
Diff ers from all other Bornean species of Chassalia in having relatively large fruits (10-12 × 10 × 5 mm); diff ers from other species of Chassalia from Mt Matang in having larger rachis and capitulum bracts, rachis bracts usually round or acuminate with long acumen which covers the fl ower buds, and in fruits becoming pale yellowish brown when dry.

Etymology
The epithet was chosen because, within Borneo, this species of Chassalia has large fruits (1.2 × 1 × 0.5 cm).

Diagnosis
Diff ers from other Bornean species of Chassalia by having secondary nerves densely distributed, 16-18 on each side of the midrib; stipule broadly fl abellate, 5-8 × 5-12 mm; infl orescence 9.5-14 cm long, longer than the average leaf, but usually shorter than the longest leaf; only found in Gunung Mulu National Park.

Etymology
The epithet alludes to Gunung Mulu National Park to which this species is endemic.

Etymology
The epithet alludes to summit moss forest where it has been collected.

Remarks
Turner (2019)  Valeton's type specimen Hallier 2703 (BO-1374040) has a hairy infructescence. Additional information provided by the drawing included in the original publication of this species also shows that it has hairy petioles and abaxial leaf midribs (Valeton 1913b). The species diff ers from other Asian species of Chassalia by having a hairy petiole, lower midrib and secondary nerves of the leaves, also the stem apices and infl orescences are densely pubescent.
Chassalia psychotriformis is restricted to a region straddling the Sarawak-Kalimantan border in Borneo based on specimen records from K and other herbaria. The most distinctive vegetative character of C. psychotriformis is the pubescent abaxial surface of the leaves and the pubescent peduncles. It is the only species that has been found with a hairy peduncle among all Asian species of Chassalia. Therefore, the identifi cation of C. psychotriformis is not diffi cult compared to that of most other Bornean species of Chassalia.

Diagnosis
Diff ers from other Bornean species of Chassalia by the stems usually dividing terminally into two fl owering branches, also in the highly contracted infl orescence rachis and fi rst branches which are less than 0.5 mm long or less, each infl orescence comprising a single capitulum.

Etymology
The epithet reflects the high level of branching in this species, making its gestalt distinctive compared with other species of Chassalia in Borneo.

Discussion
Species of Chassalia can be distinguished from those of other genera of Rubiaceae by having partly or completely corky (chaff y) stipules (note that the stipules of lower nodes are usually damaged) and pyrenes which are usually excavated ventrally. The stipules usually soon develop a brown chaff y apex, with the basal region remaining green and membranous or leathery. As the stipule gets older, the chaff y part usually extends further towards the base of the stipule, and eventually the entire stipule becomes chaff y.

Group 1. Chassalia curvifl ora
The most widespread species of Chassalia in Asia, C. curvifl ora, shares some distinctive characters with African and Madagascan species, such as the corolla tube relatively long and usually curved and winged in bud, the bract of the peduncles and partial peduncles usually small or absent, infructescence axes usually becoming succulent with vivid colour, pyrene dorsal surface with only one longitudinal crest or dorsal crest absent. It is the only Bornean species with these characteristics.

Group 2. Chassalia javanica
Chassalia javanica is another very distinctive species among all Bornean, indeed all Asian species of Chassalia. It was formerly placed in the genus Proscephaleium Korth. as the only species in this genus. Chassalia javanica was transferred to Chassalia (Piesschaert et al. 2001), however, the authors failed to validate the combination. The new combination Chassalia javanica was subsequently published . This is the only known epiphytic species of Chassalia recorded in Asia. Apart from the habit, the pyrene structure of Chassalia javanica is also diff erent from other Asian species. The pyrene dorsal surface of this species is more similar to some African species which have grooved type pyrenes, by having relatively wide endocarp outgrowths and a relatively pronounced ventral central crest (Piesschaert et al. 2001). The median part between the two endocarp outgrowths is slightly excavated, while the central crest reaches almost as high as the plane of the endocarp outgrowth.

Group 3. Chassalia 'involucrate' species
However, most of the species in Borneo and the Malay Peninsula are diff erent from C. curvifl ora and from C. javanica. The vast majority of the specimens from Borneo that were studied have stipules seldom and then only slightly bilobed, straight corolla tubes broadening conspicuously near the apex but not curved to one side (not zygomorphic), infructescence axes usually not becoming succulent, and pyrene dorsal surfaces with two adjacent longitudinal crests. This group of species usually have a capitulum with small to large capitular bracts, outer bracts usually larger than inner bracts and sometimes covering the fl ower buds. Therefore, in this study, a new informal name, the 'Involucrate group' has been given to this group of species.
The pyrene structures of 'involucrate' species are relatively consistent. In this group, species usually have a pyrene dorsal surface with two adjacent longitudinal crests. Between these two crests, there is usually a visible longitudinal groove. On the ventral surface of the pyrene, these species have two endocarp outgrowths along the margins, with a ventral excavation between them. A groove is located at the proximal margin of the pyrene dorsal surface and the endocarp outgrowth, and at the base of this groove, there is a raphal opening with a bony spine at the apex. It is hard to fi nd the preformed germination slits (PGS) (Robbrecht 1988), usually associated with Chassalia, among any of the 'involucrate' species, but the pyrenes of some species can be easily separated into two halves from the base of the groove between the dorsal surface margin and the endocarp outgrowth. This may represent an unusual variant of preformed germination slit.

Centres of diversity of 'involucrate' Bornean species
While Chassalia northiana sp. nov. and C. involucrata sp. nov. are widespread from West to East of Borneo, most 'involucrate' species are confi ned to small areas. There are two main clusters of endemics which correspond with those identifi ed for other plant groups (Ashton 2014): The Kuching-Matang area in western Borneo, in extreme western Malaysian Sarawak, with some records in neighbouring Indonesian Borneo: C. lancifolia sp. nov., C. lancifolioides sp. nov. and C. macrocarpa sp. nov.

Suggestions for further research
This study is mainly based on specimens from the herbarium of the Royal Botanic Gardens, Kew (K) and those loaned from the herbaria of Singapore Botanic Garden (SING) and the Forest Research Institute, Malaysia (KEP). The specimens that have been studied are mostly from Malaysian Borneo. Very few of the physical specimens studied (as opposed to digital images) are from Indonesian Borneo, so further work based on specimens of the Naturalis (L) and herbaria in Indonesia (principally BO) is needed for a more complete study of Bornean Chassalia. In this study much progress has been made resolving species limits in Bornean material attributed to Chassalia. It would be logical and benefi cial to continue this work for other parts of Southeast Asia where the genus has been similarly neglected, such as Sumatra. Molecular phylogenetic work would be valuable to resolve the evolutionary relationships of the diff erent species of Chassalia groups identifi ed in this study and, if samples of other genera of the tribe were included, would inform a revised classifi cation of Chassalia which is evidently needed.