Two new species of Ardisia subgenus Tetrardisia (Primulaceae-Myrsinoideae) from Borneo.

Abstract Ardisia argentiana and A. nagaensis from subgenus Tetrardisia are herein described and illustrated as new species. They are endemic to Borneo and the Indonesian province of Central Kalimantan and to the Malaysian state of Sarawak, respectively. Ardisia argentiana is unique in its linear-oblong leaves, with a long, acuminate-caudate apex, and finely serrulate margins, while A. nagaensis can be easily recognized by its elliptic-lanceolate leaves.


Introduction
Ardisia Sw. with ca. 450 to over 1000 species (Stevens 2001 onwards;Frodin 2004), is the largest tropical genus in subfamily Myrsinoideae of the enlarged Primulaceae (as circumscribed by the Angiosperm Phylogeny Group, APG 2009). The genus was last revised by Mez (1902) within his monograph of the Myrsinaceae, but there are no modern species level treatments. In South-East Asia, contemporary work on the genus has resulted in publication of several species (e.g. Stone 1989a, Stone 1990Julius et al. 2017), and regional accounts for New Guinea (Sleumer 1988) and Peninsular Malaysia (as an annotated key in Stone 1989a). For Borneo, there is no recent detailed treatment of the genus from the island, but Stone (1989b) published a provisional checklist of Ardisia with 80 species, and an additional 14 species were added to these by Hu (2002: 13 species) and Utteridge et al. (2014: 1 species).
Ardisia is classified into subgenera on characters of habit, leaf morphology, disposition of flowers at inflorescence branch apices (racemes, umbels, corymbs), inflorescence position and floral morphology (see Mez 1902), and the two new species described here are members of subgenus Tetrardisia (Mez) K. Larsen & C.M. Hu (Larsen and Hu 1995). Originally described as a monotypic genus by Mez (1902), based on the four-merous corolla and the low ovule number, Tetrardisia was reduced to a subgenus of Ardisia by Larsen and Hu (1995) due to a better understanding of character variation within Ardisia -both merosity and ovule number are now known to vary within previously described subgenera. Ståhl and Anderberg (2004) subsequently recognized it as a valid genus, using characters of merosity and ovules in their key to genera but did not discuss this opinion noting only that Tetrardisia was "treated as subgenus of Ardisia by Larsen and Hu (1995)." Initial molecular findings, however, have shown that species of Tetrardisia are nested within Ardisia (Julius et al. 2016;Julius 2019), and the classification of Larsen and Hu (1995) is followed here.
The subgenus is defined by the combination of the small, woody shrub habit, leaves lacking bacterial nodules with a finely serrulate-denticulate leaf margin, and the usually tetramerous flowers with few ovules. During study of Ardisia specimens from Borneo deposited at KEP and E, two unidentified specimens with tetramerous flowers were encountered and suspected to be undescribed taxa. After close scrutiny of the relevant literature (e.g. Furtado 1959;Larsen and Hu 1991;Stone 1992), and comparison with all other Tetrardisia species, we found the specimens do not match any of those known species. Thus, the unidentified specimens collected from Kalimantan and Sarawak are apparently new and described here as new species.
There are very few species of Ardisia with tetramerous flowers, and with few occurring in Malesia and surrounding regions: only a single taxon is recognized each from Java and Thailand, and three are known from Peninsular Malaysia, with none currently described from Borneo. With the publication of the two new species here, subgenus Tetrardisia now has six species, the previously described species, A. denticulata Blume (Blume 1826: 691), A. porosa C.B. Clarke (Clark 1882), A. tetramera K. Larsen & C.M. Hu (Larsen and Hu 1991)

Materials and methods
This study was based on examination of herbarium specimens at E, K and KEP and the relevant taxonomic literature (e.g. Furtado 1959;Larsen and Hu 1991;Stone 1992); in addition, specimen images from Global Plants JSTOR (http://plants.jstor.org/) and the BioPortal of Naturalis Biodiversity Center (http://bioportal.naturalis.nl/) were consulted. Herbaria are abbreviated according to the Index Herbariorum (Thiers 2016). All measurements were taken from herbarium specimens and rehydrated material for floral description, shape terminology follows Systematics Association Committee (1962). The conservation status was assessed using the IUCN criteria (IUCN 2012).
Etymology. The species epithet commemorates Graham Charles George Argent (1941-2019), a prominent botanist on tropical botany in South-East Asia and a leading expert on Ericaceae, especially the tropical 'Vireya' rhododendrons, and collector of the type material.
Conservation status. Data deficient (DD). The only specimen available was collected in 1993 and the species is known only from a single collection location south of Bukit Raya in Central Kalimantan, and thus meets the B1a criterion for Critically Endangered (CR) status. The species was found to the south of the Bukit Baka -Bukit Raya National Park, and, to date, there are no further collections of the species from inside this protected area. Satellite imagery in Google Earth from 2015 shows that the collection locality still has some forest coverage and was not penetrated with roads or logging tracks, and it also appears to not have been converted for agricultural use such as oil palm plantations. However, lack of collections and field observations of the species do not allow inference of decline or fluctuation in population size or EOO and AOO, and we are unable to fulfil the criteria to preliminary assess this species as Critically Endangered.
Notes. Ardisia argentiana is a distinct species on account of the combination of branches with wing-like raised lines running between the petiole bases, linear-oblong leaves less than 2 cm wide with finely serrulate margins, the terminal, laxly paniculate inflorescence with a hairy scaly rachis and few, tetramerous flowers. This new species is morphologically similar to Ardisia nagaensis, but differs from that species in leaf morphology, especially the shorter pedicels (A. argentiana 3-5 mm long; A. nagaensis 1.5-3 cm long), linear-oblong leaves less than 2 cm wide (A. nagaensis elliptic-lanceolate to 5.5 cm wide) and the fewer flowered inflorescence (A. argentiana with 8 flowers vs. A. nagaensis with 24).
The flower is described as 'white' in the specimen label which probably refers to the corolla and anthers. The leaf resembles Ardisia mystica. B.C. Stone (Stone 1982), a member of subgenus Pimelandra, but all members of that subgenus have short, axillary inflorescences. Similar wing-like ridges along the internodes are found in species of Systellantha, a genus of three species endemic to northern Borneo (Drinkell and Utteridge 2015). Members of Systellantha are understorey small trees or shrubs to only 3 m, are also tetramerous but have unisexual flowers with the plants being monoecious. The wing-like ridges may be a response to ecological conditions, such as high humidity, in the rainforest understorey.

Ardisia nagaensis Julius, T. Kajita & Utteridge, sp. nov.
urn:lsid:ipni.org:names:77209339-1 Figure 2 Diagnosis. Similar to Ardisia tetrasepala in its simple and compact inflorescence with corymbose flowers but differs mainly vegetatively in having leaves laxly arranged, long-er petioles that are covered with glandular hairs, a chartaceous lamina without marginal secondary veins, surface with scattered stellate hairs near the margin beneath and leaf base obtuse or ±cuneate. The corolla lobes are smooth without lepidote scales (vs. lepidote scales present in A. tetrasepala).
Etymology. Ardisia nagaensis is very local and was found at only one locality, Gua Naga, for which it is named.
Conservation status. Data deficient (DD). The only specimen available was collected in 2005 and the species is known only from a single collection location from Ulu Merirai in Central Sarawak, and thus meets the B1a criterion for Critically Endangered (CR) status. The Ulu Merirai region is an area of sandstone with limestone cliffs, and supports several point endemics, including several newly described species of Begonia L. (Kiew and Sang 2009) and Monophyllea R. Br. (Kiew and Sang 2013). The species was found outside of any protected area, but satellite imagery in Google Earth from 2017 shows that the collection locality has undisturbed forest coverage and was   hor, Gunung Pulai]. This is because both are characterized by a simple and compact inflorescence, whereas other taxa in subgenus Tetrardisia have compound and laxly flowers arranged. Ardisia nagaensis differs from A. tetrasepala by the leaf characters viz. the petiole length (A. nagaensis with 1.5-3 cm long vs. A. tetrasepala with 0.5-0.8 cm long), the leaf base (A. nagaensis with obtuse or ±cuneate leaf base vs. A. tetrasepala with cordate-rounded) and the venation (A. nagaensis without marginal secondary veins vs. A. tetrasepala with marginal secondary veins). Further morphological comparison between these two species is given in the Table 1.
The calyx is described as 'purplish' in the specimen label which probably refers to both the calyx and corolla. The corolla margin on one side is transparent as observed in A. denticulata.
Key to species of Ardisia subgenus Tetrardisia