A revision of the genus Osmoxylon (Araliaceae) in Palau, including two new species

Abstract Osmoxylon Miq. (Araliaceae) is revised for Palau, Micronesia including descriptions of two new taxa Osmoxylon leidichii Costion, sp. nov. and Osmoxylon ngardokense Costion, sp. nov. Full descriptions are provided for all four Palau species, along with diagnostic field keys.


Introduction
Osmoxylon Miq. (Araliaceae) is a genus of rainforest trees and shrubs from southeastern Asia and the western Pacifi c (Philipson 1979;Frodin and Govaerts 2003). All of the 60 currently recognized species are characterized by large umbelliform panicles composed of several to many three-branched infl orescence units. Each infl orescence unit terminates in a stalked head or umbellule that develops sterile baccate "pseudo-fruits" and two lateral stalked heads or umbellules with fertile bisexual fl owers and fruits. Other characteristic features of the genus include ligule-shaped stipules that clasp the stem and conspicuous rings or crests that circle or spiral around the petiole base. Like most araliads, the calyx is inconspicuous, forming a highly reduced rim around the inferior ovary, but the genus is unusual in the family in that all species have fused or united petals, forming a distinctively 2' Leaf lobes 7-9; stipules with tannish pubescence, clasping stem, fl attened; teeth inserted, each tooth located within a crenulation in the margin. Description. Small to medium sized tree, 10-12 m tall, branched. Leaf blades palmately lobed, up to 45 cm long and wide, glabrous, with 7-9 rhombic lobes; mar- gin with serrations minute and distinctly inserted or in tiny indentations of the blade, barely exceeding the margin itself, only one per secondary vein; prominent secondary veins 7 or 8 per lobe, meeting the mid-rib at a 30-45°angle; petioles up to 50 cm in length; petiolar crests 1-4, circular, re-curved, entire, with tannish pubescence along margins; stipule fl attened and appressed to the stem, broadly attenuate to the apex, fl eshy (not stiff ), with brown fl akey or papery margins and brownish pubescence, tip soft to touch, not fi rm. Infl orescence 10-15 cm in diameter, primary axis bearing 20-30 secondary infl orescence units, secondary axis (from primary axis to the point where the lateral fertile umbellules are attached) c. 2 cm long, with c. 30 pinkish to crimson, baccate pseudo-fruits, each 2 mm long and 2-3 mm in diameter; peduncles c. 3 cm long, light green, not distinctly jointed. Fertile fl owers 10-20 per umbellule, each with a yellowish, fused or united, cup-shaped calyx, c. 2 mm long; corolla tube bright orange, 5 or 6 lobed, 3 mm long; stamens alternate to the petal lobes, strongly exserted; ovary inferior, whitish-green, stigmas sessile. Fruits 10-20 per umbellule, each with 5 or 6 locules (each 1-seeded), globose-ovoid, 7-8 mm long, 6-7 mm in diameter, turning white (when immature) then maturing to pale pink, tightly clustered at maturity, forming a distinct hemispheric or mound-shaped infructescence.
Notes. Osmoxylon leidichii occurs across the limestone islands of Palau on karst and coral substrates and within this range it appears to be common. It has been previously confused with O. pachyphyllum, but is distinguished by its 7-9 rhombic leaf lobes, its appressed, fl akey stipules, and its infl orescences, which bear many more fruits that are each smaller and more globose. One of the syntypes (R. Kanehira 2452) cited in the protologue of Boerlagiodendron pachyphyllum belongs to this new species. We are happy to name this species after the Palau resident and naturalist, Ron Leidich, whose generosity enabled the discovery of this species and for his inspirational knowledge and enthusiasm about Palau's natural history.  Description. Small understory tree, 7-10 m tall, unbranched. Leaf blades large with distinct celery smell when crushed, up to 60 cm long and 75 cm wide, with 9-11 lobes; margin weakly serrated, serrations exserted or protruding from margin and spaced far apart, generally 1 per secondary vein or up to one between secondary veins; prominent secondary veins 8-13 per lobe, meeting the mid-rib at a near 90°(perpen-dicular) angle then curving to a 30-45°angle; petioles up to 92 cm in length; petiolar crests 3, circular, with papery edge and minutely toothed; stipule 3.5-4 cm long, deeply furrowed, slightly appressed to stem and mildly recurved on the axial side, tip not sharp or fi rm. Infl orescence 20-25 cm in diameter, primary axis bearing 30-40 secondary infl orescence units, secondary axis (from primary axis to where lateral umbellules are attached) c. 3.5 cm long, sterile fruits not seen; lateral peduncles jointed, c. 4-5 cm long, bottom segment 1.2-1.5 cm, top segment 2.5 cm; dark purplish-black in color. Fertile fl owers 20-30 per umbellule, with light-green, fused or united, globose to cup-shaped calyx, 2-2.5 mm long; corolla tube bright yellow-orange, 4-5 lobed, 3 mm long, 1.5-2mm wide; stamens alternate to the petal lobes, partially exserted; ovary inferior, greenish, stigmas sessile. Fruits 30-40 per umbellule, with 5 locules (each 1-seeded), globose, 3-6 mm in diameter, turning dark blackish-crimson; fruiting umbellules globose, 2-2.5 cm in diameter at maturity, spaced apart, not densely packed.
Notes. Osmoxylon ngardokense is so far known only from the type locality, with volcanic soil, near Lake Ngardok on Babeldaob, within the Ngardok Nature Reserve, for which the species is named. Th is species is clearly distinct from the other Palau taxa of Osmoxylon by its large 9-11 lobed leaves, large compound infl orescence with infl orescences widely spaced, and dark crimson globose fruits in globose clusters of 30-40.  Description. Small to medium-sized understory tree, 10-20 m tall, branched. Leaf blades palmately lobed and large, up to 80 cm long and 85 cm wide, glabrous, generally with 11-15 lobes, strongly serrated; serrations protruding from margin, 2 or 3 in between each prominent secondary vein; prominent secondary veins meeting the mid-rib at a sharp 45°angle; petioles up to 1.2 meters long, petiolar crests 3 or 4, circular, ciliate; stipule appressed to stem, shallowly furrowed on top with 1-3 ciliate crests resembling horizontal lines of teeth or wart-like projections; margin of stipule papery and tannish, expanding in towards the center as the stipule matures. Compound infl orescences 20-30 cm in diameter, primary axis bearing 20-40 secondary infl orescence units, secondary axis (from primary axis to where lateral umbels are attached) 3-6 cm long, supporting an umbel of 20-30 dark purple to blackish baccate pseudo-fruits up to 1 cm in diameter; peduncles jointed, purple, 5-7 cm long, with bottom segment 2-6 times shorter but becoming longer with maturity. Flowers 45-80 per head with yellow, angular, bright yellow calyx crowning the ovary; corolla tube bright reddishorange, 5-lobed, 4 mm long; stamens alternate to the petal lobes, strongly exserted; ovary inferior with sessile stigma. Fruits pedicelate, obpyramidal, resembling corn kernels, each c. 1 cm long, 1 cm wide, greenish to white, maturing with a dull purple apex and striations down to the base; fruiting umbellules c. 5 cm long, 4 cm wide, mulberry shaped, with up to 80 fruits densely pressed together.
Notes. Osmoxylon truncatum is common in both the limestone and volcanic islands of Palau and is often found in villages near dwellings. Th e fl owers are used for decorations in traditional and modern customs and events. Th e species is distinguished from the other Palau Osmoxylon taxa by its leaves with 11-15 lobes, ciliate crested stipules, and its much larger infl orescences with up to 80 fl owers and fruits. Description. Small to medium sized understory, tree 7-15 m tall, sparsely branched. Leaves palmately lobed, variable in size, up to 60 cm long and 65 cm wide (generally smaller), with 5-7 lobes; margins sparsely dentate with serrations exserted from margin, 1 per prominent secondary vein or alternating between veins; prominent secondary veins meeting the mid-rib (near) perpendicular then curving to a 30-45°an- gle; petiolar crests 1-2, rarely 3, fi rm with sharp edges, sparsely ciliate; stipules not appressed to stem, strongly recurved, glabrous, tip sharp to the touch. Infl orescence 7-15 cm in diameter, primary axis bearing 12-15 secondary infl orescence units, secondary axis (from primary axis to where lateral umbellules are attached) c. 2.8 cm long, with c. 30 pinkish-red baccate pseudo-fruits, c. 4 mm in diameter; peduncles jointed, c. 2.6 cm long, top segment shorter, maturing to equal the length of the bottom segment, green. Fertile fl owers 10-15, with greenish-yellow, fused calyx crowning the ovary; corolla tube yellowish-orange, 6 lobed, c. 5 mm long; stamens alternate to the petal lobes, strongly exserted; ovary inferior, stigmas sessile. Fruits fl at sided, 1.2-1.5 cm long, 0.7-0.8 cm wide, greenish with reddish-dull purple apex and striations down to the base, c. 3-5 per umbellule; fertile fruiting umbellules loosely organized with distinct peduncles, 1.5-3 cm diameter.
Notes. As circumscribed herein, Osmoxylon pachyphyllum is known only from volcanic soils on Babeldaob Island. Previous collections of this species from the limestone islands (including a syntype, R. Kanehira 2452) are now referred to the new species O. leidichii. In addition to its geography and ecology, O. pachyphyllum can easily be distinguished by its 5-7 lobed and weakly serrated leaves, its oblong, large, angled fruits, and its umbellules, which have very few (3-5) fruits, compared to all other species known from Palau. Th e stipule at the petiole base is also distinctive among the Palauan members of the genus in being strongly recurved, pointing away from the stem, and with a noticeably sharp tip.

Vegetative characters
Th e most useful characters for distinguishing among the species of Osmoxylon present in Palau are summarized in Table 1. Of these, leaf size can vary considerably between individual trees within each species. Th e number of lobes sometimes varies due to slower development of basal lobes. Th e presence of distinctive veins present at the leaf base often indicates an undeveloped lobe. We found that the number of lobes can be useful in the fi eld for distinguishing among species if caution is used in inspecting several leaves per tree. Optimally, this information should be recorded as label data for herbarium specimens, but because most specimens do not include such data, leaf-lobe number alone is not suffi cient for identifying herbarium specimens, especially given the tendency of collectors to select smaller leaves (often the reduced ones emerging directly under infl orescences) that are easier to press. Th e leaf-margin serrations can also be useful, particularly the number of teeth per prominent secondary vein. In this regard, O. leidichii is very distinct from the three other Palauan species in having teeth occurring in indentations in the margin with the tooth apex not exceeding the margin.
Th e angle of the junction between the secondary veins and primary veins cannot be used alone, but can help rule out two out of the four Palau species, and the stipule also has diagnostic value. Th e petiolar crests or rings are sometimes used to distinguish Osmoxylon species in other geographical regions. Among the Palauan taxa, the number of petiolar crests can vary within each species, but they have somewhat diff erent margins (e.g., ciliate or nearly entire). Th ese features are sometimes absent or not persistent and are therefore not suffi ciently distinct to distinguish the Palau species in the absence of other characters.

Reproductive characters
Diagnostic reproductive characters include the size of the infl orescence, the number of secondary infl orescence units, the number of fl owers per head or umbellule, and various features of the peduncles of the fertile heads. Th e fertile heads or umbellules of three out of the four Palau species have distinctly jointed peduncles, where caducous bracts are present. Th e peduncles also diff er in the proportional lengths of the upper or lower segments (above and below the bracts or bract scars). Bracts tend to subtend each segment of the compound infl orescence but are rarely persistent and thus their morphology does not provide reliable characters. Floral characters are similar among the four species, each having yellow-orange corollas with 4-6 lobes and cup-shaped or globose calyces surrounding the ovary. When present, features of the mature fertile fruits (their size, shape, and color, as well as the number of fruits per umbellule) can be used to distinguish unambiguously among all four Palau species and seem to be the most reliable diagnostic characters. Th is suggests that fruiting material is particularly important for understanding species limits within the genus and is particularly desirable for recognizing and describing new entities.

Resolving the identity of O. truncatum
Osmoxylon truncatum was previously known only from two collections, neither of which contained mature fertile parts (type: R. Kanehira 2364, and R. Kanehira 2303). Th us, resolving its correct identity required considerable eff ort. All other records attributed to this species were misidentifi ed collections of either O. oliveri or O. pachyphyllum. Th e type specimen contains only immature fl owers and no fruits are known. Kanehira (1934)  To address these uncertainties, we carefully examined the immature infl orescences of the type specimen of Osmoxylon truncatum in the Kyushu University herbarium (FU) and collected immature infl orescences of the other species recognized here for comparison. Most convincing in our assessment were recent collections of O. oliveri. Careful examination of immature infl orescences from 17 diff erent trees of O. oliveri revealed variation in the size and length of infl orescence parts, and the measurements of material ascribed to O. truncatum fi t within this range. More importantly, one collection (Costion 3985) matches the general appearance of infl orescences on the type of O. truncatum (Suppl. material 1).
To pursue this matter further, we traveled to the type locality of O. truncatum, Aimeliik on the island of Babeldaob. After observing numerous individuals of O. oliveri, we made three new collections (Costion 3987, 3988, and 3989) of this species (See Suppl. material 1), all of which contained leaves on young sprouting branches with 5-7 lobes. Costion 3987 was a mature tree along the roadside that had been pruned. All its branches were re-emergent with many 7-lobed leaves. Notably, leaves in upper or higher branches were up to 10-lobed. Costion 3988 was a sapling with 7-lobed leaves that was clearly a juvenile growing directly underneath a fully fruiting, mature individual of O. oliveri. Costion 3989 was notable in that at the base of the trunk, emerging branches contained 5-7-lobed leaves while more mature leaves in the crown of the tree were 11-13-lobed.
Although there are no known mature infl orescence characters for Kanehira's species O. truncatum, the immature characters of both the leaves and infl orescences match those of O. oliveri. Th erefore, we treat these two entities as a single species, O. truncatum, which has nomenclatural priority.

Geography
Th e distribution of the genus Osmoxylon is particularly curious, suggesting a pattern of East Malesian bird dispersal. Th e infl orescence morphology also appears to be perfectly suited for bird pollination. Because the fl eshy pseudo-fruits mature as the fertile fl owers present pollen, we hypothesize that they may act as a lure to attract birds, who then brush against the fertile fl owers of the two lateral peduncles (see also Stone 1962). To date, there have been no published accounts reporting observations on pollination or fruit/seed dispersal in Osmoxylon. Locals in Palau report that the Micronesian starling, Alponis opaca orii, frequently feeds on the fruits, but these observations do not detail eff ective pollination nor specify whether the feeding is on the sterile baccate pseudo-fruits or the fertile fruits. We suggest that birds are involved in both pollination (enticed by the pseudo-fruits) and seed dispersal (through the fertile fruits), but observations are needed to record nectar feeding and visits to Osmoxylon infl orescences to feed on the fruits and pseudo-fruits by birds or other potential pollinators. Our description of two new species from areas of Palau that have been frequented by professional plant collectors over nearly a century attests to how little is still known about this fascinating genus of plants with such a unique fl oral and fruiting morphology. We hope this study inspires further data collection on other aspects of these poorly known species.