Research Article |
Corresponding author: Craig M. Costion ( costionc@si.edu ) Academic editor: Mark Watson
© 2016 Craig M. Costion, Gregory M. Plunkett.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Costion CM, Plunkett GM (2016) A revision of the genus Osmoxylon (Araliaceae) in Palau, including two new species. PhytoKeys 58: 49-64. https://doi.org/10.3897/phytokeys.58.5292
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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.
Pacific Islands, taxonomy, Asteriids, Apiales
Osmoxylon Miq. (Araliaceae) is a genus of rainforest trees and shrubs from southeastern Asia and the western Pacific (
Three species are currently recognized in Palau: Osmoxylon oliveri Fosberg & Sachet, O. pachyphyllum (Kaneh.) Fosbeg & Sachet, and O. truncatum (Kaneh.) Fosberg & Sachet. The first record of the genus in the archipelago seems to date from the Japanese era, with a collection made in 1929, as recorded by
All three currently recognized species are known to occur on Palau’s largest island, Babeldaob. Osmoxylon truncatum is only known from Babeldaob, but both O. oliveri and O. pachyphyllum have hitherto also been recorded from Palau’s limestone islands. Recent collections, however, provide evidence that specimens identified as O. pachyphyllum from Palau’s aforementioned islands represent a distinct, undescribed species and that the original concept of O. truncatum requires revision. Furthermore, another new species of Osmoxylon was recently discovered while establishing a forest-dynamics plot on Babeldaob. Herein, we here describe these two new species and combine the two previously known species O. oliveri and O. truncatum. A dichotomous key to the four known members of the genus in Palau is provided with a list of diagnostic characters for each species so they may be more easily distinguished by non-experts. These taxa tend to be encountered infertile in the field, resulting in great confusion in their identification, and we therefore provide two separate diagnostic keys, one based solely on vegetative material and a second for fertile collections.
1 | Leaf lobes 5–9 | 2 |
2 | Leaf lobes 5–7; stipules glabrous, strongly folded or recurved with tip sharp to the touch; teeth exserted or protruding from margin | O. pachyphyllum |
2’ | Leaf lobes 7–9; stipules with tannish pubescence, clasping stem, flattened; teeth inserted, each tooth located within a crenulation in the margin | O. leidichii |
1’ | Leaf lobes 9–15 | 3 |
3 | Leaf lobes 9–11; stipules entire, glabrous; each prominent secondary vein branched, terminating in 1 or 2 serrations; junction of midrib and secondary veins nearly perpendicular, secondary veins then curving to a 30–45°angle | O. nardokense |
3’ | Leaf lobes 11–15; stipules with distinct teeth or ciliate crests, more or less glabrous; prominent secondary veins branched, terminating in 2 or 3 serrations; junction of midrib and secondary veins at 30–45°angle | O. truncatum |
1 | Inflorescence < 20 cm diameter, < 25 flowers or fruits per fertile umbellule | 2 |
2 | Fertile umbellules 20–30 per inflorescence, borne on peduncles that are not distinctly jointed; fertile fruits globose, c. 20 per umbellule | O. leidichii |
2’ | Fertile umbellules 12–15 per inflorescence, borne on distinctly jointed peduncles, fertile fruits oblong in outline, angled and flattened, 3-5 per umbellule | O. pachyphyllum |
1’ | Inflorescence c. 20–30 cm in diameter, > 25 flowers or fruits per fertile head/umbellule | 3 |
3 | Flowers 20–40 per umbellule, borne on minute pedicels; fertile fruits distinctly globose, shiny dark purplish-black | O. ngardokense |
3’ | Flowers 45-80 per head, sessile at anthesis, borne on a cone-shaped receptacle, pedicels form as fruits mature; fertile fruits obpyramidal (corn-kernel shaped), white-green maturing to dull purple from apex toward the base | O. truncatum |
Boerlagiodendron pachyphyllum Kaneh. p.p., Bot. Mag. Tokyo, 48: 401, 1934
Syntype: Palau. Aimeliik: 1933, R. Kanehira 2452
Palau. Koror: Ngeremdiu Beach, 07°15'20.22"N, 134°26'37.98"E, 18m, 6 Jun 2014 (fr), C. Costion 3711 (holotype:
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; margin 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 flattened and appressed to the stem, broadly attenuate to the apex, fleshy (not stiff), with brown flakey or papery margins and brownish pubescence, tip soft to touch, not firm. Inflorescence 10–15 cm in diameter, primary axis bearing 20–30 secondary inflorescence 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 flowers 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.
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, flakey stipules, and its inflorescences, 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.
Palau. Koror State: Ngeremdiu Beach, 6 Jun 2014 (ster.), C. Costion 3708 (
Palau. Melekeok: Ngardok forest dynamics plot, 07°30'36.97"N, 134°36'28.04"E, 50 m, 17 Jul 2014 (fr.) C. Costion 3721 (holotype:
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°(perpendicular) 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 firm. Inflorescence 20–25 cm in diameter, primary axis bearing 30–40 secondary inflorescence 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 flowers 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.
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. This species is clearly distinct from the other Palau taxa of Osmoxylon by its large 9–11 lobed leaves, large compound inflorescence with inflorescences widely spaced, and dark crimson globose fruits in globose clusters of 30–40.
Palau. Melekeok State: Ngardok Nature Reserve in Ngardok forest dynamics plot, 21 Jul 2014, 23 Jul 2014, C. Costion 3895 (
Boerlagiodendron truncatum Kaneh., Bot. Mag. Tokyo, 48: 403, fig. 2, 1934.
Osmoxylon oliveri Fosberg & Sachet, Smithsonian Contr. Bot. 45: 16. 1980.
Type. Palau. Ngardmau: near Dudui’s homestead, 2 Apr 1966 (fr.,fl.), Cheatham 54 (holotype:
Palau. Aimeliik State: 2 Aug 1933, R. Kanehira 2364 (holotype
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 inflorescences 20–30 cm in diameter, primary axis bearing 20–40 secondary inflorescence 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 reddish-orange, 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.
Osmoxylon truncatum is common in both the limestone and volcanic islands of Palau and is often found in villages near dwellings. The flowers are used for decorations in traditional and modern customs and events. The species is distinguished from the other Palau Osmoxylon taxa by its leaves with 11–15 lobes, ciliate crested stipules, and its much larger inflorescences with up to 80 flowers and fruits.
Palau. Aimeliik State: along road to power plant, Dec 2014, Costion 3987–3989, (
Boerlagiodendron pachyphyllum Kaneh., Bot. Mag. Tokyo, 48: 401, 1934.
Palau. Aimeliik: 1933 (fr.), R. Kanehira 2301 (lectotype:
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°angle; petiolar crests 1–2, rarely 3, firm with sharp edges, sparsely ciliate; stipules not appressed to stem, strongly recurved, glabrous, tip sharp to the touch. Inflorescence 7–15 cm in diameter, primary axis bearing 12–15 secondary inflorescence 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 flowers 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 flat 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.
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. The 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.
Palau. Aimeliik State: slope of Ngetchum, 28 Dec 2005 (fl.) C. Costion 894 (
The most useful characters for distinguishing among the species of Osmoxylon present in Palau are summarized in Table
Diagnostic morphological characters useful for distinguishing the Palau Osmoxylon species.
Diagnostic Characters |
O. leidichii | O. ngardokense | O. truncatum | O. pachyphyllum |
---|---|---|---|---|
LEAVES | ||||
Mature leaf lobes | 7–9 | 9–11 | 11–15 | 5–7 |
Serrations | Inserted, 1 per secondary vein | Exserted, 1 or 2 per secondary vein | Exserted, 2 or 3 per secondary vein | Exserted, 0 or 1 per secondary vein, sometimes absent |
Secondary vein orientation | 30–45°angle with midrib | Near 90°angle with midrib | 30–45°angle with midrib | Near 90°angle with midrib |
Stipule | Appressed to stem, tannish pubescence | Appressed to semi-recurved, glabrous | Appressed to stem, ciliate crests/teeth | Recurved, glabrous, tip sharp |
FLOWERS | ||||
Compound umbel dia. | 10–15 cm | 20–25 cm | 20–30 cm | 7–15 cm |
No. inflor-escences per compound umbel | 20–30 | 30–40 | 20–40 | 12–15 |
No. flowers per head | 10–25 | 20–40 | 45–80 | 10–15 |
Fertile peduncles | c. 3 cm, not distinctly jointed | 4–5 cm, jointed, bottom segment shorter maturing to equal top segment | 5–7 cm, jointed, bottom segment 2–6 times shorter | c. 2.6 cm, jointed, top segment shorter maturing to equal bottom segment |
FRUITS | ||||
No. per head | 10–25 | 20–40 | Up to 80 | 3–5 |
Head shape | Globose | Globose | Oblong | Umbel |
Head size | 1.5–2 cm dia. | 2–2.5 cm dia. | 4–5 cm dia. | 1.5–3cm dia. |
Fruit size | 0.7–0.8 × 0.6–0.7 cm | 0.3–0.6 cm | 1.0 × 1.0 cm | 1.2–1.5 × 0.8 cm |
Shape | Globose-ovoid slightly angled | Distinctly globose | obpyramidal (corn kernel) | Oblong, flattened sides |
Color | White maturing to pinkish | Shiny dark purple-blackish | White-green maturing to dull purple from apex | Yellow-green with maroon-dull purple striations and apex |
Compound inflorescence shape at fruit maturity |
Hemispheric | Globose | Globose | Umbel-shaped |
Diagnostic reproductive characters include the size of the inflorescence, the number of secondary inflorescence units, the number of flowers per head or umbellule, and various features of the peduncles of the fertile heads. The fertile heads or umbellules of three out of the four Palau species have distinctly jointed peduncles, where caducous bracts are present. The peduncles also differ 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 inflorescence 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. This suggests that fruiting material is particularly important for understanding species limits within the genus and is particularly desirable for recognizing and describing new entities.
Osmoxylon truncatum was previously known only from two collections, neither of which contained mature fertile parts (type: R. Kanehira 2364, and R. Kanehira 2303). Thus, resolving its correct identity required considerable effort. All other records attributed to this species were misidentified collections of either O. oliveri or O. pachyphyllum. The type specimen contains only immature flowers and no fruits are known.
To address these uncertainties, we carefully examined the immature inflorescences of the type specimen of Osmoxylon truncatum in the Kyushu University herbarium (
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
Although there are no known mature inflorescence characters for Kanehira’s species O. truncatum, the immature characters of both the leaves and inflorescences match those of O. oliveri. Therefore, we treat these two entities as a single species, O. truncatum, which has nomenclatural priority.
The distribution of the genus Osmoxylon is particularly curious, suggesting a pattern of East Malesian bird dispersal. The inflorescence morphology also appears to be perfectly suited for bird pollination. Because the fleshy pseudo-fruits mature as the fertile flowers present pollen, we hypothesize that they may act as a lure to attract birds, who then brush against the fertile flowers of the two lateral peduncles (see also
This work was made possible through the funding of the Institute of Pacific Islands Forestry,
Supporting evidence for the treatment of O. oliveri and O. truncatum as one species
Data type: species images
Explanation note: Photos of flowers taken under microscope. Photos of leaves taken in the field at the type locality of O. truncatum.