Areca riparia

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Areca (ah-REHK-ah)
riparia (ree-pahr-EE-ah)
Ar0033.JPG
Photo by Dr. C.D.Heatubun, Seven new species of Areca (Arecaceae) in Phytotaxa 28. 2011.
Scientific Classification
Genus: Areca (ah-REHK-ah)
Species:
riparia (ree-pahr-EE-ah)
Synonyms
None set.
Native Continent
Asia
Asia.gif
Morphology
Habit: Clustering
Leaf type: Pinnate
Culture
Survivability index
Common names
None.

Habitat and Distribution

Photo by Dr. C.D.Heatubun, Seven new species of Areca (Arecaceae) in Phytotaxa 28. 2011.
This palm is only known from one collection made by Dr. Tom Evans in Tatai Chveng waterfall in Koh Kong Province, Cambodia. This species grows as a rheophyte in a wet, vegetated rocky river bank at elevation less than 20 m above sea level, downstream of the falls and outside the splash zone, but below the peak flood level. (C.D.Heatubun, Seven new species of Areca (Arecaceae) in Phytotaxa 28. 2011)/Palmweb.

Description

Clustering, small and slender palm, snaking, rarely vertical. Stem to 2.5 m tall, 1-1.5 cm in diam.; internodes 2.5-3 cm long, green near the crown, shiny, brown near the base, nodal scars conspicuous, white to whitish brown. Leaves about 7 in the crown, pinnate, to 80 cm long (including petiole); sheath tubular, about 19 × 4 cm long, smooth, greenish cream, speckled with reddish scurf-like ramenta; crown shaft well defined, 28-30 cm long and 1-2 cm in diam.; petiole short, 12-13 cm long, 5 × 5 mm at the base, channelled adaxially, rounded abaxially; rachis somewhat arching, very slender with adaxial longitudinal ridge, rounded abaxially, covered by rusty brown indumenta; blade with regularly arranged leaflets, about 25 leaflets on each side; leaflets somewhat arching, single fold, linear, 20-30 cm long, acuminate, with pointed tips, papery, green adaxially and light green abaxially, slightly discolorous when dried; continuous white woolly ramenta present along the mid-ribs on abaxial surface. Inflorescence infrafoliar, erect, small and slender, 13-15 cm long and 6-8 cm wide, protandrous, branching to 1 order and sometimes the basalmost rachilla branches to 2 order; peduncle 1-1.5 cm long; prophyll about 14 × 4.5 cm, lanceolate, 2-keeled, papery, cream coloured, entirely enclosing the inflorescence, then splitting longitudinally and falling before staminate flower anthesis; rachis cream to greenish; rachillae about 16, divaricate, elongate, yellowish. Floral clusters uniseriate, only one complete triad including female flower occurring at the base of each rachilla, rarely several triads. Staminate flowers small, triangular, about 2 × 0.75 mm (young stage), asymmetric, cream to pale brown; calyx low, triangular cup-shaped, about 0.75 mm wide and 0.5 mm high (at young stage), 3-lobed; petals 3, strongly keeled, valvate, ca. 1.5 × 0.75 mm (at young stage); stamens 3, about 1 mm long (at young stage), basifixed; anthers about 0.8 mm long (at young stage), white to cream-coloured, longer than the filaments; filaments about 0.2 mm (at young stage), orange to brown; pistillode lacking. Pistillate flowers larger than the staminate, triangular, about 6.2 mm long and 4 mm wide (at young stage); sepals 3, strongly imbricate, about 4 × 3 mm (at young stage), triangular; petals 3, imbricate, triangular, about 5 × 3 mm at anthesis; gynoecium about 3 mm (at young stage), lacking staminodes. Fruits 2-2.8 × 1-1.5 × 1-1.5 cm, elongate, fusiform, apical stigmatic remains, epicarp smooth, mesocarp fleshy, endocarp fibrous, bright crimson when ripe. Seeds about 1.6 × 1.2 cm, obovoid, endosperm ruminate, embryo basal. (C.D.Heatubun, Seven new species of Areca (Arecaceae) in Phytotaxa 28. 2011)/Palmweb. Editing by edric.

Areca riparia is the third species of Areca that has been reported from Cambodia and the fifth for Indo-China (Govaerts & Dransfield 2005, Henderson 2009, Henderson et al. 2010). This palm differs from other Areca species in the region by its growth-form being truly rheophytic (van Steenis 1981, Dransfield et al. 2008) in the flood zone of fast-flowing rocky rivers. This species has three stamens, similar to A. songthanhensis Henderson et al. (2010: 34), A. triandra Roxb. in Buchanan-Hamilton (1826: 310) and A. montana Ridley (1907: 136)-this last species was synonymized by Andrew Henderson in his recent monograph of palms of Southern Asia (Henderson 2009), a taxonomic decision questioned here. However, A. riparia can easily be distinguished from A. songthanhensis, A. montana and A. triandra by having narrow and flexible single-folded leaflets adapted to flowing flood water. Additional morphological characters found in A. songthanhensis and A. montana, such as solitary habit and pistillate flowers borne only on distal rachillae, further distinguish this new species, which is clustering and bears pistillate flowers on all rachillae. Differences from A. triandra are in dimension and inflorescence structure in general. Areca riparia is a small and slender palm with a stem diameter to 1.5 cm and stem height to 2.5 m, inflorescences branched to two orders, rachillae relatively few with only one pistillate flower on each rachilla. In contrast, A. triandra is typically medium-sized with a stem diameter to 7 cm or more and stem height to 6 m, inflorescences branched to three orders, numerous rachillae and 3 to 5 (sometimes up to 10) pistillate flowers on each rachilla. Also the inflorescence of A. triandra also has a powerful lemon scent as emphasized by Lim & Whitmore (2001), which has not been recorded for A. riparia. A rheophytic form of A. triandra has been reported recently from Vietnam (Henderson 2009, Henderson et al. 2010). One herbarium specimen collected by Andrew Henderson and his colleagues (Henderson et al. 3623) from a lowland area of Vietnam has been studied at Kew and is clearly not the same taxon as A. riparia. In this specimen, the typical features of A. triandra can be observed, such as the relatively broad leaflets with multiple folds (at least two-folded with prominent nerves in the leaflet, in contrast to the single-folded, smooth leaflets without prominent nerves in A. riparia), the relatively large inflorescence (about 2.5 times longer and wider than A. riparia) that is branched to three orders, as opposed to two orders in A. riparia. Areca riparia is the second rheophytic species in the genus, the other species is A. rheophytica Dransfield (1984: 18) from Borneo. Besides being rheophytes, these two species are different from each other in many ways. (C.D.Heatubun, Seven new species of Areca (Arecaceae) in Phytotaxa 28. 2011)/Palmweb.

Culture

Cold Hardiness Zone: 10a

Comments and Curiosities

Etymology: The specific epithet reflects the habitat where this species grows.

This medium-sized, slender, clustering palm found growing in the flood-zone of rivers in southern Vietnam may be a form of Areca riparia known from Cambodia or the closely related Areca triandra. Its fruits are nearly twice as large as regular A. triandra and the seeds are covered in long, open fibers. It will make a splendid ornamental for the humid tropics and should do well in wet soils. (RPS.com)



External Links

References

Phonetic spelling of Latin names by edric.

Special thanks to Geoff Stein, (Palmbob) for his hundreds of photos.

Special thanks to Palmweb.org, Dr. John Dransfield, Dr. Bill Baker & team, for their volumes of information and photos.

Glossary of Palm Terms; Based on the glossary in Dransfield, J., N.W. Uhl, C.B. Asmussen-Lange, W.J. Baker, M.M. Harley & C.E. Lewis. 2008. Genera Palmarum - Evolution and Classification of the Palms. Royal Botanic Gardens, Kew. All images copyright of the artists and photographers (see images for credits).

C.D.Heatubun, Seven new species of Areca (Arecaceae) in Phytotaxa 28. 2011


Many Special Thanks to Ed Vaile for his long hours of tireless editing and numerous contributions.

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