Metroxylon Rottboell (PROSEA)
- Protologue: Nye Saml. K. Danske Vidensk. Selsk. Skrift. 2: 527 (1783).
- Family: Palmae
- Chromosome number: x= 13
Major species and synonyms
- Metroxylon sagu Rottboell - see separate article.
- Metroxylon amicarum (H.A. Wendland) Beccari, Ann. Royal Bot. Gard. Calc. 12(2): 187 (1918), synonyms: Sagus amicarum H.A. Wendland (1878), Coelococcus carolinensis Dingler (1887), C. carolinense Beccari (1913).
- Metroxylon salomonense (Warburg) Beccari, Denkschr. K. Akad. Wiss. M. Nat. Kl. (Rechinger ed.), Wien, 84: 60 (1913), synonyms: Coelococcus salomonensis Warburg (1896), Metroxylon bougainvillense Beccari (1913).
- Metroxylon warburgii (Heim) Beccari, Ann. Royal Bot. Gard. Calc. 12(2): 182 (1918), synonyms: Coelococcus warburgii Heim (1904), ? [ Metroxylon upoluense Beccari (1918)].
- Metroxylon vitiense (H.A. Wendland) Beccari, Ann. Royal Bot. Gard. Calc. 12(2): 185 (1918), synonyms: Coelococcus vitiensis H.A. Wendland (1862), Sagus vitiensis H.A. Wendland (1865-1868).
- General: Metroxylon (En, Fr).
- M. amicarum : Caroline ivory-nut palm, Polynesian ivory-nut palm (En). Micronesia: osh (Pohnpei), rupung (Truk).
- M. salomonense : Solomons' sago palm, ivory-nut palm, hebe-nut palm (En)
- Papua New Guinea: bia. Solomon Islands: koko, sao.
- M. warburgii : Fiji: ota, oat (Rotuma). Western Samoa: niu (o)lotuma. Vanuatu: natangora, tenebee.
- M. vitiense : Fiji: songa, soqo, sogo.
Origin and geographic distribution
The origin of Metroxylon is disputed: Moluccan, Moluccan and New Guinean, or Melanesian. It only occurs in South-East Asia and several island groups of Micronesia and Melanesia. The distribution areas of the five species in this genus within this area are neighbouring but hardly overlap. M. sagu is the most widespread and naturalized throughout South-East Asia. The other four occur from the Federated States of Micronesia and the Bismarck Archipelago north of New Guinea to Western Samoa far to the east of it.
- M. amicarum is found in the Federated States of Micronesia (native in Pohnpei and Truk, probably planted in Nukuoro, and in Kusaie), and cultivated in the Philippines and in Guam.
- M. salomonense grows in Papua New Guinea (north-eastern New Guinea, Bismarck Archipelago, Bougainville Island), throughout the Solomon Islands (e.g. Guadalcanal, Santa Cruz Islands), and in Vanuatu (probably imported).
- M. warburgii is found in Vanuatu (indigenous), in West Samoa, and in the Fijian depency Rotuma, while its occurrence on Tikopia in Solomon Islands is uncertain.
- M. vitiense only occurs in Fiji (e.g. on Viti Levu, Vanua Levu, and Ovalau).
The main useful products from Metroxylon palms are starch from the pith of the trunks, roof thatch from leaflets, and vegetable ivory from the hard endosperm of seeds.
The four more easterly occurring Metroxylon species have always been used far less for starch than M. sagu , and since the 1950s their use has died out in most places where they occur. There are no reports of the starch in M. amicarum trunks being exploited. At present, Rotuma (Fiji) is the easternmost point where starch is produced from these species, i.e. M. warburgii .
Metroxylon thatch is widely preferred to Nypa and Cocos thatch in many parts of South-East Asia and the Pacific where these palms are available together.
Vegetable ivory is derived not only from the corozo nuts ( Phytelephas macrocarpa Ruiz & Pavon) of Ecuador and Peru, but also from the fruits of Metroxylon species. The most usual source used to be M. salomonense , with much smaller amounts coming from M. amicarum and even less from the fruits of M. warburgii and M. vitiense . Buttons were the main objects manufactured, though chessmen, umbrella handles and other ornaments were also made. The demand for vegetable ivory has dwindled since the introduction of plastics.
Other uses and main site-specific uses are:
- M. salomonense . There are recent reports of the use of starch extracted from the pith in East Futuna, Anuta, Tikopia and Kolombangara in the western Solomons, and from the Santa Cruz Islands. Unprocessed pith from the trunk is fed to domestic pigs. Building material from the leaves is the most important use (the universal thatch provider). Starch from the trunk is collected and processed throughout Bougainville Island (Papua New Guinea). Fronds are used for thatching throughout the islands of Bougainville and Buka. The plant is an item of the nursery trade in ornamental plants in northern Queensland, Australia.
- M. warburgii . In Vanuatu (New Hebrides), the people only know of the tradition of extracting and using the starch from this palm as food. Nowadays they value M. warburgii solely for its leaves and trunks, which they use to build houses. In Rotuma (Fiji), starch extracted from the trunk is used to thicken soups and stews, to starch clothing, and is cooked (e.g. baked or fried after being wetted and wrapped in a banana leaf); the midribs of leaflets are used to make brooms; immature fruits are eaten raw after the pericarp has been removed. In western Samoa the practice of using starch from this palm has almost been forgotten.
- M. vitiense . In Fiji, starch from the trunk is only occasionally extracted; leaves used to be, and indeed still are, applied in thatching.
- Metroxylon spp. On Tikopia (Santa Cruz Islands in the Solomons), an undocumented Metroxylon species ("rakau ota") - probably M. warburgii , but possibly M. salomonense - is used: its trunk for starch, its leaflets for thatch, and its petioles to make rafts. The starch is used in times of scarcity, but it is also regarded as a delicacy (a food particularly for infants and the elderly). It is often used as an emollient to mix with other foods to give them bulk, flavour, and smoothness.
In Bougainville Island (Papua New Guinea), an undocumented Metroxylon species - M. salomonense or M. sagu - is regularly used for starch; sago beetle grubs are also collected from this palm, and burnt petioles were an important source of salt until the 1950s.
Production and international trade
- M. amicarum . During the German and Japanese occupation, fruits were harvested, dried, and exported as vegetable ivory under the name of Tahiti nuts, Polynesian ivory nuts, or Südsee Steinnüsse, for use in the manufacture of buttons. This species is native to the Caroline group, and it is from those islands, and especially from Pohnpei, that seeds were exported to Germany. Vegetable ivory is no longer an item of commercial importance.
- M. salomonense . In the Solomon Islands, fruits used to be gathered and sold as a source of vegetable ivory to traders for export for button manufacture, under the names ivory nut and "hebe nut" (pidgin English). The export of this commodity from the (formerly British) Solomon Islands fell steadily from 1570 t, worth £ 23 400 in 1925, to only £ 3600 from half this quantity in 1935, when the price was only £ 5 per t. The latest figures are for 1955, when only 19.5 t worth £ 525 were exported.
In Rotuma (Fiji), leaflets of M. warburgii ("rau ota") are regarded as superior to those of coconut for thatching, and last 5-10 years. Midribs of leaflets of this species provide the material for a broom called "taufere"; these brooms are considered to be more durable than those constructed from coconut leaflets and are used for sweeping outdoors. Rotumans much prefer the taste of the starch from this palm ("mar ota") than the taste of starch prepared from cassava, East Indian arrowroot ( Tacca leontopetaloides (L.) O. Kuntze), or sweet potato. Older Rotumans are said to enjoy "mar ota" particularly, but it is rarely prepared nowadays because it is more difficult to process than cassava starch.
Robust to massive, solitary or clustered, armed or unarmed, hapaxanthic or pleonanthic, polygamous tree palms. Stem erect, cylindrical, up to 20 m tall, usually partly obscured by remaining leaf bases, bearing circular leaf scars on the nodes, the internodes sometimes having usually spine-like adventitious roots; cortex hard, pith soft and rich in starch. Leaves large, pinnately compound, erect, spreading or sometimes horizontal, marcescent or sometimes neatly abscising; sheath clasping the stem, splitting opposite the petiole, covered with caducous indumentum, smooth or having semicircular transverse ridges bearing series of more or less conspicuous spines; petiole well developed, largest in tillers, armed or unarmed like the sheath, rounded abaxially, deeply grooved adaxially in the proximal part; rachis like the petiole but angled adaxially; leaflets numerous, single-fold, linear, acuminate, straight to drooping, arranged regularly or in clusters, usually armed with inconspicuous short spines along the margin and main vein, green and shiny, slightly paler beneath. Inflorescence a panicle, branched to 2 or 3 orders, interfoliar (axillary) in pleonanthic species, in hapaxanthic species suprafoliar (terminal) and aggregated into a compound, multibracteate inflorescence with branches equivalent to axillary inflorescences, each subtended by a reduced leaf or bract and sometimes emerging through a split in its mid-line; peduncle very short; rachis much longer than peduncle; bracts armed or unarmed; rachillae catkin-like, robust, cylindrical, with a short stalk-like portion and a dense spiral of imbricate, membranous bracts, each enclosing a pair of flowers (dyad), one male and one bisexual; male flowers open before the somewhat flatter bisexual ones; calyx 3-lobed, corolla twice the length of the calyx, 3-lobed; stamens 6, filaments united at base, forming a tube around the ovary in bisexual flowers; pollen grains elliptical, dicolpate; in bisexual flowers pistil tricarpellate, triovulate, style conical with 3 stigmatic angles. Fruit drupe-like, subglobose, usually large and containing 1 seed; exocarp covered in neat vertical rows of shiny yellowish to brown, reflexed, imbricate, rhomboidal scales; mesocarp rather thick, corky or spongy; endocarp not differentiated. Seed globose, deeply invaginated apically, sarcotesta thin to thick fleshy; endosperm homogeneous, hard, bony; germination adjacent-ligular, eophyll bifid or pinnate.
- M. amicarum . Solitary, pleonanthic; stem 6-8(-20) m tall, 30-40 cm in diameter, brown-corky, pith fibrous, upper part often with stubs of old inflorescences; leaves 5-6 m long, spiny, about 85 pinnae on each side of the strong woody rachis, petiole 25 cm long, sheath 90 cm long and closed in its lower 30 cm, median pinnae 110 cm × 10 cm; inflorescence interfoliar, axillary, up to 125 cm long with about 12 primary (first-order) branches, the lower ones each with 6 rachillae 10-14 cm long (rachillae here second-order branches); fruit 7-11(-13) cm long, 8-9.5(-12) cm wide, covered with 24-28 rows of brown-red scales, bearing a prominent tubercle at apex.
- M. salomonense . Solitary, hapaxanthic; stem 9-20 m tall, up to 55 cm in diameter; leaves 9-11 m long, bearing spines in transverse series; inflorescence suprafoliar, first-order branches erecto-patent, 3-4 m long, second-order branches spreading, rachillae (here third-order branches) pendulous and about 20 cm long; fruit 5.5-6.5 cm long, 7-9 cm wide, with 24-27 rows of yellowish scales, depressed at base and apex.
- M. warburgii . Solitary, hapaxanthic; stem 6-7 m tall, up to 30 cm in diameter; leaves up to 3 m long, bearing spines in transverse ridges or series; inflorescence suprafoliar, branched to 3 orders, all branches erecto-patent, first order branches 1-1.5 m long; flowers large, corolla up to 1 cm long; fruit inversely pear-shaped, (4-)7-12 cm long, (3.5-)6-9 cm wide, with 24 rows of red-brown scales; seed in upper, wider part.
- M. vitiense . Solitary, hapaxanthic; stem 5-10(-15) m tall, up to 50 cm in diameter; leaves up to 5 m long, with brown spines, leaflets in one plane, petiole short (long in seedling leaves); inflorescence suprafoliar, first-order branches erecto-patent, 2-2.5 m long, second-order branches pendulous, 20 cm long, each with 8-9 rachillae (third-order branches); fruit conical, 7 cm long, 5 cm wide, with 27-28 rows of yellow-brown scales.
Growth and development
All Metroxylon palms accumulate starch in the pith of their trunks. The starch in M. amicarum is not exploited by people, probably because its pleonanthic habit prevents starch accumulating in sufficient quantity.
The stout trunks of the hapaxathic Metroxylon species take about 15 to 20 years to reach the flowering stage.
- M. salomonense . When cultivated for its leaves for house thatching, the "pruning" effect of harvesting seems to retard tree development; flowering and fruiting are delayed.
- M. warburgii . A 13-year-old non-flowering specimen, growing on a rich, well drained loam in Papeari on the south-east side of Tahiti (annual rainfall 2500 mm, 22-32°C), started developing a trunk when 9 years old, had an overall height of 10 m and a trunk height of 2 m, bearing 35 leaves and 21 leaf scars.
Other botanical information
The genus Metroxylon is divided into two sections: Metroxylon (clustering palms with fruits covered with 18 vertical rows of scales), comprising only M. sagu , and section Coelococcus (solitary palms with fruits covered with (22-)24-28 vertical rows of scales), comprising the other 4 species. This division based on fruit characters is not in accordance with the division based on the two pollen types occurring in Metroxylon .
When more information is available on the length and abundance of spines, the other Metroxylon species could possibly be subdivided similarly to M. sagu .
In the Santa Cruz Islands (Solomon Islands), two types of M. salomonense are recognized: a large type up to 10 m tall at maturity, with wide leaflets and large flattened globose fruits, and a smaller type with obviously narrower leaflets and smaller pointed fruits. Both types are said to breed true from seed.
Metroxylon palms usually occur in lowland swamps, but occasionally also on hillsides.
- M. amicarum . Thrives inland on hill slopes, and in dry conditions.
- M. salomonense . Is often cultivated well away from swampy areas, sometimes on high ridges.
- M. vitiense . On Viti Levu Island (Fiji) it is very abundant on lowland gley soil, and occurs occasionally in dryland forest. In one of the major wetland sites, the Vunimoli wetlands, there are almost pure stands on the wet gley soils on the colluvium and alluvium in the valleys upstream of the coastal plain. This is the only extensive wetland forest (262 ha) in Fiji and includes most of the M. vitiense population. On wet gley soils, M. vitiense forms a distinctive vegetation type. It also occurs on adjacent hillsides in association with a variety of trees.
Propagation and planting
Unlike M. sagu , which is propagated almost exclusively from basal offshoots, the other Metroxylon species can only be propagated from seed. M. salomonense is normally propagated by sowing in situ, but sometimes germinated seed is transplanted.
Although M. salomonense may once have been planted for exploitation on Bougainville Island (Papua New Guinea) and the Solomon Islands, no information is available on husbandry.
Diseases and pests
The cane weevil borer, Rhabdoscelus obscurus , is gaining importance as a pest in M. salomonense in northern Queensland where this palm is grown as an ornamental. Larvae mine in the leaf bases and adjoining trunk tissue, killing young palms and making older ones unmarketable.
- M. salomonense . For its - now obsolete - use as vegetable ivory, the fallen fruits used to be gathered and brought to trading centres.
- M. warburgii . Trees just about to start flowering are preferred for the production of starch in Rotuma (a dependency of Fiji), because they have a maximum of available starch. A tree's suitability is tested by plunging a machete into the trunk: the ideal tree is hard outside and very soft inside; a hard inner trunk indicates that the palm does not have much starch. After selection the tree is felled close to the ground and the trunk split longitudinally with a single well-placed blow of an axe. A halved coconut shell, sharpened along the edge, is used to scrape out the tissue in the inner part of the trunk. A person straddling the trunk, holds the shell firmly in both hands, and pulls it towards him. The trunk gratings are piled up and put into a sack or basket for transport and processing.
Handling after harvest
- M. salomonense . Seed for export as vegetable ivory has to be dried sufficiently, because fresh seed contains up to 50% moisture. Incompletely dried seed is a useless product for trade.
- M. warburgii . In Rotuma, for the production of starch, the grated pith from the trunk is put into a clean fabric bag (which used to be made from the textile-like base of coconut fronds or fibres from banana petioles). The bag is then placed in a container of water, and the starch is wrung out. After wringing, new gratings are placed in the bag. The water in the container soon becomes cloudy with the starch. After wringing the gratings, the cloudy water is covered with leaves (to exclude flies or other undesirable particles), the starch is allowed to settle for approximately 10 minutes, and is then decanted. The starch is dried in the sun, and in this form may either be stored for future use, or used immediately.
Genetic resources and breeding
No germplasm collections or breeding programmes are known to exist for the Metroxylon species considered here.
As starch plants, Metroxylon species other than M. sagu are not very productive, and cannot be readily propagated vegetatively. The effort needed to produce food from these palms compares unfavourably with other ways of procuring food energy. They are, however, useful as emergency food; during the life cycle of the once-flowering species, harvestable quantities of starch remain stored in the live trunk for several years.
Their importance as providers of thatch will slowly decline as more durable building materials take over.
The role of vegetable ivory in manufacturing has since long been taken over by synthetic products. Vegetable ivory from Metroxylon species may play a modest role in local tourist industries as a material for carving souvenirs.
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