Robust to massive, solitary or clustered, armed or unarmed, hapaxanthic or pleonanthic, polygamous tree palms. Stem erect, usually partly obscured by the marcescent leaf bases, the internodes sometimes bearing adventitious roots, these usually spine-like; cortex hard, pith soft, rich in starch. Leaves large, pinnate, marcescent or sometimes neatly abscising; sheath splitting opposite the petiole, unarmed, or armed with partial whorls of rather slender spines united by their bases to form low collars, and covered with caducous indumentum; petiole well developed, unarmed or armed as the sheath, channelled adaxially in proximal part, becoming rounded distally, rounded abaxially throughout; rachis like the petiole, but angled adaxially; leaflets numerous, single-fold, linear, regularly arranged or grouped and fanned within the groups to give the leaf a plumose appearance, rarely bearing white wax on abaxial surface, usually armed with inconspicuous short spines along the margins and main vein, midribs prominent adaxially, transverse veinlets usually conspicuous. Inflorescences branched to 2 orders, either interfoliar in pleonanthic Metroxylon amicarum or aggregated into a suprafoliar, compound 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; prophyll tubular, tightly sheathing, 2-keeled, 2-lobed; peduncular bract 1–several, tubular; rachis much longer than peduncle; rachis bracts ± distichous, tubular, tightly sheathing, with a triangular limb, unarmed or rarely with a few scattered spines; first-order branches horizontal or pendulous, each with a basal tubular, 2-keeled, 2-lobed empty prophyll and ± distichous, tightly sheathing, tubular bracts, unarmed or armed with few scattered spines, all but the proximal 1–ca. 3 subtending a catkin-like rachilla (second-order branch of inflorescence); rachillae robust, cylindrical, with a short proximal, bare, stalk-like portion, and a dense spiral of imbricate wide, rounded or apiculate, striate bracts, the proximal and distal few empty, the rest each enclosing a dyad of a small staminate and a similar hermaphroditic flower, in bud partly obscured by a dense pile of hairs, except in M. amicarum where hairs sparse, dyad prophyll completely tubular, with 2 keels and 2 triangular lobes, usually bearing dense hairs on the abaxial surface, inner bracteole with 2 keels and dense hairs. Staminate flowers opening before the hermaphroditic; calyx tubular, ± striate, with 3 triangular lobes; corolla usually ± twice the length of the calyx, divided to ± 2/3 its length into 3 oblong, valvate, smooth petals with triangular tips; stamens 6, borne on the base of the corolla, filaments fleshy, abruptly contracted and reflexed, anthers medifixed, oblong, latrorse; pistillode conical. Pollen ellipsoidal, bi-symmetric; apertures equatorially disulcate; ectexine tectate, completely psilate, finely to coarsely perforate, or coarsely reticulate, aperture margins similar to surrounding ectexine; infratectum columellate; longest axis 44–64 µm, post-meiotic tetrads tetragonal [5/7]. Hermaphroditic flowers superficially similar to the staminate but somewhat fatter; calyx and corolla like the staminate; stamens like the staminate, but with filaments united proximally to form an androecial tube surrounding the ovary; gynoecium tricarpellate, triovulate, rounded, covered in vertical rows of minute scales, and bearing a conical style with 3 stigmatic angles, ovule basally attached, anatropous. Fruit rounded, usually large, 1-seeded, with apical stigmatic remains; epicarp covered in neat vertical rows of straw- to chestnut-coloured reflexed scales, mesocarp rather thick, corky or spongy, endocarp not differentiated. Seed globose, basally attached, deeply invaginated apically, enveloped in a thin to thick sarcotesta, endosperm homogeneous; embryo basal. Germination adjacent-ligular; eophyll bifid or pinnate. Cytology: 2n = 26.
The sago palms. Massive solitary or clustered tree palms of the Moluccas, New Guinea and the western Pacific islands, although widely cultivated elsewhere; sheaths, rachis and leaflet margins are armed with spines, rarely sheaths unarmed; hapaxanthic or pleonanthic, the inflorescences are interfoliar or more usually suprafoliar; rachillae are catkin-like, bearing spirally arranged bracts, filled with hairs, each subtending a pair of flowers, one staminate the other hermaphroditic.
About seven species native to east Malesia, the Solomon Islands, New Hebrides, Samoa, Fiji, and the Carolines; one species Metroxylon sagu, thought to be native to New Guinea and the Moluccas, is now widespread and naturalised throughout the Southeast Asian region as a source of sago and thatching material.
The two major uses of Metroxylon species are as sources of sago and as materials for house construction, leaves are used for thatching and woven walling is made from split petioles. Sago from Metroxylon is of great importance as a staple in parts of the Moluccas, New Guinea, and the western Pacific. The apparatus used in the production of sago is, itself, often made from Metroxylon. Seeds of some species furnish a form of vegetable ivory used in the past but of little value now. Felled trunks are used for rearing sago grubs, which are curculionid beetle larvae, a highly esteemed food.