Clustered robust palm climbing to 20-35 m. Stems without sheaths, 18-30 mm in diameter, with 28-40 mm; internodes 18-24 cm long. Leaf sheaths very lightly striate, light moderately to profusely armed with upward-pointing brown spines; armature often concentrated on the sheath apex and often sloughing off at base leaving very feint circular scars; profuse white caducous indumentum present on mature and juvenile sheaths; ocrea ± horizontally truncate, somewhat striate, sometimes with a very slight (0.5-1 cm) rounded lobe abaxial to the leaf, armed as the sheath, although spines often concentrated at apex, extending up to 3 cm. Spear leaf dull yellow to bright green. Juvenile sheaths, armed as the mature sheaths only spines more profuse; leaves bifid, soon becoming pinnate; elaminate rachis up to 2 m long. Leaves on mature stems sessile, or with short (less than 2.5 cm long) petiole; rachis 1.5-2.0 m long, abaxially rounded, adaxially concave, becoming trapezoid then triangular in cross section distally, armed as the sheath, spines particularly profuse on underside, sparse on upper surface; cirrus up to 1.5 m long, ± triangular in cross section, unarmed; leaflets 30-40 on each side of the rachis, lightly pendulous, composed of a single-fold, linear-lanceolate, or ± sigmoid, boadly attenuate at the base, broadly to finely acuminate at apex, 35-45 cm long by 1.5-2.5 cm broad at the widest point, concolourous, armed along the margin with robust spines, particularly at base of leaflet, single-nerved, with 5-7 conspicuous secondary nerves on each side, lowermost leaflets smaller than the rest, somewhat erect or stiffly swept back across stem; acanthophylls up to 5 cm long. Inflorescences in successive, axils, ca. 3 m from stem apex; peduncle 15-20 cm long; peduncular bracts ca. 3-4, 6-7 cm long; rachis up to 1.2 m in length, arching, pendulous; rachis bracts 2.5-4.5 cm long; rachillae, circular in cross-section, 25-35 cm long, pendulous, bracts bright yellow prior to anthesis, becoming dry, grey; prophyll subtending flower cluster 5-7 mm long, striate. Flower cluster with one central pistillate flower subtended by two lateral cincinni with one pistillate and 3-4 staminate flowers; staminate flowers 5 mm by 3.5 mm; calyx longer than the corolla, with short (less than 1 mm) stalk, tubular for the basal 2 mm; corolla, enclosed within the calyx, ca. 4 mm long, tubular for the basal 1 mm; staminodial tube ca. 2 mm long; anthers 0.8 mm by 0.2 mm; pistillode 2 mm by 0.5 mm; pistillate flowers similar to the staminate, 3.5-4.5 mm wide; ovary, 2.5 mm by 0.8 mm, tipped with a ca. 1.5 mm long style. Fruit at maturity ± globose, 1.8-2 cm., with 18-20 vertical rows of scales. Seed sub-globose, 1.5-1.8 cm by 1.6-1.9 cm, smooth, lightly flattened to depressed below; sarcotesta less than 0.3 mm.
This species is commonly found in forest margins, gaps of fallen trees and other open areas and is particularly common in seasonally inundated forest and alongside water courses.
Heimisch in, Angola, Cameroon, Congo, Equatorial Guinea, Gabon, Democratic Republic of the Congo
Oncocalamus macrospathus is distributed from Cameroon, south of the Sanaga River, to Cabinda (Angola). This species is more commonly encountered in coastal forest although it also occurs in the lowland riverine forests of the Congo Basin.