Elaeocarpus polystachyus is a medium-sized rainforest tree to 23 m tall that occurs from lowland to hill dipterocarp forest to 840 m altitude, including peat swamp and coastal forest, but rarely up to montane forest. This sun-loving species usually grows in small populations, on hill slopes and ridges but is more common in secondary forest than in primary forest.
The outer bark of E. polystachyus is grey-brown with pale brown inner bark and cream sapwood. The stipules are narrowly triangular, dark brown or black, and sometimes persist even after leaf maturity. The simple leaves are spirally arranged and loosely grouped towards the twig tips. The long petioles, which grow up to 18 cm long and are kneed at both ends, are one of the diagnostic characters for E. polystachyus. The leaf blades are elliptic to ovate, usually to 19.5 × 10 cm, with 6–12 pairs of lateral veins. The hairy racemose inflorescences bear many brownish yellow flowers with sometimes as many as 38 flowers per inflorescence. The flowers are 4-merous, but mutation in flower part number is recorded in two specimens: one from Slim Hills Forest Reserve, Perak (Whitmore FRI 717) has 5-merous flowers, and the other from Bukit Timah, Singapore (Samsuri Ahmad SA 1353) has a mixture of 4- and 5-merous flowers. The annular disc is hairy and weakly-developed, thus is structurally less obvious. The stamens are numerous, between 50–65 per flower, and are densely-arranged in multiple tiers; they are brown turning blackish brown after anthesis. The filaments are short, between 0.5–1 mm long. The anthers are a little longer, between 1.5–2 mm long, and are not awned. The 2-locular ovary is densely covered by yellowish brown hairs and each locule contains 10–12 ovules. The oblong or ellipsoid fruits are green and hairy when young, turn glabrescent and once were recorded as yellow when mature (Suppiah FRI 11360) instead of blue as in most Elaeocarpus fruits. Often only one seed is developed in each fruit and the seed is enclosed within a hard stone that develops from the inner mesocarp. The embryo is curved and the endosperm is ruminate.
Elaeocarpus polystachyus is endemic to Peninsular Malaysia and Singapore and is locally abundant in Bukit Timah Nature Reserve, Singapore, but becomes less common from Johor northwards to Kelantan and Perak. The species belongs to the Polystachyus group and the latter consists of five other West Malesian endemics, i.e. E. cupreus Merr., E. clementis Merr., E. multinervosus R.Knuth, E. polyanthus Ridl. and E. integripetalus Miq. Members of this group share high morphological similarities, particularly their floral characters, such as petals that are entire or very shallowly fimbriate with up to 6 divisions, the numerous stamens (35–80 per flower) densely-arranged in multiple tiers and anthers that are not awned (Coode 1996).
