Shrub or small “columnar” tree 1.8–6(–9) m. high, with horizontal branches all the way up the 1–3 main stems; bark red-brown. Young branchlets grey-puberulous or pubescent; then epidermis flaking away to expose rusty-red, powdery inner bark (but see note below). Stipules spinescent, long, straight, whitish, about 2–5(–9) cm. long, some fused at base into round, blackish “ant-galls” up to 2.5(–3) cm. in diameter. Leaves with grey, inconspicuous puberulence on the 2–4 mm. long petiole and the 2–10 cm. long rhachis; pinnae of well-developed leaves of mature shoots 15–22 pairs (reduced leaves with fewer pairs usually also present), 1–3 cm. long; leaflets very numerous, 1.5–4.5 mm. long, 0.75–1 mm. wide, acute or subacute at apex, ciliolate near base or glabrous. Flowers creamy-white, in heads; involucel at base of the tomentellous to densely puberulous, nearly or quite eglandular, 0.75–1 mm. thick peduncle. Calyx 1.75–2 mm. long. Corolla glabrous, or slightly puberulous only near apex outside, 4–5 mm. long, 2–3 times as long as calyx. Pods (Fig. 16/49, p. 67) falcate, grey-puberulous, acute at both ends, 2–8 cm. long, 0.4–1.0 cm. wide. Seeds grey, smooth, elliptic with an irregular outline, compressed, often curved, 9–11 mm. long, 4–6 mm. wide; areole 6–7 mm. long, 3 mm. wide.
