Global description
Macroptilium atropurpureum is a creeping or climbing annual or perennial herb with training stems reaching up to 2 m in length. The leaves are alternate, trifoliate, with oval- triangular stipules, 4 to 5 mm long. The leaflets are ovate to rhombic, the lateral ones more or less lobed to one lobe, oblique. The inflorescence is a loose terminal cluster, with long peduncle, 10 to 30 cm. The Papilionaceous flower is dark purple with a velvety sheen, reddish at the base. Fruits are linear pods, cylindrical, 5 to 9 cm long and 3 to 4.5 mm wide, finely pubescent, dehiscent containing up to 12 seeds.
General habit
Macroptilium atropurpureum is an annual or perennial herb, creeping or climbing, more or less lianescent, with training stems reaching up to 2 m in length.
Underground system
Taproot system and adventitious roots at the nodes in contact with the ground.
Stem
The stem is cylindrical, full, creeping or climbing, branched, reaching up to 2 m in length; the young stems are voluble, lianescent, pubescent to densely hairy with white hairs.
Leaf
The leaves are alternate, compound trifoliate. The stipules are ovate-triangular, 4 to 5 mm long, veined. The leaflets are oval to rhombic, 2 to 7 cm long and 1.5 to 5 cm wide. The terminal leaflet are broadly elliptical, the maximum width located in the lower third. The lateral leaflets are highly unsymmetrical, with margin entire and more or less strongly lobed margin. The apex is rounded, the base is rounded to more or less unsymmetrical for lateral leaflets. The upper surface is dark green and finely pubescent, the underside is gray-green, pubescent.
Inflorescence
The inflorescences are very loose terminal clusters, with peduncles, 10 to 30 cm long.
Flower
The papilionaceous flower is dark purple with a velvety sheen, reddish at the base: the calyx is tubular, 7 to 8 mm long with 5 triangular lobes, 2 to 3 mm long; blackish purple corolla is 1.5 to 2.5 cm long.
Fruit
The fruit is a linear, straight, cylindrical pod, 5 to 9 cm long and 3 to 4.5 mm wide, rostrate, pubescent, dehiscent, containing up to 12 seeds.
Seed
The seeds are oval to oblong-ellipsoid, 4 mm long, streaked with brown and black.