Acorus calamus is an aquatic and aromatic reed-like perennial herb which grows up to 1.8 metres tall. It has long, thin, sword-shaped leaves growing from long creeping rhizomes. The bright green leaves are reddish towards their bases. Inconspicuous flowers are compactly arranged on a long, fleshy axis and surrounded by a large leaf-like spathe. There are several varieties differing mainly in chemical composition and chromosome number. 

Reference:

1 Lander, V. and Schreier, P. (1990). Acorenone and γ-asarone: Indicators of the origin of calamus oils (Acorus calamus, L.). Flavour and Fragrance Journal, 5 (2): 75-79.