Description from Flora of China
Perennial herbs, glabrous, aromatic, growing in marshes or as emergent aquatics; laticifers and raphides absent; aerenchyma present. Rhizome creeping, much branched, lacunose, with specialized aromatic oil cells; roots on lower side and bearing leaves at apex. Leaves distichous, bases overlapping, unifacial, ensiform, not differentiated into petiole and blade; intravaginal squamules present in leaf axils; venation parallel. Inflorescence solitary, terminal, borne laterally on leaflike scape (usually interpreted as peduncle and spathe; peduncle with 2 separate vascular systems); continuous shoot arising in axil of leaf preceding inflorescence; spathe much longer than spadix, erect, persistent (appearing merely as a vertical extension of leaflike peduncle); spadix jutting out at an angle from peduncle, sessile, conoid, cylindric and fingerlike or slender and tail-like, flowering from base to top. Flowers bisexual, with perigone, densely arranged, bractless, 3-merous; tepals 6, in 2 whorls of 3, free, thin, arched; stamens 6, in 2 whorls of 3, free, filaments linear-oblong and flattened, anthers introrse, thecae globose-ellipsoid, subopposite, dehiscing by longitudinal slit, connective inconspicuous; pollen monosulcate, ellipsoid, small (15-20 µm), exine shallowly and remotely or more densely foveolate, otherwise psilate, apertural exine subpsilate; gynoecium (pistil) obconic-cylindric, only slightly exceeding tepals, ovary 2- or 3-locular, ovules several per locule, orthotropous (atropous), pendent on apical placenta, both integuments bearing trichomes and inner integument longer than outer, forming micropyle, stigma minute (± punctate), subsessile (a broad stylar region especially seen by longitudinal cut). Fruit a few-seeded berry, oblong-obovoid with thinly leathery pericarp, enclosed by tepals, ± whitish with brownish stigma remnant when fresh, soon drying to straw-brown, 1-5(-9)-seeded. Seed oblong to ellipsoid; testa light brown, with small pits (slightly foveolate) (Acorus calamus) or smooth (A. gramineus), long integumentary trichomes (bristles) present at micropyle (A. gramineus) or absent (A. calamus); embryo axile, cylindric or conoid (A. gramineus), with perisperm and abundant endosperm. 2n = (22), 24, 36, (44), 48.
Li Hen. 1979. Acorus. In: Wu Cheng yih & Li Hen, eds., Fl. Reipubl. Popularis Sin. 13(2): 4-9.
One genus and two species: temperate and subtropical Asia and North America, tropical Asia; introduced and naturalized in Europe, New Guinea (at least partly), and North America (partly); both species in China.
(Authors: Li Heng (李恒 Li Hen), Zhu Guanghua (朱光华) ; Josef Bogner )