|Peppermint is a perennial herbaceous plant, producing creeping stolons. The steins are quadrangular, channelled, purplish, somewhat hairy and branching towards the top. The leaves are opposite, petiolate, ovate, sharply seriate, pointed, smoother on the upper than on the under surface, and of a dark green color, which is paler beneath. The flowers are small, purple, and in terminal obtuse spikes, interrupted below, and cymosely arranged. Late in the season, the growth of the lateral lower branches often gives to the inflorescence the appearance of a corymb. The calyx is tubular, often purplish, furrowed, glabrous below, and fivetoothed, the teeth being hirsute. The corolla is purplish, tubular, with its border divided into four segments, of which the uppermost is broadest, and notched at its summit. The four short stamens are concealed within the tube of the corolla; the style projects beyond it, and terminates in a bifid stigma.
Leaves more or less crumpled and frequently detached from the stems; stems quadrangular, from 1 to 2 mm. in diameter, glabrous except for a few scattered deflexed hairs; leaves when entire ovate-oblong to oblong-lanceolate, petioles from 4 to 15 mm. in length, slightly pubescent, laminae from 1 to 9 cm. in length, acute and sharply serrate, light green to purplish-brown, upper surfaces nearly glabrous, lower surfaces glandular hairy, especially on the veins; flower-whorls in oblong or oval spikes which are usually compact, or somewhat interrupted at the base, from 1 to 1.5 cm. in breadth, rounded at the summit, and in fruit attaining a length of from 3 to 7 cm.; bracts oblong-lanceolate, very acuminate, 7 mm. in length; calyx tubular, equally 5-toothed, pubescent and glandular punctate, often dark purple in color; corolla tubular-campanulate, 4- cleft, about 3 mm. in length and often light purple; stamens 4, short and equal; nutlets ellipsoidal, about 0.5 mm. in diameter, blackish-brown[.]
Source: United States Dispensatory (1918)