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See list of 5 species in this genus

A difficult genus complicated by hybridization and polyploidy. Misapplication of names has caused tremendous confusion, and often the names that authors have used actually apply to different species or hybrids (the correct name can sometimes be determined by examining the characters they used to identify the taxa). Many species of Mentha produce small-flowered, carpellate plants in addition to the normal ones with bisexual flowers. The bisexual flowers have exserted stamens at anthesis, the carpellate ones do not. Mentha suaveolens Ehrh. was reported from ME by Gleason and Cronquist (1991), but specimens are unknown. Further, reports of this species are complicated by the fact that usage of this name and its hybrid with M. longifolia ( M. ×‌rotundifolia) are frequently confused. References: Harley (1972), Tucker and Naczi (2007).

  • 1a. Inflorescence consisting largely of verticillasters of flowers in the axils of normal foliage leaves separated by internodes of normal length [Fig. 700]
    • 2a. Leaf blades narrow-oblanceolate to rhombic-elliptic (infrequently to ovate), usually cuneate at the base [Fig. 700], (1.5–) 1.7–3.3 (–4.1) times as long as wide, gradually decreasing in size toward the apex of the plant; calyx 2–3 mm long, the subulate to narrow-triangular (rarely triangular) lobes (0.6–) 0.7–0.9 (–1.1) mm long; fresh plants with pennyroyal or peppermint odor
    • 2b. Leaf blades ovate to suborbicular, rounded at the base, 1.3–2.3 times as long as wide, not gradually descreasing in size toward apex of plant, the uppermost 1 or 2 nodes with conspicuoulsy reduced leaves; calyx 1.5–2.5 mm long, the narrow-triangular to broad-triangular lobes 0.5–0.8 (–0.88) mm long; fresh plants with sweet fruit odor (rarely pennyroyal-scented)
  • 1b. Inflorescence consisting of congested (at least in part) verticillasters of flowers that are subtended by reduced, bracteal leaves and separated by highly shortened internodes [Fig. 699]
    • 3a. Inflorescence composed of 2 or 3 closely aggregated verticillasters of flowers, collectively forming a globose to short-ovoid cluster of flowers 15–25 (–30) ×12–20 mm (sometimes also with 1–3 additional separate verticillasters of flowers from the upper axils of foliage leaves) [Fig. 699]
    • 3b. Inflorescence composed of 4–19 or more verticillasters of flowers, elongate and cylindric, (30–) 40–100 (–150) ×5–10 (–15) mm
      • 4a. Leaf blades ovate to suborbicular, 1–2 times as long as wide, mostly shorter than 
45 mm, rugose-veiny abaxially, usually strongly so (in part)
      • 4b. Leaf blades lanceolate or oblong-lanceolate to oblong-ovate, 2–3.5 times as long as wide, mostly longer than 45 mm, not or scarcely rugose-veiny abaxially
        • 5a. Leaf blades oblong-lanceolate to oblong-elliptic or oblong-ovate, widest near the middle, pubescent entirely with simple hairs; stems pubescent; pollen-bearing anthers 0.28–0.38 mm long; mericarps 0.54–0.79 mm long; fresh plants with 
musty odor
        • 5b. Leaf blades lanceolate to narrow-ovate, widest near the base, sometimes with 
a few scattered dendroid hairs on the abaxial surface; stems glabrous (rarely sparsely pubescent); pollen-bearing anthers 0.38–0.58 mm long; mericarps 0.74–0.94 mm long; fresh plants with spearmint odor (in part)

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