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Lycopus

Lycopus superficially resembles some species of Mentha. The two genera can be separated using a number of characters, including stamen number (2 in Lycopous, 4 in Mentha), mericarp shape (truncate at apex in Lycopus and sometimes with a crest or tubercles, rounded at apex in Mentha), and scent of fresh foliage (weakly or not aromatic in Lycopus, strongly mint- or fruit-scented in Mentha). Reference: Henderson (1962).

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1a. Lobes of the calyx broad-triangular, shorter than 1 mm, less than 2 times as long as wide, obtuse to subacute at the apex, equaled or surpassed by the mericarps at maturity; lower leaf blades serrate to coarsely serrate
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  L. uniflorus
  2a. Corolla 5-lobed, with flaring lobes; calyx with 5 lobes; plants with tubers; interior angles of the mericarps shorter than the 2 lateral angles, therefore, the apex of the combined mericarps concave [Fig. 696]; leaf blades and stem glabrous to sparsely pubescent
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  L. virginicus
  2b. Corolla 4-lobed (sometimes the upper lobed notched), with forward-oriented lobes [Fig. 698]; calyx with 4 lobes; plants without tubers; interior angles of the mericarps as long as the 2 lateral angles, therefore, the apex of the combined mericarps flat across the apex [Fig. 697]; leaf blades and stem pubescent
See list of 5 species in 1b
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1b. Lobes of the calyx narrow-triangular, 1–2.1 (–2.5) mm long, greater than 2 times as long as wide as long, acuminate at the apex, surpassing the mericarps at maturity [Fig. 696]; lower leaf blades serrate to pinnately lobed
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  3a. Corolla 4-lobed; interior angles of the mericarps shorter than the 2 lateral angles, therefore, the apex of the combined mericarps concave [Fig. 696]; plants without tubers (except in L. asper, a rare introduction)
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    L. asper
    4a. Leaf blades sessile or subsessile; rhizomes tuberous-thickened at tip; mericarps (1.3–) 1.4–1.8 mm wide
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    4b. Leaf blades borne on evident petioles; rhizomes without tubers; mericarps  0.7–1.3 mm wide
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      L. americanus
      5a. Leaf blades pubescent abaxially with hairs 0.01–0.5 mm long; fresh anthers 0.26–0.5 mm long; calyx 2–3.3 mm long; mericarps 1–1.37 mm long, the collar-like corky crest separated by a distance of 0.1–0.3 mm at the base on the adaxial  (i.e., inner) surface
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      L. europaeus
      5b. Leaf blades pubescent abaxially with hairs 0.5–1.6 mm long; fresh anthers  0.5–0.7 mm long; calyx 3–4.5 mm long; mericarps 1.3–1.73 mm long, the collar-like corky crest completely encircling the mericarp or separated by up to 0.2 mm at  the base on the adaxial surface
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  3b. Corolla 5-lobed or 4-lobed with the upper lobe prominently notched; interior angles of the mericarps as long as the 2 lateral angles, therefore, the apex of the combined mericarps flat across the apex [Fig. 697]; plants with tubers
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    L. rubellus
    6a. Leaf blades lanceolate to elliptic, either concavely narrowed from the basal-most teeth to a narrow base or with an evident petiole
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    L. amplectens
    6b. Leaf blades oblong to oblong-lanceolate, convexly narrowed from the basal-most teeth to a broad-cuneate to rounded base

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Show photos of: Each photo represents one species in this genus.