Lythrum

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  1a. Inflorescence an elongate thyrse borne at the summit of the stem and branches, the primary one mostly 10–40 cm tall; flowers with 12 stamens, trimorphic; petals 7–12 mm long; leaf blades 3–10 cm long, 2 or 3 at each node
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    L. salicaria

    2a. Intersepalar appendages 2.5–3 mm long, usually longer than the sepals; plants generally pubescent on the stem and/or leaf blades (very rarely glabrous); leaf blades truncate to subclasping at the base (very rarely tapering)
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    L. virgatum

    2b. Intersepalar appendages ca. 1 mm long, about as long as or shorter than the sepals; plants glabrous throughout; leaf blades tapering to the base
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  1b. Inflorescence composed of solitary or paired flowers in the axils of leaves; flowers with 4–6 stamens, mono- or dimorphic (with 12 stamens and trimorphic in L. junceum); petals 2–6 mm long; leaf blades 0.8–4 cm long, usually 1 at each of the middle and upper nodes (regularly opposite in L. lineare)
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    L. junceum

    3a. Flowers with 12 stamens; hypanthium spotted with red near the base; petals 
5–6 mm long
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    3b. Flowers with 4–6 stamens; hypanthium without red spots; petals 2–5 mm long 
(up to 6 mm long in L. alatum)
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      L. alatum

      4a. Leaf blades narrow-oblong to lanceolate or narrow-ovate, 4–10 mm wide, rounded to truncate or subcordate at the base; hypogynous disk present, appearing as a thickened ring around the base of the ovary; plants perennial
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      4b. Leaf blades linear to narrow-oblong, 1–6 mm wide, cuneate at the base [Fig. 715]; hypogynous disk absent; plants annual or perennial
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        L. hyssopifolia

        5a. Vegetative leaves chiefly alternate (the lower nodes sometimes with opposite leaves), mostly longer than the associated internodes; plants annual, usually bearing flowers from near the stem base and upward; intersepalar appendages definitely longer than the calyx lobes; flowers monomorphic as to style length
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        L. lineare

        5b. Vegetative leaves chiefly opposite, mostly shorter than the associated internodes; plants perennial, usually without flowers at the lower nodes; intersepalar appendages ± as long as the calyx lobes; flowers dimorphic—pin flowers with long styles and thrum flowers with short styles

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