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Key to Platanthera

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1 Leaves basal, (1-) 2, orbicular, (5-) 8-25 cm wide, prostrate on the ground, the stem naked or with a few bladeless bracts; [subgenus Platanthera, section Lysias].
  2 Stem lacking bracts; flowers sessile; spur tapering from base to apex
  2 Inflorescence with bracts below the inflorescence; flowers on pedicels 5-10 mm long; spur clavate (enlarged near the tip).
    3 Spur > 28 mm long; pollinaria > 4.6 mm long
    3 Spur < 28 mm long; pollinaria < 4.6 mm long
1 Leaves cauline, 1-10 or more, mostly lanceolate, the largest 1-5 cm wide, ascending, the stem with at least 1 (usually several) bladed leaves.
      4 Lip deeply divided into 3 similarly-sized lobes (these often further deeply fringed or sometimes merely toothed or jaggedly incised); [subgenus Fimbriella].
        5 Lobes of lip merely toothed or jaggedly incised, few (if any) of the segments > 1 mm long; flowers rose-purple
        5 Lobes of lip shallowly or deeply fringed, most or all of the segments > 1 mm long; flowers rose-purple, white, cream, or greenish.
          6 Flowers purple (or rarely white in albino forms).
             7 Lobes of the lip fringed < 1/3 of the way to the base of the lobes; opening to nectary dumbbell-shaped (the pollen sacs close together); spur 12-20 mm long
             7 Lobes of lip fringed from 1/3 to nearly all the way to the base of the lobes; opening to nectary widely rounded (the pollen sacs spread widely apart); spur 20-35 mm long.
               8 Lip segments moderately to deeply fringed; isthmus of the lip stout, about 2× as long as wide; spur 20-26 mm long, ca. 1.25× as long as the lip; orifice round; flowering mid Jun-early Jul
               8 Lip segments deeply and compoundly lacerate; isthmus of the lip slender, about 4× as long as wide; spur 21-35 mm long, 2-2.5× as long as the lip; orifice angled at top; flowering mid Jul-early Aug
          6 Flowers greenish-white or yellowish-white.
                 9 Perianth greenish-white, cream, or white; lateral petals linear-spatulate, < 2 mm wide, blunt, entire to inconspicuously crenulate; lateral sepals deflexed
                 9 Perianth white or cream; lateral petals cuneate to broadly obovate, 4-12 mm wide, toothed; lateral sepals divergent
                   10 Rostellum lobes parallel, rounded when viewed laterally
                   10 Rostellum lobes spreading, angular when viewed laterally
      4 Lip unlobed or very shallowly 3-lobed at the apex only (the sinuses < 1 mm deep), the lip's margin entire, minutely toothed, or deeply fringed.
                     11 Lip margin deeply fringed; flowers white, yellow, or orange; [subgenus Blephariglottis, section Blephariglottis].
                          13 Spur 15-26 mm long, ca. 1× as long as the ovary; lip descending and thence curved back toward the stem, narrowed at its base to a very short isthmus (the section between the base and the fringed portion); lip fringing short and relatively coarse; [NL (Newfoundland) west to MI and IL, south to GA]
                          13 Spur 30-50 mm long, ca. 2× as long as the ovary; lip projected forward; lip narrowed to an extended isthmus; lip fringing elongate and delicate; [e. NC south to c. peninsular FL, west to e. TX]
                       12 Flowers yellow to orange; spur 5-33 mm long.
                            14 Spur 20-33 mm long, exceeding the 12-27 mm long ovary; undivided portion of lip 8-12 mm long
                            14 Spur 4-17 mm long, equal to or shorter than the ovary; undivided portion of lip 4-6 mm long.
                     11 Lip margin minutely denticulate or entire; flowers white, yellow-orange, green, or greenish-white.
                                16 Spur 35-60 mm long; lip 10-15 mm long; flowers white; [subgenus Blephariglottis, section Blephariglottis]
                                16 Spur 2-18 mm long; lip 3-12 mm long; flowers white, yellow-orange, green, or greenish-white.
                                  17 Lip uppermost (flowers not resupinate); spur 10-18 mm long, borne horizontally, parallel to the pedicel and in side view of the inflorescence crossing the rachis; [subgenus Blephariglottis, section Gymnadeniopsis].
                                  17 Lip lowermost (flowers resupinate); spur 2-13 mm long, borne horizontally or descending, in side view crossing the rachis only in P. clavellata.
                                    18 Flowers not purely white, instead green, greenish white, orange- to golden-yellow, yellowish-green, or yellowish-white.
                                       19 Flowers orange- to golden-yellow; lip minutely crenulate; [subgenus Blephariglottis, section Gymnadeniopsis]
                                       19 Flowers green, greenish-white, yellowish-green, or yellowish-white; lip entire, shallowly trilobed, proximally dilated, or eroded at the tip.
                                         20 Lip tapering to a narrow tip distally, slightly or not dilated basally (proximally), the margins and tip neither trilobed nor eroded; [of n. PA northwards]; [subgenus Limnorchis].
                                           21 Anther sacs oriented horizontally from one another, the apices almost touching; lateral petals and lip green; lip not dilated at base; flowers essentially scentless; plants to 60 cm tall
                                           21 Anther sacs oriented upright, the apices parallel and not almost touching; lateral petals and lip greenish-white; lip slightly dilated at base; flowers with sweet fragrance; plants to 100 cm tall
                                         20 Lip rounded or somewhat rectangular, about as wide near the tip as at the base, shallowly trilobed and/or eroded at the tip; [collectively widespread].
                                             22 Larger stem leaves usually 1, rarely 2, near the middle of the stem; raceme 2-9 cm long, 2-3.5 cm in diameter; lip without a tubercle on the upper surface near the base, also lacking lateral auricles near the base; [subgenus Blephariglottis, section Gymnadeniopsis]
                                             22 Larger stem leaves usually 2 (-5), near the middle of the stem or toward its base; raceme 5-20 cm long, 1.2-2 cm in diameter; lip with a tubercle on the upper surface near the base, also usually with 2 lateral auricles; [subgenus Tulotis].
                                               23 Most bracts of the inflorescence shorter than the flowers they subtend (the lowermost few exceeding the flowers); flowers sparsely distributed; lip orbicular, 1-1.5× as long as broad, yellowish-green
                                               23 Most bracts of the inflorescence exceeding the flowers they subtend (the uppermost few sometimes shorter than the flowers); flowers densely distributed; lip oblong, 2-3× as long as broad, green
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