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Key to Cypripedium

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1 Plant scapose, with 2 basal leaves; pouch-like lip of flower with a longitudinal fissure, pink (rarely nearly white); [section Acaulia]
1 Plant caulescent, with (2) 3-7 leaves alternate on the stem; pouch-like lip of flower with a rounded orifice, yellow, pink, or white.
  2 Pouch-like lip of flower pink and white (rarely all white); lateral petals and dorsal sepal white, not twisted, obtuse to acute; [section Obtusipetala]
  2 Pouch-like lip of flower yellow or white; lateral petals and dorsal sepal brown, purplish brown, or yellow, slightly to strongly twisted, acuminate-attenuate; [section Cypripedium].
    3 Pouch-like lip of flower white, 1.5-2.5 cm long; orifice margin acute on the apical margin; [of calcareous barrens]
    3 Pouch-like lip of flower bright yellow, pale yellow, or creamy white, (2.0-) 2.2-6.3 cm long (if pale yellow or white, then > 4 cm long); orifice margin rounded-obtuse on the apical margin; [of various habitats].
      4 Dorsal sepal 3.5-5.0 cm wide; pouch orifice 2.7-4.5 cm long; pouch-like lip 4.5-6.3 cm long, pale yellow or creamy white; plants robust, typically 5-8 dm tall
      4 Dorsal sepal 1.1-2.9 cm wide; pouch orifice 0.5-1.7 cm long; pouch-like lip 1.5-5.8 cm long, medium to rich yellow; plants not as robust, typically 2-5 (-6) dm tall.
        5 Lower surface of the uppermost sheathing bract sparsely and inconspicuously pubescent to glabrous when young; pouch-like lip 15-29 mm long; sepals and petals usually suffused with dark reddish brown or madder; scent intense, sweet.
        5 Lower surface of the uppermost sheathing bract densely and conspicuously silvery-pubescent when young (later sometimes glabrescent); pouch-like lip 20-54 mm long; sepals and petals variably spotted, striped, blotched, and reticulately marked with reddish brown or madder (rarely unmarked); scent moderate to faint, rose or musty.
          6 Pouch-like lip 2.2-3.4 cm long; sepals and lateral petals usually densely and minutely spotted with dark reddish brown or purple, thus appearing uniformly dark
          6 Pouch-like lip (2.0-) 3.0-5.8 cm long; sepals and lateral petals unmarked (greenish-yellow), or more often streaked, blotched, striped or reticulately marked with dark reddish brown or purple (but generally not extensively blotched)