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Key to Trifolium

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1 Flowers bright yellow (fading brown); [subgenus Chronosemium].
  2 Leaves palmately trifoliolate (all leaflets essentially sessile); heads 10-13 mm in diameter; flowers 5-7 mm long
  2 Leaves pinnately trifoliolate (the lateral leaflets essentially sessile, the terminal leaflet with a petiolule 0.8-3 mm long); heads 5-13 mm in diameter; flowers 2.5-5 mm long.
    3 Standard with 5 obvious diagonal veins (striations); heads 8-13 mm in diameter, generally with 20-30 flowers; flowers 3.5-5 mm long; petiolule of the terminal leaflet 1-3 mm long
    3 Standard inconspicuously veined; heads 5-8 mm in diameter, generally with 5-15 (-20) flowers; flowers 2.5-3.5 mm long; petiolule of the terminal leaflet ca. 1 mm long
1 Flowers white, pink, purplish, or red; [subgenus Trifolium].
      4 Flowers borne on distinct pedicels, (1-) 2-10 mm long, these often curving or reflexing in age; flowers white, fading pink with age in most species; [native and exotic species]; [section Lotoidea and Trifolium (T. alexandrinum)].
        5 Plants stoloniferous, all or some of the leaves alternate from ground level and long petioled.
          6 Calyx lobes narrowly triangular, about as long as the calyx tube; peduncles axillary along the stolons; stipules scarious-membranaceous; [plant an abundant introduced weed]
          6 Calyx lobes subulate, distinctly longer than the calyx tube; peduncles terminal, either at tips of the stolons, or at tips of erect flowering branches; stipules green, foliaceous; [plants rare natives].
             7 Peduncle terminal, at the tip of the stolon, lacking leaves; pedicels 2-3 mm long
             7 Peduncle terminal at tip of erect flowering branches, subtended by a pair of opposite or subopposite, short-petioled leaves; pedicels (2-) 4-8 mm long
        5 Plants not stoloniferous, clumped (though sometimes with prostrate or lax stems).
               8 Calyx lobes narrowly triangular, about as long as the calyx tube (or longer in T. hybridum); stipules scarious-membranaceous; [plants introduced].
                 9 Calyx lobe margins sparsely plumose; [section Trifolium, subsection Alexandrina]
                 9 Calyx lobe margins sometimes scarious, but not sparsely plumose; [section Lotoidea]
                   10 Calyx lobes not scarious-margined, straight, equal to or longer than the tube
                   10 Calyx lobes scarious-margined, becoming divergent and twisted, about equal to the tube
               8 Calyx lobes subulate to lanceolate, distinctly longer than the calyx tube; stipules green, foliaceous; [plants rare natives].
                     11 Flowers 4-6 mm long; calyx lobes lanceolate, foliaceous, 3-nerved, 0.4-0.8 mm wide
                     11 Flowers 8-12 mm long; calyx lobes subulate, setaceous, 1-nerved, < 0.4 mm wide.
                       12 Leaflets 3-7× as long as wide; stems prostrate; flowers creamy white and purple-veined; plant a perennial; [plants of shale barrens and other rock outcrops, from VA northward]
                       12 Leaflets 1-2.8× as long as wide; stems erect, ascending, or decumbent; flowers white or purplish; plant an annual or biennial; [plants of a variety of natural woodlands, collectively widespread in our area]
                          13 Plant decumbent; terminal petiole 5.7-8.2 cm long; sepal lobes 0.2-0.37 cm long; peduncles 0.23-0.31× as long as the terminal petiole; [dry limestone woodlands, Bluegrass region of KY (so far as is known)]
                          13 Plant ascending to erect; terminal petiole 0.3-3.5 cm long; sepal lobes 0.3-0.7 cm long; peduncles 0.4-4.2× as long as the terminal petiole; [various woodlands and barrens, widespread in our region (at least formerly)]
      4 Flowers sessile or on very short pedicels (usually < 1 mm long); flowers pink, purplish, white, or scarlet; [exotic species].
                            14 Plants stoloniferous, all or some of the leaves alternate from ground level and long petioled.
                              15 All flowers with petals; fruiting heads enlarging, becoming a reddish brown, pubescent ball ca. 2 cm in diameter, remaining aerial; [section Vesicaria]
                              15 Only 2-5 outer flowers of the head with petals, the others lacking petals and sterile; fruiting heads becoming a subterranean bur, buried by downward curvature and growth of the peduncle; [section Trichocephalum]
                            14 Plants not stoloniferous, the leaves clustered at or near ground level and/or produced on aerial stems.
                                16 Heads subtended by a pseudo-involucre of 2 (-3) enlarged stipules and/or opposite or subopposite leaves; [section Trifolium].
                                  17 Flowers white (fading pink), 7-8 mm long; calyx tube both externally glabrous and 20-nerved
                                  17 Flowers red, pink-purple, or bicolored, either 11-20 mm long or 4-6 mm long; calyx tube not both externally glabrous and 20-nerved (externally pubescent, or 10-nerved, or both).
                                    18 Flowers 4-6 mm long
                                    18 Flowers 11-20 mm long.
                                       19 Stipules gradually tapering to a long slender tip, longer than the fused part; calyx densely hirsute; stem soft pubescent with deflexed to spreading hairs
                                       19 Stipules abruptly narrowed to a short awn; calyx glabrous to pilose; stem appressed pubescent
                                16 Heads not subtended by a pseudo-involucre of leaves or expanded stipules.
                                         20 Heads axillary, sessile, in the axils of subtending leaves; calyx tube glabrous (except for a few hairs at apex); [section Lotoidea]
                                         20 Heads terminal or axillary; calyx tube pubescent.
                                           21 Calyx bladdery-inflated in fruit; corolla resupinate (inverted 180 degrees, such that the standard is lowermost); [section Vesicaria].
                                             22 Inflorescence with a prominent peduncle; head lobed in fruit
                                             22 Inflorescence subsessile to shortly peduncled; head spherical in fruit
                                           21 Calyx not bladdery-inflated in fruit; corolla orientation normal (standard uppermost).
                                               23 Corolla 3-6 mm long; [section Trifolium]
                                               23 Corolla 10-18 mm long.
                                                 24 Corolla crimson, 10-13 (-15) mm long; floral bracts absent; heads 1-1.5 (-2) cm in diameter; [section Trifolium]
                                                 24 Corolla white, 15-18 mm long; floral bracts present; heads 2.5-3 cm in diameter; [section Mistyllus]