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Key to Silphium

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1 Leaves basally disposed, the basal leaves large and persistent, the stem with very few to many leaves, but these definitely reduced upward in size; leaves entire to toothed, to deeply cut; plants with definite taproots (except S. brachiatum, S. confertifolium, S. gracile, S. mohrii, S. simpsonii, and S. wasiotense).
  2 Stem relatively leafy, with 4-5 nodes or more, the stem leaves smaller than the basal, but not merely bracteal.
      4 Rays white; stems less than 1 m tall
      4 Rays yellow; stems usually 1-3 m tall
    3 Leaves nearly entire to coarsely toothed (but not pinnatifid).
        5 Basal and lower stem leaves cordate, subcordate, or truncate-sagittate; [Interior provinces: KY, TN, n. AL, and nw. GA].
          6 Stem hispid; peduncles hispidulous with hairs ca. 1 mm long; phyllaries obtuse.
        5 Basal and lower stem leaves rounded or cuneate at the base.
             7 Stem, inflorescence, and phyllaries glabrous or very nearly so; [AL and perhaps MS]
             7 Stem, inflorescence, and phyllaries obviously scabrous, hirsute, or hispid; [collectively more widespread].
               8 Stem, inflorescence, and phyllaries densely shaggy-hirsute, hairs > 5 mm long; inflorescence corymbose with many heads (around 3-15); pales eglandular; [TN, north AL, and northwest GA]
               8 Stem, inflorescence and phyllaries scabrous to hispid, hairs < 2.5 mm long; inflorescence consisting of a 1-5 heads subtended by a long, wand-like scape; pales glandular or eglandular.
                 9 Pales glandular; [Coastal Plain, SC, GA, FL, AL, and MS]
  2 Stem nearly naked, bearing only a few bracteal (very reduced) leaves.
                   10 Heads relatively large (involucre 13-25 mm high, disk 15-25 mm wide), with 14-40 ray flowers; [calcareous or mafic glades or woodlands].
                     11 Principal leaves shallowly to deeply pinnatifid; leaf blade base cuneate, rounded or shallowly cordate; leaf blade often > 2× as long as wide
                     11 Principal leaves only toothed (or subentire); leaf blade base cordate or truncate at the base (rarely abruptly narrowed); leaf blade < 1.5× as long as wide
                   10 Heads relatively small (involucre 6-11 mm high, disk 8-15 mm wide), with 6-12 ray flowers; [a wide range of mostly dry, often acidic habitats]; ["Silphium compositum group"].
                       12 Blades of basal leaves unlobed (or with a single obscure basal lobe on one or both sides), reniform, usually wider than long, often > 25 cm wide; leaves usually puberulent beneath; achenes shorter than the phyllaries at maturity; [upper Piedmont and Mountains]
                       12 Blades of basal leaves divided or shallowly to deeply lobed, with several lobes on each side, about as wide as long, or longer than wide, < 25 cm wide; leaves usually glabrous (or sparsely scabrous) beneath; achenes longer than (or as long as) the phyllaries at maturity; [collectively widespread].
                          13 Involucre mostly 1.0-1.5 cm wide; achenes 6-9 mm long at maturity; achene wings < 1 mm wide, the wing tips long acute to acuminate, the sinus between the wing tips V-shaped; [VA south to GA, and west to AL]
                          13 Involucre mostly 1.5-3.0 cm wide; achenes 8-14 mm long at maturity; achene wings 1-2 mm wide, the wing tips either acute to acuminate or obtuse, the sinus between the wing tips either V-shaped or narrowly U-shaped.
                            14 Achene wing tip obtuse, the sinus between the wing tips narrowly U-shaped; leaf blade usually longer than wide; petiole short, as long as or shorter than the leaf blade (midrib); [se. SC south to c. peninsular FL and FL Panhandle]
                            14 Achene wing tip acute to acuminate, the sinus between the wing tips V-shaped; leaf blade usually as long as wide; petiole long, as long as or longer than the leaf blade (midrib); [se. NC south to se. GA and FL Panhandle]
1 Leaves primarily on the stem, basal leaves usually absent or soon withering, the stem with many leaves, these similar in size; leaves entire or toothed; plants fibrous-rooted from a crown, rhizome, or caudex.
                              15 Stem square in ×-section; upper leaves connate, fused basally, the stem thus perfoliate.
                                16 Stem spreading-hispid (rarely nearly glabrous); heads with usually ca. 8 or ca. 13 rays; hairs on lower leaf surface or veins 1-2 mm long
                                16 Stem glabrous or glabrescent; heads with usually ca. 21 or ca. 34 rays; hairs on lower leaf surface absent or < 1 mm long
                                    18 Ray flowers 20-30 per head (or more); pales eglandular; [west of the Mississippi River].
                                    18 Ray flowers 12-22 per head; pales eglandular or stipitate-glandular (glands often mixed with other pubescence, use 20× magnification); [east of the Mississippi River, except for S. asperrimum]
                                           21 Mid-stem leaves predominately alternate (lower leaves often opposite); inflorescence eglandular except for inconspicuous dull yellow-brown stipitate-glands on pales; stem glabrous, hispid, or hirsute with hairs >1 mm.
                                             22 Stem hispid or hirsute, with hairs often > 1mm long (rarely nearly glabrous); mid-stem leaves clasping, sessile, or short-petiolate; inflorescence open or congested.
                                               23 Phyllary abaxial surface hispid; inflorescence congested, with peduncles usually less than 4 cm long; basal leaves withered by flowering
                                               23 Phyllary abaxial surface scabrous or glabrous; inflorescence open, with peduncles up to 13 cm long; basal leaves sometimes persisting at flowering; [Coastal Plain, SC, GA, FL, AL, and MS]
                                           21 Mid-stem leaves predominately opposite or whorled (occasionally alternate in damaged re-growth); inflorescence eglandular, or if glandular then glands conspicuous and not restricted to pales; stem various, but never hirsute with hairs >1 mm.
                                                 24 Mid-stem leaves petiolate or subsessile (not clasping), with a cuneate or rounded base; leaves typically lanceolate, gradually tapering towards the tip; stem and inflorescence branches eglandular.
                                                    25 Mid and lower stem leaves opposite; stem conspicuously hispid (or at least scabrous); inflorescence congested, with less numerous heads
                                                    25 Mid and lower stem leaves opposite or in whorls of 3; stem glabrous or nearly so; inflorescence open, with more numerous heads (up to 50 per inflorescence, although often much less).
                                                 24 Mid-stem leaves clasping to short-petiolate, with a subcordate to rounded base; leaves typically ovate, widest at base and abruptly tapering to an acute tip; stem and inflorescence branches eglandular or stipitate-glandular.
                                                        27 Stems, leaves, and phyllaries with yellow-brown and shining stipitate glands, usually well-evident on leaves and stem in addition to phyllaries (in addition to the eglandular pubescence).
image of plant
Show caption*© Scott Ward
                                                            29 Plants mostly 8-15 dm tall, with usually 6 or 7 nodes below the inflorescence; glandular hairs of the stems and leaves longer than the eglandular hairs; rays (8-) 12-14 (-16) per head; [dolomite or limestone in Bibb County, c. AL]
                                                            29 Plants mostly 15-20 dm tall, with usually 9-12 nodes below the inflorescence; glandular hairs of the stems and leaves about as long as the eglandular hairs; rays (17-) 19-23 (-33) per head; [chalk in c. AL]
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