1 Leaves linear, lanceolate, or oblancolate, > 4× as long as wide.

  2 Leaves phyllodial (septate, hollow or flat, segmented); flowers white or purple; [plants of wetlands].

    3 Umbels simple; leaves spatulate, broader towards the tip, often somewhat flattened in ×-section, rounded or obtuse at the apex

    3 Umbels compound; leaves tapering to a pointed tip, either flat or terete in ×-section.

      4 Leaves flat in ×-section, with obvious parallel venation and less obvious septae / cross-partitions

      4 Leaves terete in ×-section, the septae obvious.

        5 Plants 1-11 dm tall, annuals, sometimes mat-forming and adventitiously perennial; fruits 1-3 mm long, with lateral ribs; rays 3-15

        5 Plants 6-24 dm tall, perennials from rhizomes or tubers; fruits 4-9 mm long, with lateral wings; rays 5-20

  2 Leaves “normal” (non-septate, flat, continuous, and in some cases lobed, toothed, or spinose-margined); flowers blue, yellow, white, or whitish-green; [plants of wetlands or uplands].

          6 Flowers borne in involucrate heads; corolla blue or greenish-white

          6 Flowers in compound umbels; corolla yellow or white.

             7 Stem leaves similar to the basal, all simple; corollas yellow; [exotic, of disturbed areas]

             7 Stem leaves palmately 3-5-foliolate with linear leaflets, differing from the simple basal leaves; corollas white; [native, of blackland prairies and associated disturbed areas, from AL and TN westward]

1 Leaves orbicular, ovate, or elliptic, < 4× as long as wide.

               8 Leaves orbicular, about as wide as, or wider than, long; base peltate or cordate.

                 9 Leaves leathery, with spinose margins; inflorescence a head; flowers blue; [rare introduction]

                 9 Leaves herbaceous or somewhat fleshy. toothed or lobed, but not spinose; inflorescence umbellate or verticillate; flowers white, greenish, or purplish; [collectively common and widespread].

                   10 Foliage and fruits (or ovaries) stellate-pubescent or glochidiate; leaves lobed, but otherwise entire; [rare exotic]

                   10 Foliage and fruits (or ovaries) glabrous; leaves toothed, sometimes also lobed; [common natives and exotics]

               8 Leaves ovate or elliptic, 1.2-4× as wide as long; base cordate, peltate, or truncate.

                     11 Leaves perfoliate; flowers yellow; [rare exotics].

                       12 Stem leaves entire; umbellets subtended by broad foliaceous bractlets

                       12 Stem leaves serrate; umbellets lacking bractlets

                     11 Leaves cordate or truncate at the base; flowers white, green, yellow, blue, or purple;

                          13 Flowers greenish or blue; leaves all simple (sometimes stem leaves lobed); inflorescence a head or very congested (subcapitate) umbel; [plants of wetland situations, prostrate, creeping, or erect].

                            14 Inflorescence a very congested (subcapitate) umbel, with 4-9 flowers; leaves cordate at the base, long-petiolate, the petioles characteristically 2× as long as the leaf

                            14 Inflorescence a head, with > 20 flowers; leaves cuneate to truncate at the base, sessile to short petiolate, the petioles < 1× as long as the leaf (except E. prostratum)

                          13 Flowers yellow or purple; basal leaves simple, stem leaves usually compound; Inflorescence a compound umbel; [erect plants of upland situations].

                              15 Fruits ribbed (with rounded, cordlike ribs), lacking thin-edged wings; flowers yellow; central flower of each umbellet either staminate and pedicelled, or pistillate and sessile; fruits all pedicelled in some umbellets (those with a staminate central flower), or the central fruit sessile in some umbellets (those with a pistillate central flower)

1 Leaflets entire.

  2 Leaflets linear (resembling winged rachises); corolla white

  2 Leaflets broad, elliptic, ovate, or obovate, sparingly lobed; corolla yellow

1 Leaflets variously serrate, dentate, and/or incised.

    3 Plants in flower.

      4 Corolla yellow, maroon, or pale creamy yellow.

        5 Flowers yellow; central flower of each umbellet either staminate and pedicelled, or pistillate and sessile; fruits all pedicelled in some umbellets (those with a staminate central flower), or the central fruit sessile in some umbellets (those with a pistillate central flower); developing fruits laterally compressed, all of the ribs rounded and cordlike

      4 Corolla white.

          6 Ovary hispid or pubescent.

             7 Rays 16-25; leaves somewhat coriaceous; leaflet bases often cuneate or obliquely truncate; [plants of dry habitats]

             7 Rays 3-5; leaves thin in texture; leaflet bases rounded, subcordate, or broadly cuneate; [plants of moist forests]

          6 Ovary glabrous.

               8 Plants 1-9 dm tall at maturity; [rarely naturalized exotics].

                 9 Ovary (and later the fruits) terete, the ribs corky-winged; [plants of wetlands]

                 9 Ovary (and later the fruits) laterally compressed, not winged; [plants of uplands]

                   10 Umbellets (rays) 9-15 per umbel

                   10 Umbellets (rays) (25-) 50-100 per umbel

               8 Plants 6-18 dm tall at maturity; [collectively common and widespread natives].

                     11 Sheaths of the upper leaves dilated, > 1 cm wide when flattened; [plants of moist habitats, from GA northward in the Mountains, extending into adjacent provinces towards the northern edge of our area]

                     11 Sheaths of the upper leaves not dilated, < 1 cm wide; [plants either of wetlands or of moist to dry forests].

                       12 Veins of the leaflets directed to the sinuses; leaflets mostly 2.5-5× as long as wide, acuminate at the tip; [of wetlands]

                       12 Veins of the leaflets directed to the tips of the teeth or lobes; leaflets mostly 1.3-1.8× as long as wide, acute to obtuse at the tip; [of moist to dry forests]

    3 Plants in fruit.

                          13 Fruit hispid or pubescent (regardless of winging).

                            14 Fruits 4-6 mm long, hispid across the surfaces; rays 16-25; leaves somewhat coriaceous; leaflet bases often cuneate or obliquely truncate; [plants of dry habitats]

                            14 Fruits 18-24 mm long, oblanceolate or linear, appressed-pubescent on the ribs; rays 3-5; leaves thin in texture; leaflet bases rounded, subcordate, or broadly cuneate; [plants of moist forests]

                          13 Fruit glabrous (sometimes winged or prominently ribbed as well).

                              15 Fruits dorsally compressed (strongly to slightly) or subterete, either thin-winged or corky-winged.

                                16 Ribs very corky; fruits 2-3 mm long; [rare aquatic or semiaquatic exotic]

                                16 Ribs thin-winged; fruits 3-8 mm long; [common natives of most to submesic forests].

                                  17 Rays 12-30; fruits 4-8 mm long; plant 6-20 dm tall; sheaths of the upper leaves dilated, > 1 cm wide when flattened

                              15 Fruits laterally compressed, not winged (except thin-winged in Ligusticum).

                                    18 Veins of the leaflets directed to the sinuses; leaflets mostly 2.5-5× as long as wide, acuminate at the tip; [of wetlands]

                                    18 Veins of the leaflets either directed to the tips of the teeth or lobes, or reticulating extensively and becoming obscure before reaching the margin; leaflets mostly 1.3-1.8× as long as wide, acute to obtuse at the tip; [of moist to dry forests].

                                       19 Leaflet venation palmate-ternate, each leaflet with 3 (-5) main veins from the base (the central vein then with pinnate secondary veins); plants from rhizomes

                                       19 Leaflet venation pinnate, each leaflet with a dominant central vein (it then with pinnate secondary veins); plants from a caudex, taproot, or cluster of fibrous or fleshy roots.

                                         20 Lateral veins of the leaflet parallel and straight, clearly extending to the tip of each tooth; leaflet base often strongly oblique; leaflet margin entire in the lower ¼ to 1/3

                                         20 Lateral veins of the leaflet arcing, reticulating and becoming obscure before reaching the leaflet margin; leaflet base symmetrical or slightly oblique; leaflet margin toothed to or very close to the base

                                           21 Umbellets (rays) per umbel (25-) 50-100

                                           21 Umbellets (rays) per umbel 4-25