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| 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 adventiously 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; [alien, 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 alien] |
| ............ ......10 Foliage and fruits (or ovaries) glabrous; leaves toothed, sometimes also lobed; [common natives and aliens] |
| ............ ..8 Leaves ovate or elliptic, 1.2-4× as wide as long; base cordate, peltate, or truncate. |
| ............ ........11 Leaves perfoliate; flowers yellow; [rare aliens]. |
| ............ ..........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 (partly to fully mature) with thin-edged wings; flowers yellow or purple; central flower of each umbellet staminate and pedicelled; fruits all pedicelled in all umbellets |
| ............ ............ ....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) |