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Key S1: herbaceous dicots with opposite, simple, and unlobed leaves on the stem

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1 Inflorescence an involucrate head subtended by phyllaries, the heads solitary or many and variously arrayed in secondary inflorescences, the ovary inferior, the corolla connate and tubular at least basally, the calyx absent, the stamens 5, the fruit a cypsela
1 Inflorescence, flower, and fruit structure various, but not with the combination of features as above (sometimes the flowers in a head, e.g. Pycnanthemum in LAMIACEAE, but then with other features differing, such as stamens 4, or green calyx present, or fruit a schizocarp of mericarps, etc.).
..2 Leaves scale-like, stems fleshy; flowers embedded in the fleshy stem, no perianth present; [saline environments (coastal or rarely inland)].
..2 Leaves small to large; stems not fleshy; flowers sessile or on pedicels; [collectively of many habitats, saline and not].
....3 Ovary inferior or partially inferior.
......4 Perianth of a single whorl (petals absent) (note that in Mirabilis in NYCTAGINACEAE the petaloid calyx is subtended by a 5-lobed fused set of involucral bracts).
........5 Leaves herbaceous, suborbicular, about as long as wide or wider than long; calyx 3- or 4-merous; stamens 4, 8, or 12.
..........6 Plant ascending, with a single node (2 leaves); leaves > 6 cm long and wide; calyx 3-merous, brown to yellowish; stamens 12
..........6 Plant creeping, with many nodes; leaves 3-15 mm long and wide; calyx 4-merous, yellow to greenish; stamens 4 or 8
........5 Leaves fleshy, linear, lanceolate, to broadly ovate, at least slightly longer than broad; calyx 5-merous; stamens 3, 5, or 10.
............ 7 Flowers axillary, sessile or nearly so, solitary or a few; petaloid sepals widely spreading, separate; leaves linear to oblanceolate; stamens 5 or 30-50
............ 7 Flowers in terminal cymose panicles; petaloid sepals connate into a narrow tube (reminiscent of the corolla of Ipomoea); leaves lanceolate, elliptic, ovate, or broadly ovate; stamens 3 or 5
......4 Perianth in 2 whorls (sepals and petals both present).
............ ..8 Petals distinct; inflorescence diffuse.
............ ....9 Leaves distinctly 3-veined from the base, the 3 veins converging again at the leaf apex
............ ....9 Leaves with prominently pinnate venation
............ ..8 Petals connate into a tube (at least basally); inflorescence often a head or dense terminal cyme (also axillary, or solitary on long peduncles).
............ ......10 Petals 5; stamens 3, 4, or 5
............ ........11 Creeping subshrub; flowers paired at tip of an upright peduncle
............ ........11 Upright herb; flowers in axils or terminal corymbs
............ ..........12 Flowers in terminal corymbs; fruit dry
............ ..........12 Flowers axillary; fruit fleshy
............ ......10 Petals 4 (or 6 or 8 in Richardia in RUBIACEAE); stamens 4, 6, or 8.
............ ............ 13 Inflorescence a head, subtended by 4 large bright white bracts
............ ............ 13 Inflorescence a head or more diffuse (see below), sometimes subtended by green bracts.
............ ............ ..14 Leaves serrate; corolla bilaterally symmetrical (especially the flowers near the outer edge of the head); inflorescence a head
............ ............ ..14 Leaves entire; corolla radially symmetrical; inflorescence a head or more diffuse (see below).
............ ............ ....15 Petals acute; flowers in terminal panicles, cymes, or panicles, or axillary; plant habit various, not simultaneously with all the characters below
............ ............ ....15 Petals broadly rounded; flowers axillary, solitary; plant a diffusely branched herb with linear leaves
....3 Ovary superior.
............ ............ ......16 Perianth of a single whorl (petals absent) or missing entirely (petals and sepals both absent). {key lead number needs adjusting}
............ ............ ........17 Inflorescence a cyathium, consisting of a single pistillate flower (reduced to a single 3-carpellate pistil) and 2 or more staminate flowers (each reduced to 1 stamen), borne in a cup-like involucre, the involucre bearing pointed or rounded glands, these sometimes brightly colored and petaloid, mimicking an individual flower (the cyathia then secondarily arranged in terminal cymes, or solitary and axillary, etc.); fresh plants with milky juice; fruit a 3-lobed, 3-locular capsule
............ ............ ........17 Inflorescence not a cyathium (and staminate or bisexual flowers with > 1 stamen, except Callitriche in PLANTAGINACEAE); fresh plants lacking milky juice; fruit various, not as above.
............ ............ ..........18 Flowers 1 (or 2) in leaf axils; leaves entire.
............ ............ ............ 19 Flowers unisexual; sepals 0; flowers (staminate) with 1 stamen
............ ............ ............ 19 Flowers bisexual; sepals 4; flowers with 2, 4, or 6 stamens.
............ ............ ..........18 Flowers many, in axillary spikes, cymes, or glomerules, or in terminal spikes, heads, cymes, or panicles; leaves entire or serrate.
............ ............ ............ ....21 Fruit single-seeded, an achene or utricle; {YYYY}.
............ ............ ............ ......22 Leaves serrate, regularly and sharply so; plants with stinging hairs (or not)
............ ............ ............ ......22 Leaves entire, or with a few very obscure crenations (Iresine) or basally disposed rounded lobe-like teeth (Atriplex); plants without stinging hairs.
............ ............ ............ ........23 Leaves of a pair slightly to strongly different in size
............ ............ ............ ........23 Leaves of a pair the same size.
............ ............ ............ ..........24 Styles 1-2 (-3); leaves 2-30 mm long, 0.5-8 mm wide, linear or narrowly elliptic
............ ............ ............ ..........24 Style 1; leaves generally either longer than 30 mm, or wider than 8 mm (if linear and smaller than those dimensions, then fleshy).
............ ............ ............ ............ 25 Tepals scarious; inflorescence of heads, spikes, or panicles
............ ............ ............ ............ 25 Tepals herbaceous; inflorescence of glomerules, these axillary or arrayed in spikes or panicles
............ ............ ......16 Perianth in 2 whorls (sepals and petals both present).
............ ............ ............ ............ ..26 Gynoecium of 4-many pistils, each 1-carpellate.
............ ............ ............ ............ ....27 Pistils 4-5, in a single whorl; stamens 4, 5, 8, or 10; fruit an aggregate of follicles
............ ............ ............ ............ ....27 Pistils many, spiral; stamens many; fruit an aggregate of plumose achenes
............ ............ ............ ............ ..26 Gynoecium either of 1 pistil (with 1 or more carpels), or of 2 pistils, united only by the style and stigma (APOCYNACEAE).
............ ............ ............ ............ ......28 Petals not at all connate, not even at their bases.
............ ............ ............ ............ ........29 Leaves with pellucid punctate glands (most easily visible with transmitted light); stamens often fascicled into 3, 4, or 5 fascicles; petals yellow or pinkish
............ ............ ............ ............ ........29 Leaves lacking pellucid punctate glands; stamens not fascicled; petals variously colored.
............ ............ ............ ............ ............ 31 Petals 3; sepals 5, dimorphic, the 2 outer sepals narrower than the 3 inner and concave sepals; stamens (3-) 5-15 (-25)
............ ............ ............ ............ ............ 31 Petals 4-7; sepals 4-7, normally monomorphic; stamens 4, 5, 6, 8, 10, or 12 (or sometimes rarely 2 or 3).
............ ............ ............ ............ ............ ..32 Capsule 2-5 (-6) locular; style 1; perianth 4-7-merous; stamens 4, 6, 8, 10, or 12
............ ............ ............ ............ ............ ..32 Capsule either 1-locular of 10-locular; styles 2-5; perianth 4-5-merous; stamens 4, 5, 8, or 10 (or rarely 2 or 3).
............ ............ ............ ............ ............ ....33 Capsule 1-locular, dehiscent apically by teeth or valves; sepals connate into a tube or separate; styles 2-5; perianth 4-5-merous; stamens 4, 5, 8, or 10 (or rarely 2 or 3)
............ ............ ............ ............ ............ ....33 Capsule 10-locular (each of the 5 carpels divided at maturity), septicidal; sepals distinct or nearly so; styles 5, perianth 5-merous; stamens 5
............ ............ ............ ............ ......28 Petals connate at least for a short distance at their bases.
............ ............ ............ ............ ............ ......34 Corolla radially symmetrical (or so slightly bilaterally symmetrical as to be mistakable as radially symmetrical); stamens as many as the corolla lobes (or 1 less in Ruellia in ACANTHACEAE, Buchnera in OROBANCHACEAE, Trichostema in LAMIACEAE, and Verbena in VERBENACEAE); carpels 2 or 3.
............ ............ ............ ............ ............ ........35 Pistils 2, united only by the style and stigma; fruit a schizocarp of 2 1-carpellate follicles (often single by abortion); plant with milky juice when fresh (except Catharanthus); leaves entire
............ ............ ............ ............ ............ ........35 Pistil 1 (of 2-5 fused carpels); fruit either a 2-5-carpellate capsule or of 2 or 4 1-seeded nutlets derived from 2 carpels; plant lacking milky juice; leaves entire or serrate.
............ ............ ............ ............ ............ ..........36 Ovary and capsule 3-5-carpellate; capsule 3- or 1-locular.
............ ............ ............ ............ ............ ............ ..38 Inflorescence a terminal cyme; corolla salverform, with an elongated and very narrow tube, pink or white; capsule 3-locular
............ ............ ............ ............ ............ ............ ..38 Inflorescence various but not cymose, of terminal or axillary racemes or panicles, or of solitary axillary flowers; corolla connate only at the base, the petals appearing nearly separate (not salverform); capsule 1-locular
............ ............ ............ ............ ............ ..........36 Ovary and capsule 2-carpellate; fruit either a 2-locular capsule or of 2 or 4 1-seeded nutlets derived from 2 carpels.
............ ............ ............ ............ ............ ............ ....39 Stamens 4-12, the same number as the corolla lobes; corolla (and the flower as a whole) strictly radially symmetrical.
............ ............ ............ ............ ............ ............ ......40 Capsule septicidal; corolla white, pink, blue, yellowish white, or greenish white; inflorescence either a terminal or axillary cyme, or a terminal panicle or raceme, or a terminal or axillary cyme reduced to 1 or a few flowers
............ ............ ............ ............ ............ ............ ......40 Capsule loculicidal and also deeply 2-lobed; corolla white, pink, or scarlet with a yellow interior; inflorescence of cymosely arranged spikes
............ ............ ............ ............ ............ ............ ....39 Stamens either 4, 1 fewer than the 5 corolla lobes, or 2 (with 2 staminodes); corolla usually slightly bilaterally symmetrical (the flower as a whole made bilaterally symmetrical by the 2 or 4 stamens).
............ ............ ............ ............ ............ ............ ........41 Leaves entire; corolla tube flaring for all of its length
............ ............ ............ ............ ............ ............ ..........42 Fruit a schizocarp of 4 1-seeded nutlets; inflorescence terminal, of cymes; corolla ca. 5 mm long; leaves prominently 3-veined
............ ............ ............ ............ ............ ............ ..........42 Fruit a capsule; inflorescence axillary, of cymes or clusters (often reduced to a solitary flower); corolla > 12 mm long; leaves with single primary vein
............ ............ ............ ............ ............ ............ ........41 Leaves serrate; corolla salverform, the tube narrow and nearly the same diameter for most of its length; inflorescence a terminal spike, raceme, raceme of racemes, or head.
............ ............ ............ ............ ............ ............ ............ 43 Fruit a 2-locular capsule; stamens inserted near the base of the corolla tube
............ ............ ............ ............ ............ ............ ............ 43 Fruit a schizocarp of 4 mericarps; stamens inserted near or above the middle of the corolla tube
............ ............ ............ ............ ............ ......34 Corolla bilaterally symmetrical (or the corolla 2-lipped but the corolla lobes twisted so as to make the flower asymmetrical); fertile stamens fewer than the corolla lobes (except Plantago in PLANTAGINACEAE, which is equal, with 4 each; a few genera have a 5th, sterile, stamen which is obviously different in form than the 4 fertile stamens) (note that many corollas are bilabiate and the number of corolla lobes, 4 or 5, may be difficult to interpret); carpels 2.
............ ............ ............ ............ ............ ............ ............ ..44 Carpels 2, each carpel slightly to deeply lobed, separating at maturity into 4 half-carpellate units (not separating in Phyla in VERBENACEAE); fruit a schizocarp of 4 mericarps (or 2 nutlets in Phyla in VERBENACEAE).
............ ............ ............ ............ ............ ............ ............ ....45 Inflorescence a thyrse, verticillaster, or terminal cyme, the flowers borne in cymose lateral branches; corolla strongly bilaterally symmetrical (rarely nearly radially symmetrical); stems square in ×-section (or sometimes rounded, especially on older growth); fresh plants often (but not always) aromatic
............ ............ ............ ............ ............ ............ ............ ....45 Inflorescence of spikes, racemes, or heads, the flowers or fruits single at nodes; corolla often nearly radially symmetrical; stems rounded in X-section (rarely square); fresh plants usually not aromatic
............ ............ ............ ............ ............ ............ ............ ........47 Corolla 4 lobed, almost radially symmetrical; corolla scarious, white, or bluish
............ ............ ............ ............ ............ ............ ............ ........47 Corolla 4-5-lobed, either strongly bilabiate or salverform (Pseuderanthemum in ACANTHACEAE); white, blue, or yellow.
............ ............ ............ ............ ............ ............ ............ ..........48 Inflorescence of solitary (rarely 2) axillary flower. {add [Hypoestes] ACANTHACEAE}
............ ............ ............ ............ ............ ............ ............ ............ ..50 Corolla barely bilaterally symmetrical, the lobes about as long as the tube; outer sepals ovate, much wider than the inner sepals
............ ............ ............ ............ ............ ............ ............ ............ ..50 Corolla distinctly bilabiate, the lobes shorter than the tube; sepals of nearly the same width.
............ ............ ............ ............ ............ ............ ............ ............ ......52 Corolla 5-lobed, distinctly bilabiate or in some nearly radially symmetrical; corolla colored.
............ ............ ............ ............ ............ ............ ............ ............ ........53 Flowers in terminal racemes, panicles, or spikes, the inflorescence not interspersed with large, leaf-like bracts.
............ ............ ............ ............ ............ ............ ............ ............ ..........54 Sepals connate for at least 0.3× their length to form a tube (this cup-like and flaring in Scrophularia in SCROPHULARIACEAE).
............ ............ ............ ............ ............ ............ ............ ............ ............ 55 Inflorescence a diffuse panicle; corolla 5-11 mm long, reddish-brown (sometimes with some yellow); fruit a septicidal capsule
............ ............ ............ ............ ............ ............ ............ ............ ............ 55 Inflorescence of 1 or more terminal (and sometimes upper axillary) spikes or racemes; corolla 10-50 mm long (6-8 mm long in Phryma in PHRYMACEAE), white, pink, blue, purple, or yellow; fruit either a loculicidal capsule (OROBANCHACEAE) or a single seeded achene (Phryma in PHRYMACEAE).
............ ............ ............ ............ ............ ............ ............ ............ ............ ..56 Corolla 6-8 mm long, white to pale pink; fruit a 1-seeded achene contained in the accrescent calyx, this “lopping down” against the inflorescence axis
............ ............ ............ ............ ............ ............ ............ ............ ........53 Flowers axillary and solitary, borne in the axils of normally-sized leaves or somewhat reduced but still large and leaf-like bracts [some taxa keyed here and below].
............ ............ ............ ............ ............ ............ ............ ............ ............ ........59 Leaves entire, either mostly larger or smaller [see below]; plant not blackening on drying
............ ............ ............ ............ ............ ............ ............ ............ ............ ..........60 Leaves 0.6-2.8 cm long, round, obovate, or broadly elliptic, < 1.8× as long as wide; plants creeping, ascending to 3 dm tall; [plants of wet places]
............ ............ ............ ............ ............ ............ ............ ............ ............ ..........60 Leaves 3-30 cm long, narrowly elliptic to lanceolate, > 2× as long as wide; plants erect or the stems sprawling; [plants mostly of uplands]
............ ............ ............ ............ ............ ............ ............ ............ ............ ............ ..62 Calyx lobes longer than the tube, or as long as the tube, corolla 25-50 mm long; plants usually blackening on drying
............ ............ ............ ............ ............ ............ ............ ............ ............ ............ ..62 Calyx lobes shorter than the tube; corolla 14-22 mm long; plants not blackening on drying.
............ ............ ............ ............ ............ ............ ............ ............ ............ ............ ....63 Corolla red or orange, with a very narrow, cylindrical tube, the lobes then flaring into a limb about 1 cm across; plants blackening on drying; [rare alien, in crop fields, a noxious hemiparasitic weed under quarantine]
............ ............ ............ ............ ............ ............ ............ ............ ............ ............ ....63 Corolla yellow, not narrowly cylindrical, the lower lip arched; plants not blackening on drying; [rare, in seepage wetlands]
............ ............ ............ ............ ............ ............ ............ ............ ............ ............ ......64 Corolla pink (sometimes almost white), often lined with yellow inside; leaves narrowly linear, often filiform (except lanceolate in A. auriculata); plants usually blackening on drying (some species do not); corolla not strongly bilabiate
............ ............ ............ ............ ............ ............ ............ ............ ............ ............ ......64 Corolla white, blue, or combinations of blue and white (sometimes with some yellow markings); leaves broader, mostly lanceolate; plants not blackening on drying; corolla strongly bilabiate.
............ ............ ............ ............ ............ ............ ............ ............ ............ ............ ........65 Upper lip of the corolla not hooded and enclosing the anthers; plants not blackening on drying.
............ ............ ............ ............ ............ ............ ............ ............ ............ ............ ..........66 Corolla blue; lower lip of the corolla arched upwards into the throat; plants perennial from rhizomes or crowns, 3-13 dm tall
............ ............ ............ ............ ............ ............ ............ ............ ............ ............ ..........66 Corolla bicolored, the upper lip white or very pale blue, the lower lip bright blue; lower lip of the corolla folded downward into a pouch enfolding the anthers; plants annuals, 0.5-4 dm tall