Fabaceae subfam. Caesalpinioideae
(synonym of Fabaceae subfam. caesalpinioideae)
Senna
senna
erect, glabrous or pubescent.
bipinnate, sometimes pinnate or tripinnate, 2-foliolate, or phyllodic.
alternate, even-pinnate, mesophyllous to sclerophyllous, rarely phyllodic;
stipules present, caducous or persistent;
petiolate;
extrafloral nectaries 0 or 1–8, on petioles, on rachis between proximal pair of leaflets only, or between proximal and subsequent pairs of leaflets, sessile or stipitate, glandlike and concave;
leaflets (or phyllodes) 2–28, blade margins usually entire, surfaces glabrous or pubescent.
1–40+-flowered, axillary, racemes, sometimes aggregated into compound racemes, erect;
bracts present, usually early or late caducous, rarely persistent at anthesis;
bracteoles absent.
caesalpinioid (noncaesalpinioid in Ceratonia) or mimosoid, bilateral or radial;
sepals distinct or connate;
petals (0 or)5(or 6), distinct or connate;
stamens (2–)5–250+, filaments distinct or connate, heteromorphic or some or all sometimes modified or staminodial, anthers basifixed or dorsifixed;
pollen in monads, tetrads, or polyads.
caesalpinioid, monosymmetric or asymmetric, enantiostylous;
calyx usually early or late caducous, rarely persistent after anthesis, bowl- or vase-shaped, lobes 5, greenish to yellowish;
corolla yellow to yellow-orange, petals subequal, blades narrowed to claw;
stamens 6–10, often heterantherous, with 1 adaxial set of 3 staminodes, 1 middle set of 4 short stamens and 1 abaxial set of 2 or 3 long stamens, or not heterantherous and stamens similar in shape and only slightly different in size;
filaments stiff, or arcuate, glabrous, nearly same length as anthers;
anthers stiff, basifixed, dehiscing by 1 or 2 apical pores or short slits;
ovary shortly stipitate, incurved, linear, usually hairy, rarely glabrous;
style linear or incurved, usually terminating in minute stigmatic tip.
legumes, dehiscent or indehiscent, or loments (in Prosopis).
legumes, stipitate, flat to cylindrical, straight or curved [spiral], often corrugated over seeds, indehiscent or partly dehiscent, glabrous or pubescent.
usually with an apical hilum, complex hilar valve absent, pleurogram on each face, open or closed;
embryo straight.
10–20, ovoid, obovoid, ellipsoid to rhomboid, or pyriform, usually areolate.
nectaries often present on petiole or rachis, sometimes stipular or bracteal.
lateral, free, or absent.
= 7, 8, 9, 11, 12, 13, 26, 28.
= 12, 13, 14.
Fabaceae subfam. Caesalpinioideae
Senna
Genera 148, species ca. 4400 (38 genera, 174 species in the flora).
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Species 300–350 (25 in the flora).
Senna was segregated from the large Cassia and placed in subtribe Cassiinae, together with Cassia in the narrow sense and Chamaecrista (H. S. Irwin and R. C. Barneby 1981, 1982). A number of Senna species have been used as purgatives or laxatives, several other species have been cultivated as ornamentals (L. T. F. Colladon 1816; M. A. Luckow 1996), and some have become invasive weeds (Irwin and Barneby 1982). Senna is a remarkable example of floral specialization in relation to buzz pollination (that is, pollen-collecting bees vibrate flowers; S. L. Buchmann 1974). The nectarless flowers of Senna display a wide range of traits typical of buzz-pollinated flowers: poricidal anthers, heteranthery, point-tipped stigma, and enantiostyly (deflection of the carpel to the left or right) often accompanied by asymmetric corolla and androecium (B. Marazzi et al. 2007; Marazzi and P. K. Endress 2008). Another characteristic feature of Senna is the conspicuous glandlike extrafloral nectaries (EFNs) on leaves and, sometimes, also at the base of pedicels in 80% of the species (Irwin and Barneby 1982; Marazzi and M. J. Sanderson 2010). Less well known are recently reported cryptic EFNs embedded within stipules, bracts, and sepals in species previously thought to lack extrafloral nectaries (Marazzi et al. 2013). Extrafloral nectaries offer nectar to ants in return for their protection from insect herbivores (R. R. Fleet and B. L. Young 2000). Senna is a relatively old genus with several fossil fruits described from the Eocene (P. S. Herendeen 1992; L. Calvillo Canadell and S. R. S. Cevallos-Ferriz 2005), and probably diverged from its sister Cassia in the narrow sense about 50 million years ago (Marazzi and Sanderson). The current classification of Senna (Irwin and Barneby 1982), recognizing 35 series in six sections [Astroites H. S. Irwin & Barneby, Chamaefistula (de Candolle ex Colladon) H. S. Irwin & Barneby, Paradictyon H. S. Irwin & Barneby, Peiranisia (Rafinesque) H. S. Irwin & Barneby, Psilorhegma (Vogel) H. S. Irwin & Barneby, and Senna], has been adopted worldwide but received little support in molecular phylogenetic studies, as only sect. Psilorhegma is monophyletic (Marazzi et al. 2006).
In North America, all of the sections are represented except for the monospecific Astroites and Paradictyon from central Mexico and South America, respectively. Several ornamental species are cultivated in warmer areas of North America and may escape, especially in Arizona, California, and Florida, including Senna bacillaris (Linnaeus f.) H. S. Irwin & Barneby, S. corymbosa, S. didymobotrya (Fresenius) H. S. Irwin & Barneby, S. multijuga (Richard) H. S. Irwin & Barneby, S. spectabilis (de Candolle) H. S. Irwin & Barneby, and S. surattensis. Currently, S. corymbosa and S. surattensis are considered naturalized in North America, along with at least another six introduced species. A few of the introductions have regionally been recognized as problematic invasive plants (S. multiglandulosa in California and S. pendula in Florida), and two other species (S. obtusifolia and S. occidentalis) are considered noxious weeds in many countries worldwide.
Two species, Senna bicapsularis (Linnaeus) Roxburgh and S. italica Miller, have been excluded since all specimens examined were misidentified. Specimens known in North America as S. bicapsularis have been confirmed to represent misidentifications or misappli
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Key to Genera of Subfamily Caesalpinioideae, Excluding Mimosoid Clade
2. Corollas absent; inflorescences often on older branches and trunk; leaves even-pinnate; legumes flat with thickened margins. | Ceratonia |
2. Corollas present; inflorescences not usually on older stems or trunk; leaves even- or odd-pinnate, often bipinnate; legumes flat or cylindric, without thickened margins. | → 3 |
3. Trees, often armed (with straight or branching thorns); inflorescences spicate clusters; flowers polygamous or dioecious; corollas not obvious; leaves clustered from spurs. | Gleditsia |
3. Trees, shrubs, or herbs, armed or unarmed; inflorescences not spicate clusters; flowers bisexual; corollas obvious; leaves clustered on spurs or not clustered. | → 4 |
4. Stamen filaments proximally villous, much longer than anthers; stems photosynthetic, yellowish green; leaves clustered, alternate from spurs, bipinnate but sometimes appearing pinnate; legumes constricted between seeds. | Parkinsonia |
4. Stamen filaments glabrous, usually nearly same length as anthers; stems not photosynthetic, brownish (except Senna armata); leaves not clustered on spurs, pinnate. | → 5 |
5. Trees; leaf petiole and rachis without glands; stamens 10, filaments of 3 abaxial stamens sigmoidally incurved, usually longer than anthers; legumes indehiscent, not corrugated over seeds; inflorescences usually terminal. | Cassia |
5. Herbs, shrubs, or trees; leaf petiole and rachis with or without glands; stamens (2 or)3–10, all filaments straight; legumes dehiscent or indehiscent, often corrugated over seeds; inflorescences terminal or axillary. | → 6 |
6. Bracteoles absent; petals subequal; stamens 6–10, usually dwindling from one side to the other; legumes either indehiscent or tardily dehiscent through 1 or both sutures, if the latter then not coiling, or valves breaking into 1-seeded joints; stipules inconspicuous; flowers in axillary racemes, sometimes aggregated into compound racemes; root nodules absent. | Senna |
6. Bracteoles present; petals unequal; stamens (2 or)3–10, radially symmetric or bilateral, equal or irregularly unequal; legumes elastically dehiscent, valves coiling; stipules conspicuous, striate, often persistent; flowers in reduced few-flowered axillary racemes; root nodules present. | Chamaecrista |
1. Leaves bipinnate (sometimes also pinnate, rarely unifoliolate). | → 7 |
7. Trees; flowers usually unisexual, appearing apetalous, polygamous or dioecious; petals and sepals small and similar, greenish yellow; legumes usually pulpy between seeds. | → 8 |
8. Trees unarmed; leaves bipinnate, leaflet blade margins entire; inflorescences terminal, racemes or panicles; legumes thick, turgid, woody; seeds 2–5, subglobose. | Gymnocladus |
8. Trees often armed with simple or branched thorns (cultivars sometimes unarmed); leaves bipinnate in terminal growth, pinnate from spur shoots, leaflet blade margins often crenulate; inflorescences axillary, spicate racemes; legumes usually thin, flat, flexible; seeds 1–25(–30), compressed to subterete. | Gleditsia |
7. Trees, shrubs, herbs, or vines; flowers bisexual, obviously petalous; petals and sepals easily differentiated; legumes not pulpy between seeds. | → 9 |
9. Trees, 8–35 m, unarmed. | → 10 |
10. Corollas yellow, less than 25 mm diam., petals not clawed; calyx imbricate; stigma peltate, broad; stipules 1 mm, triangular, entire; legumes 4–12 cm, margins winged, indehiscent; seeds 1–4. | Peltophorum |
10. Corollas scarlet and yellow, 80–100 mm diam., petals clawed; calyx valvate; stigma capitate; stipules 5–15 mm, pinnate; legumes 30–60 cm, margins not winged, dehiscent; seeds 20–40. | Delonix |
9. Herbs, shrubs, subshrubs, or small trees to 6 m, armed or unarmed. | → 11 |
11. Leaves odd-bipinnate. | → 12 |
12. Leaflet blades not glandular-punctate (inflorescences often conspicuously glandular-pubescent); calyx persistent in fruit. | Hoffmannseggia |
12. Leaflet blades glandular-punctate; calyx deciduous or persistent in fruit. | → 13 |
13. Shrubs, trees, or perennial herbs; stipules ovate, lanceolate-ovate to deltate or suborbiculate, usually early deciduous, persistent, or subpersistent; sepals ovate-lanceolate to orbiculate, persistent in fruit; androecium and gynoecium free, not cupped in lower sepal; legumes lanceolate-oblong; leaflet blades eglandular or with conspicuous black, sessile glands along margin, these sometimes sunken in sinuses of crenulated margin. | Erythrostemon |
13. Shrubs, subshrubs, or perennial herbs; stipules filiform or linear to lanceolate, persistent or tardily deciduous; sepals linear, deciduous in fruit; androecium and gynoecium cupped in the lower sepal; legumes oblong, oblong-lanceoloid, lunate, or ovate; leaflet blades with multiple orange, glandular dots on abaxial surfaces (drying black). | Pomaria |
11. Leaves even-bipinnate. | → 14 |
14. Legumes covered with prickles on faces; seeds ovoid or globose to subglobose, 15–25 mm wide; flowers functionally unisexual, segregated on separate male and female racemes. | Guilandina |
14. Legumes not covered with prickles; seeds ovoid or ellipsoid, 4–10 mm wide; flowers bisexual. | → 15 |
15. Legumes indehiscent, compressed or subterete, valves not twisting. | → 16 |
16. Branches brown or gray-brown; legumes oblong, compressed, ± fleshy, leathery; calyx irregular, abaxialmost sepal covering others in bud; petals yellow, banner with red medial markings. | Tara |
16. Branches green or yellowish green; legumes oblong to linear, compressed or subterete, sometimes torulose; calyx nearly radially symmetric, valvate, sepal lobes nearly distinct; petals all yellow, banner without red markings. | Parkinsonia |
15. Legumes dehiscent, valves twisting, laterally-compressed. | → 17 |
17. Corollas yellow to orange or red; petals glabrous. | Caesalpinia |
17. Corollas consistently yellow, banner sometimes with red marks; petals pubescent. | Denisophytum |
Key to Genera of Subfamily Caesalpinioideae, Mimosoid Clade
1. Stamens 5–10. | → 2 |
2. Shrubs or trees, usually armed with nodal spines or thorns; fruits legumes or loments, indehiscent or dehiscent, elongate, turgid, and often irregularly moniliform, or coiled springlike, rarely irregularly twisted or contorted. | → 3 |
3. Pinnae 1 or 2 pairs; fruits loments, straight or spirally coiled, seeds distinct nearly to base; inflorescences axillary, spikes or globose heads, 40–100+-flowered, corollas yellow, cream-yellow, purple-brown, greenish white, or yellow-green, all bisexual; sc, sw United States. | Prosopis |
3. Pinnae 7–15 pairs; fruits legumes, contorted or coiling at maturity; inflorescences axillary, pendent spikes, 50–200-flowered, corollas pale green, distally bisexual with bright yellow anthers, and proximally sterile with purple or white staminodes; introduced, Florida. | Dichrostachys |
2. Herbs, shrubs, or trees, unarmed, or with internodal prickles which are generally curved and flattened; fruits legumes, dehiscent or segmenting. | → 4 |
4. Perennial herbs, unarmed, terrestrial or floating-aquatic; stems ascending, decumbent, prostrate, prostrate-ascending, or floating; corollas yellow-green to white; distal flowers often bisexual, proximal flowers sterile with petaloid staminodes (monomorphic in N. lutea). | Neptunia |
4. Trees, shrubs, subshrubs, or perennial herbs, usually unarmed, sometimes armed, terrestrial; stems erect, ascending, decumbent, prostrate, or sprawling; corollas white, cream, yellow, greenish white, pink, purple, or purple-pink; flowers usually bisexual. | → 5 |
5. Trees, 5–15(–40) m, unarmed; leaflet blades 15–45 mm, alternate; inflorescences in terminal panicles or axillary spikelike racemes; corollas white to cream; legumes straight to falcate or contorted, swollen around seeds, valves twisting; seeds glossy red, flattened; stipitate anther glands present; introduced, Florida. | Adenanthera |
5. Trees, shrubs, subshrubs, or herbs, 0.1–3 m, unarmed or armed; leaflet blades 1.5–26(–30) mm, opposite; inflorescences in heads, spikes, or racemes; corollas white, greenish white, pale green, yellow, or pink to purple; legumes straight or curved, not swollen around seeds; seeds not glossy red; stipitate anther glands absent. | → 6 |
6. Petiole nectar glands absent; legumes often craspedial, segmented into 1-seeded portions separating from persistent sutures (replum), sometimes unsegmented; plants armed with recurved or straight prickles or unarmed; corollas white or pink to purple. | Mimosa |
6. Petiole nectar gland present (sometimes minute); legumes not craspedial; plants without prickles; corollas white, greenish white, or yellow. | → 7 |
7. Shrubs or trees, 2–18(–20) m; stems erect; stipules ovate, inconspicuous; inflorescences globose heads; corollas yellow, white, or greenish white; legumes shortly stipitate, linear or oblong, compressed or flat, valves sometimes curling. | Leucaena |
7. Herbs or shrubs, to 3 m; stems erect to prostrate or decumbent; stipules subulate, small but evident; inflorescences condensed heads or spikes; corollas pale green or white; legumes sessile, linear or falcate, subterete to flattened, splitting along margins at maturity. | Desmanthus |
1. Stamens 14–250. | → 8 |
8. Filaments distinct (except rarely connate basally in Acacia); leaves bipinnate or simple phyllodia. | → 9 |
9. Leaves simple phyllodia or bipinnate; stipular spines usually absent (except A. paradoxa); seeds usually with pulpy aril forming a cap or encircling seed. | Acacia |
9. Leaves bipinnate; stipular spines present or absent; seeds usually without pulpy aril (except sometimes in Vachellia). | → 10 |
10. Stipular spines present, sometimes enlarged and inhabited by ants; ovaries sessile or subsessile; seeds sometimes surrounded by pulp. | Vachellia |
10. Stipular spines absent; ovaries stipitate; seeds not surrounded by pulp. | → 11 |
11. Petiolar glands absent; prickles absent; inflorescences usually in globose heads; corollas greenish white, drying to pink-rose; stamens 175–250, creamy white, anther glands absent. | Acaciella |
11. Petiolar glands usually present; prickles present or absent; inflorescences spikes or heads; corollas white to creamy white or yellow; stamens 35–160, white, small anther glands sometimes present. | → 12 |
12. Prickles usually present; stipules caducous; inflorescences terminal or axillary, heads or spikes, usually in pseudoracemes or pseudopanicles; ovaries stipitate or sessile, with nectariferous disc at base; stamens creamy or yellow, fading to reddish brown. | Senegalia |
12. Prickles absent; stipules persistent; inflorescences axillary, cylindrical spikes; ovaries short-stipitate, nectariferous disc absent; stamens white. | Mariosousa |
8. Filaments fused into a tube, monadelphous; leaves bipinnate. | → 13 |
13. Legumes usually dehiscent into 2 valves, sometimes craspedial; nectary glands present or absent. | → 14 |
14. Trees, shrubs, or herbs, unarmed; stipular spines absent. | → 15 |
15. Petiole nectary glands present; inflorescences axillary spikes or racemes; legumes straight, flat, membranous, with 2 prominent suture ribs surrounding periphery and detaching after dehiscence (craspedial); exocarp dark purple to blackish, exfoliating. | Lysiloma |
15. Petiole nectary glands absent; inflorescences axillary or terminal heads sometimes in short pseudopanicles; legumes straight or slightly curved, dehiscent longitudinally along sutures, valves strongly recurving; exocarp dark brown to green, not exfoliating. | → 16 |
16. Heads globose; corollas greenish white; stamens 30–60; legumes dehiscing elastically from distal end to proximal end. | Zapoteca |
16. Heads ± hemispheric or obconic; corollas reddish or whitish; stamens (14–)19–28; legumes dehiscing longitudinally along sutures but not elastically. | Calliandra |
14. Shrubs or trees, usually armed; stipular spines present. | → 17 |
17. Legumes recurved to coiled into a circle; seeds with aril; petiolar nectary gland between lowest pair of pinnae; leaflet blade venation pinnate. | Pithecellobium |
17. Legumes straight; seeds without an aril; petiolar nectary gland borne between distalmost pair(s) of pinnae; leaflet blade venation brochidodromous. | Havardia |
13. Legumes indehiscent, dehiscent, or late dehiscent; nectary glands present. | → 18 |
18. Legumes falcate or forming a nearly complete flattened spiral, valves woody, dull light brown to maroon or blackish; inflorescences racemes, pseudoracemes, or heads (fascicles) or flowers solitary. | → 19 |
19. Pinnae 2 or 3 pairs; inflorescences pseudoracemes from short shoots; legumes falcate, light brown to maroon. | Ebenopsis |
19. Pinnae 4–10(–15) pairs; inflorescences heads; legumes forming a nearly complete flattened spiral, blackish. | Enterolobium |
18. Legumes straight or slight curved, valves papery, membranous, leathery, or if woody, glossy red-brown; inflorescences umbels, spikes, racemes, or heads. | → 20 |
20. Legumes turgid, fleshy, pulpy, margins thickened, septate between seeds; flowers dimorphic, peripheral flowers smaller than central; introduced, Florida. | Samanea |
20. Legumes flat, not fleshy or pulpy, not septate between seeds; flowers sometimes dimorphic; Florida (Lysiloma), California (Paraserianthes), or widespread (Albizia). | → 21 |
21. Legumes with prominent sutural ribs surrounding periphery and persistent, not separating from the valves (or tardily), seeds released through decay of valves; Florida. | Lysiloma |
21. Legumes without prominent peripheral ribs, raised over seeds; widespread. | → 22 |
22. Pinnae 20–30+ pairs; inflorescences axillary, racemes, flowers homomorphic, corollas greenish; stamens ± 90, equaling petals, greenish to lemon yellow; legumes indehiscent or late-dehiscent; introduced, California. | Paraserianthes |
22. Pinnae (1 or)2–12 pairs; inflorescences axillary or terminal, heads or corymbs; flowers dimorphic, corollas whitish or pink to yellow-green; stamens 20–70, usually long-exserted, pink or white; legumes dehiscent or late-dehiscent; widespread. | Albizia |
1. Extrafloral nectaries absent from leaves. | → 2 |
2. Stipules persistent; inflorescences spikelike racemes, often apparently terminal, bearing flowers in conelike head; bracts conspicuous, covering buds and caducous as pedicel elongates, 10+ mm; corollas monosymmetric, lower and upper petals similar in shape; shrubs, rarely arborescent shrubs. | S. alata |
2. Stipules caducous or persistent; inflorescences not spikelike racemes; bracts early caducous, to 5 mm; corollas highly asymmetric, shape of 1 or both lower petals highly modified, flag-shaped; shrubs or trees. | → 3 |
3. Leaves 8.5–28.5 cm, leaflet pairs 2–5. | S. atomaria |
3. Leaves 1.2–3.7 cm, leaflet pairs 3 or 4. | S. wislizeni |
1. Extrafloral nectaries present on leaves at petiole base, or between 1 or more leaflet pairs, or along dorsal (upper) margin of phyllodes, where putative leaflets should be inserted. | → 4 |
4. Leaves sclerophyllous or xerophytic, usually modified as phyllodes, rarely absent; shrubs. | → 5 |
5. Stamens 7; staminodes 3; flowers monosymmetric; leaves often irregularly present or absent on same plant; branch tips often acuminate. | S. armata |
5. Stamens 10; staminodes 0; flowers slightly asymmetric, enantiostylous; leaves always present; branch tips not acuminate. | S. artemisioides |
4. Leaves mesophyllous or slightly sclerophyllous, not modified as phyllodes; herbs, shrubs, or trees. | → 6 |
6. Extrafloral nectaries usually at base of or along petiole or both, rarely also with nectary between leaflet pairs; leaves mesophyllous, leaflet pairs (2–)4–10. | → 7 |
7. Anthers of abaxial stamens elongated beyond pores, apical appendage conspicuous and linguiform, thickened; extrafloral nectaries usually at base of petiole only (sometimes along petiole or leaflet pairs in S. ligustrina). | → 8 |
8. Bracts longer than bud, often blackish green; leaflets pairs 4 or 5(or 6); branches dark green and blackish. | S. occidentalis |
8. Bracts equal to or shorter than bud, green; leaflet pairs 4–8; branches green. | → 9 |
9. Legumes 120–230 × 2–5 mm; extrafloral nectaries globose egg-shaped; Arizona, New Mexico. | S. hirsuta |
9. Legumes 70–140 × 5–9 mm; extrafloral nectaries globose-flattened; Florida. | S. ligustrina |
7. Anthers of abaxial stamens truncate, apical appendage inconspicuous or absent; extrafloral nectaries at base of or along petiole, sometimes between first leaflet pair. | → 10 |
10. Styles apically dilated; extrafloral nectaries usually between first leaflet pair, sometimes on petiole near first pair. | S. mexicana |
10. Styles not apically dilated; extrafloral nectaries at base of or along petiole. | → 11 |
11. Ovaries densely hairy; ovules 10–16; racemes to 25–37-flowered; legumes 5.5–8 mm wide, each seed compartment nearly as wide as long. | S. hebecarpa |
11. Ovaries glabrous or slightly hairy; ovules 20–28; racemes usually 5–15-flowered; legumes 7.5–11 mm wide, each seed compartment wider than long. | S. marilandica |
6. Extrafloral nectaries not at base or along petiole, usually between leaflet pair; leaves mesophyllous or slightly sclerophyllous, leaflet pairs 1–10. | → 12 |
12. Stamens 10, staminodes 0; androecium heterantherous. | S. surattensis |
12. Stamens 7, staminodes 3; androecium heterantherous or not. | → 13 |
13. Stamens heterantherous, middle stamens 1/2 as long as abaxial or smaller; legumes indehiscent, usually pendulous (erect or curved downward in S. obtusifolia); herbs, shrubs, or trees, to 1.2–5(–6) m; leaves usually glabrous or slightly hairy, rarely densely hairy. | → 14 |
14. Leaflet pairs 3. | → 15 |
15. Shrubs or trees, to 3.5 m; leaflet blades more than 3 times as long as wide; racemes 4–18-flowered; petals same shape and size, monosymmetric. | S. corymbosa |
15. Herbs, to 1.5(–2.4) m; leaflet blades usually less than 3 times as long as wide; racemes 1 or 2(or 3)-flowered; petals: 1 lower petal conspicuously larger, asymmetric. | S. obtusifolia |
14. Leaflet pairs 4–9. | → 16 |
16. Leaflet pairs 5–9; extrafloral nectaries between many or all leaflet airs. | S. multiglandulosa |
16. Leaflet pairs 4 or 5(–7); extrafloral nectaries between first leaflet pair, rarely also second pair. | S. pendula |
13. Stamens usually not heterantherous, lengths about equal, 3 abaxial ones slightly longer (except heterantherous in S. lindheimeriana and S. orcuttii); legumes usually dehiscent, erect; herbs, usually perennial (biennial in S. durangensis), to 1.5 m; leaves hairy, often densely. | → 17 |
17. Leaflet pairs 2–8. | → 18 |
18. Leaflet pairs 2–4; stipules to 1 mm wide; racemes (2–)4–8-flowered. | S. covesii |
18. Leaflet pairs (3 or)4–8; stipules 1–3 mm wide; racemes usually 5–25-flowered. | → 19 |
19. Leaflets pallid green; petals to 10.5–16 mm; legumes 5–9 mm wide. | S. lindheimeriana |
19. Leaflets dull glaucescent; petals to 8–10.5 mm; legumes 3.5–6.5 mm wide. | S. orcuttii |
17. Leaflet pairs 1. | → 20 |
20. Leaflet blades broadly obovate or oblong-obovate, length usually to 1.8 times width. | → 21 |
21. Calyces caducous; ovules 38–44; legumes with 2 series of seeds. | S. durangensis |
21. Calyces persistent into developing fruit; ovules 16–26; legumes with 1 series of seeds | S. pilosior |
20. Leaflet blades lanceolate, oblanceolate, oblong, oblong-elliptic, obovate-elliptic or ovate-oblong, length often more than 2 times width. | → 22 |
22. Calyces often persistent into developing fruit; ovules 6–12; leaflet blades 2–5 mm wide; herbs to 0.2 m. | S. pumilio |
22. Calyces caducous; ovules 18–40; leaflet blades (3–)4–16 mm wide; herbs to 0.2–0.7 m. | → 23 |
23. Leaflet blades lanceolate or lanceolate-oblong, length usually more than 4 times width. | S. roemeriana |
23. Leaflet blades oblong, ovate-oblong, oblong-elliptic, or obovate-elliptic, length usually less than 4 times width. | → 24 |
24. Styles to 1.5 mm, distally dilated; racemes 1–3-flowered. | S. bauhinioides |
24. Styles 3–3.5 mm, linear; racemes 1(or 2)-flowered. | S. ripleyana |
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