Herbs, annual, biennial, or perennial, shrubs, or trees, unarmed; roots without bacterial nodules. Stems erect, glabrous or pubescent. Leaves 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. Inflorescences 1–40+-flowered, axillary, racemes, sometimes aggregated into compound racemes, erect; bracts present, usually early or late caducous, rarely persistent at anthesis; bracteoles absent. Flowers 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. Fruits legumes, stipitate, flat to cylindrical, straight or curved [spiral], often corrugated over seeds, indehiscent or partly dehiscent, glabrous or pubescent. Seeds 10–20, ovoid, obovoid, ellipsoid to rhomboid, or pyriform, usually areolate. x = 12, 13, 14.

Distribution

North America, Mexico, West Indies, Bermuda, Central America, South America, Australia, introduced in Asia, Africa, Atlantic Islands, Indian Ocean Islands, Pacific Islands.

Discussion

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

Selected References

None.

Lower Taxa

Senna alata, Senna armata, Senna artemisioides, Senna atomaria, Senna bauhinioides, Senna corymbosa, Senna covesii, Senna durangensis, Senna hebecarpa, Senna hirsuta, Senna ligustrina, Senna lindheimeriana, Senna marilandica, Senna mexicana, Senna multiglandulosa, Senna obtusifolia, Senna occidentalis, Senna orcuttii, Senna pendula, Senna pilosior, Senna pumilio, Senna ripleyana, Senna roemeriana, Senna surattensis, Senna wislizeni

Key

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.	Senna 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.	Senna atomaria
3	Leaves 1.2–3.7 cm, leaflet pairs 3 or 4.	Senna 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.	Senna armata
5	Stamens 10; staminodes 0; flowers slightly asymmetric, enantiostylous; leaves always present; branch tips not acuminate.	Senna 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.	Senna 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.	Senna hirsuta
9	Legumes 70–140 × 5–9 mm; extrafloral nectaries globose-flattened; Florida.	Senna 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.	Senna 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.	Senna 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.	Senna 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.	Senna 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.	Senna 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.	Senna obtusifolia
14	Leaflet pairs 4–9.	> 16
16	Leaflet pairs 5–9; extrafloral nectaries between many or all leaflet airs.	Senna multiglandulosa
16	Leaflet pairs 4 or 5(–7); extrafloral nectaries between first leaflet pair, rarely also second pair.	Senna 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.	Senna 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.	Senna lindheimeriana
19	Leaflets dull glaucescent; petals to 8–10.5 mm; legumes 3.5–6.5 mm wide.	Senna 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.	Senna durangensis
21	Calyces persistent into developing fruit; ovules 16–26; legumes with 1 series of seeds	Senna 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.	Senna 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.	Senna 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.	Senna bauhinioides
24	Styles 3–3.5 mm, linear; racemes 1(or 2)-flowered.	Senna ripleyana