Senna is a large genus of flowering plants in the legume family Fabaceae, placed in the subfamily Caesalpinioideae and tribe Cassieae. The genus comprises over 300 species — estimates range from about 260 to 350 — encompassing shrubs, small trees, and perennial herbs distributed across tropical and subtropical regions worldwide, with a handful of species extending into temperate zones.
Members of Senna share a recognisable floral structure: five usually yellow petals (occasionally cream or pale orange) and five egg-shaped green or yellowish sepals arranged in racemes. Leaves are alternately arranged and pinnately compound, with up to 25 pairs of leaflets. The fruit is a leathery pod containing several seeds. Many species bear extrafloral nectaries on their leaves or flower stalks, which attract ants and form ecologically important associations.
Pollination in the genus is specialised. Flowers are heterantherous, bearing distinct feeding stamens and pollinating stamens. The predominant mechanism is buzz pollination, performed mainly by large female bees of the genus Xylocopa, which vibrate at the precise frequency needed to release pollen. Some species additionally employ ricochet pollination, where pollen bounces off petals onto the bee's body. Senna plants also serve as larval hosts for numerous butterfly and moth species, including the black witch, two-barred flasher, and emigrant butterflies.
The genus is economically and culturally significant. Senna alexandrina has been used as a medicinal laxative for centuries, stimulating bowel contractions, and remains an ingredient in modern pharmaceutical preparations. Cassia gum, derived from Senna obtusifolia seeds, is approved as a food thickening agent. Senna siamea leaves are eaten in Southeast Asian cuisines. Approximately 50 species are cultivated as ornamentals, valued for their prolific yellow flowers and adaptability to warm climates.
Etymology
The genus name Senna traces to the Arabic word sanā, which historically described plants valued for their cathartic and laxative properties. The Arabic term entered medieval Latin pharmacopoeia and was adopted into European botanical literature before Philip Miller formally applied it as the accepted genus name in 1754. Prior to that, most species now placed in Senna were grouped under the broader Linnaean genus Cassia, and vernacular names in several languages still reflect this older arrangement.
Distribution
Senna is predominantly a genus of the tropics, native across Africa, the Americas, Asia, and Australasia, with its greatest diversity in the Neotropics — particularly South America and Central America. A small number of species extend into warm-temperate regions. The genus is characteristic of disturbed habitats, open woodland, savanna, and secondary vegetation, and several species have become naturalised well beyond their native ranges through cultivation and accidental introduction.
Ecology
Senna species occupy a distinctive ecological role through their specialised pollination systems. Flowers are structurally heterantherous, bearing separate feeding stamens and pollinating stamens, a trait that rewards visiting bees while ensuring pollen transfer. The primary pollinators are large female bees of the genus Xylocopa (carpenter bees), which perform buzz pollination by gripping the anthers and vibrating their flight muscles at a resonant frequency that expels pollen. Some species additionally show ricochet pollination, in which pollen grains bounce from petals onto the pollinator's body.
Many Senna species maintain extrafloral nectaries on leaves or flower stalks, secreting sugars that recruit ants as indirect protectors against herbivores. The genus also functions as a larval host plant for numerous Lepidoptera, including the black witch moth, the two-barred flasher, and several emigrant butterflies (Catopsilia spp.).
Cultivation
Approximately 50 Senna species are cultivated as ornamentals, prized for their bright yellow flowers and tolerance of hot, sunny conditions. They are used in tropical and subtropical landscaping as specimen shrubs, small trees, or screening plants. Most cultivated species prefer well-drained soils and full sun, and many are drought-tolerant once established. Several species — including Senna alata (candle bush) and Senna macranthera — are grown in temperate gardens as large annuals or tender perennials where winter frosts occur.
Cultural & Economic Uses
Senna has a long history of medicinal and culinary use. The most pharmacologically important species is Senna alexandrina (alexandrina senna or tinnevelly senna), whose dried leaves and pods have been used as a stimulant laxative since at least medieval times. The active compounds, sennosides A and B, stimulate peristalsis in the large intestine and are incorporated into modern over-the-counter laxative preparations.
Cassia gum, produced from the endosperm of Senna obtusifolia seeds, is used commercially as a food thickening and gelling agent. In Southeast Asia, the young leaves and shoots of Senna siamea (Siamese cassia, Thai: khi-lek) are cooked as a vegetable or used in curries. Various Senna species have been used in traditional medicine across tropical regions for treating skin diseases, fungal infections, and fever, though the efficacy and safety of many such uses have not been formally validated.
Taxonomy
Senna was formally established by Philip Miller in 1754, published in Gardeners Dictionary Abridged, 4th edition. Miller segregated it from Cassia sensu Linnaeus on the basis of morphological and chemical distinctions. The type species is Senna alexandrina Mill.
Molecular phylogenetic studies have confirmed that Senna, together with Chamaecrista and Cassia, forms a monophyletic group within tribe Cassieae (subfamily Caesalpinioideae, family Fabaceae). However, the precise internal relationships among these three genera remain incompletely resolved. GBIF records 562 descendant taxa under the accepted genus concept. The genus has historically been treated under varying circumscriptions, and the vernacular names "cassia" and "senna" are still used interchangeably in commerce and horticulture.
