Overview

Senna is a large genus of flowering plants in the legume family. It belongs to the broad grouping often called a genus (genus overview) of flowering plants and is placed within the family Fabaceae (Fabaceae, subfamily Caesalpinioideae, tribe Cassieae). Species of Senna occur widely; they are primarily native to the tropics with a smaller number reaching temperate regions. Estimates of total species vary, typically put between about 260 and 350 (species estimates); the type species is Senna alexandrina.

Characteristics

Senna species range from herbs to shrubs and small trees. Common features include pinnate leaves, yellow or sometimes white flowers with 4–6 petals, and elongated pods containing several seeds. As members of the legume family, many fix nitrogen in association with soil bacteria, which can improve poor soils.

Taxonomy and history

The circumscription of Senna has changed over time. Historically many species were classified in the broadly defined genus Cassia; modern taxonomic revisions split that group into Senna, Cassia (strict sense) and Chamaecrista based on floral and genetic differences. The genus has been important in regional traditional medicine and in botanical trade for centuries.

Uses and cultivation

Several Senna species are cultivated for different reasons:

  • Medicinal use: Some species' leaves and pods contain compounds used as stimulant laxatives; these preparations are used short-term in herbal medicine but should be used cautiously.
  • Ornamental horticulture: Brightly flowering species are grown in gardens and as street trees in warm climates.
  • Soil improvement: As legumes many species contribute to nitrogen enrichment and erosion control.

Safety, ecology and notable facts

Active compounds in medicinal sennas are anthraquinone glycosides (commonly called sennosides) that stimulate bowel activity; prolonged or excessive use can cause adverse effects and is not recommended during pregnancy. Ecologically, some species are valuable for pollinators, while others can naturalize outside their native range. About fifty species are commonly in cultivation for one or more purposes, and botanists continue to describe and reclassify taxa as molecular studies refine relationships.

For general reference and further reading, see botanical treatments and regional floras (genus overview, flowering plants, Fabaceae resources) or specialized accounts on species such as Senna alexandrina.