Senna (plant)

Plants in the genus Senna are shrubs, sometimes small trees or perennial herbs. The leaves are arranged alternately, paripinnate with up to 25 pairs of leaflets, each leaf with a stipule at the base, but that often falls off as the leaves mature. The flowers are arranged in racemes in leaf axils with bracts at the base. The flower has five egg-shaped green or yellowish sepals and five usually yellow, usually glabrous petals. There are usually ten stamens that are free from each other, the filaments of different lengths, and some are staminodes. The fruit is a leathery pod containing several seeds.[7][8][9][10]

Chamaecrista, Cassia, and Senna form a monophyletic group which some authors have called Cassia sensu lato.[11] In 1982, the group was named Cassiinae and classified as a subtribe of the tribe Cassieae.[12] The tribe Cassieae contains 21 genera and is now known to be polyphyletic,[11] but the classification is still accepted because a revision of Fabaceae has yet to be published.[11]

The genus Senna has had a complex taxonomic history.[13] What is now known as Senna was included by Linnaeus in his concept of Cassia in Species Plantarum in 1753.[14] Philip Miller segregated Senna from Cassia in 1754 in the fourth edition of The Gardeners Dictionary.[15] Until 1982, many authors, following Linnaeus, did not recognize Senna and Chamaecrista, but included them in a broadly circumscribed Cassia sensu lato. Phylogenetic analyses of DNA have shown that Chamaecrista, Cassia, and Senna are all monophyletic, but the relationships between these three genera have not been resolved.[4] They are therefore shown in phylogenetic trees as a tritomy.

The genus name derives from the Arabic sanā, describing plants whose leaves and pods have cathartic and laxative properties.[16]

As of June 2023, Plants of the World Online accepted the following species:[17]

Senna alexandrina

Senna artemisioides

Senna bicapsularis

Senna didymobotrya

Senna hirsuta

Senna macranthera

Senna multiglandulosa

Senna occidentalis

Senna pendula

Senna spectabilis

Senna surattensis

Senna odorata

The caterpillars of many species feed on Senna plants. The black witch (Ascalapha odorata), two-barred flasher (Astraptes fulgerator), common emigrant (Catopsilia pomona), and mottled emigrant (C. pyranthe) have all been recorded on candle bush (S. alata), for example.

Some species have extrafloral nectaries on the leaves or flower stalks that are visited by ants.[4]

Some Senna species are used as ornamental plants in landscaping. The genus is adapted to many climate types.

Cassia gum, an extract of the seeds of Chinese senna (S. obtusifolia), is used as a thickening agent. The leaves and flowers of Siamese cassia (S. siamea) are used in some Southeast Asian cuisines, such as Thai, Shan/Burmese and Lao cuisines. They are known as khi-lek in Thai, and are used in curries.[20]

A fossil seed pod of Senna sp. from the middle Eocene epoch has been described from the Rancho clay pit in Henry County, Tennessee (United States).[28]