Senna
Senna alexandrina
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Angiosperms
Clade:	Eudicots
Clade:	Rosids
Order:	Fabales
Family:	Fabaceae
Subfamily:	Caesalpinioideae
Tribe:	Cassieae
Genus:	Senna Mill.
Type species
Senna alexandrina Mill.
Species
Over 300; see § Species
Synonyms[1]
List Adipera Raf. 1838 Cassia (Mill.) Benth. 1871 Cassia sect. Senna (Tournefort) DC. ex Colladon 1816 Cassia sect. Senna sensu Benth. 1865 Cassia subgen. Senna Benth. 1870 Cathartocarpus (partim) Chamaefistula (DC.) G. Don Chamaesenna (DC.) Raf. ex Pittier Desmodiocassia Britton & Rose Diallobus Earleocassia Britton Echinocassia Britton & Rose Gaumerocassia Britton Herpetica (DC.) Raf. Isandrina Leonocassia Britton Palmerocassia Britton Peiranisia Raf. Phragmocassia Britton & Rose Pseudocassia Britton & Rose Pterocassia Britton & Rose Sciacassia Britton Senna sensu Gaertn. 1791 Senna sensu Link 1831 Senna sensu Roxb. 1832 Sericeocassia Britton Tharpia Britton & Rose Vogelocassia Britton Xerocassia Britton & Rose

Senna, the sennas, is a large genus of flowering plants in the legume family (Fabaceae, subfamily Caesalpinioideae,^[2] tribe Cassieae ser. Aphyllae ^[3]). This diverse genus is native throughout the tropics, with a small number of species in temperate regions. The number of species is estimated to be from about 260^[4] to 350.^[5] The type species for the genus is Senna alexandrina. About 50 species of Senna are known in cultivation.^[6]

Description

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]}

Systematics

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.

Etymology

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

Species

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

Ecology

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]

Uses

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]

Fossil record

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]

References