Senna (plant)

Senna includes herbs, shrubs, and trees. The leaves are pinnate with opposite paired leaflets. The inflorescences are racemes at the ends of branches or emerging from the leaf axils. The flower has five sepals and five usually yellow petals. There are ten straight stamens. The stamens may be different sizes, and some are staminodes. The fruit is a legume pod containing several seeds.[6]

Chamaecrista, Cassia, and Senna form a monophyletic group which some authors have called Cassia sensu lato.[7] In 1982, the group was named Cassiinae and classified as a subtribe of the tribe Cassieae.[8] The tribe Cassieae contains 21 genera and is now known to be polyphyletic,[7] but the classification is still accepted because a revision of Fabaceae has yet to be published.[7]

The genus Senna has had a complex taxonomic history.[9] What is now known as Senna was included by Linnaeus in his concept of Cassia in Species Plantarum in 1753.[10] Philip Miller segregated Senna from Cassia in 1754 in the fourth edition of The Gardeners Dictionary.[11] 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.[3] 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 catharitic and laxative properties.[12]

The caterpillars of many Lepidoptera 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.

Senna species are pollinated by a variety of bees, especially large female bees in genera such as Xylocopa.[3] They rely on "buzz pollination" and some within that on "ricochet pollination", which is a secondary pollen presentation where the pollen is not deposited on the pollinator's body by direct contact with the anthers.[13] The flowers have two sets of stamen: feeding stamens, which are longer, and pollinating stamens, which are smaller in size.[14] Due to buzz pollination, the pollens from the pollinating stamens get thrown from the anthers and ricochets against the petals multiple times before it settles on the dorsal side of the pollinating bee. The roughness on the petal walls causes the pollen to slow down its speed. The ricocheting effect alone cannot ensure effective pollen dissemination. It is aided by static charges wherein the flying bees become positively charged owing to the friction in the air and the pollen becomes negatively charged because of which they naturally get attracted to the bees body. The pollinator bee ends up carrying the pollen and also gets to feed on the pollen which is on the feeding stamens.[13] Some species also have extrafloral nectaries on the leaves or flower stalks, which attract ants, but do not benefit pollinators.[3]

Some Senna species are used as ornamental plants in landscaping. The species 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.[15]

As of December 2019, Plants of the World Online accepts the following species:[22][23][24]

Senna alexandrina

Senna artemisioides

Senna bicapsularis

Senna didymobotrya

Senna hirsuta

Senna lindheimeriana

Senna macranthera

Senna multiglandulosa

Senna occidentalis

Senna pendula

Senna pinheiroi

Senna spectabilis

Senna surattensis

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).[29]