Mimosa is a large genus of herbs and shrubs in the legume family Fabaceae, placed in the subfamily Caesalpinioideae and the mimosoid clade. The genus was described by Carl Linnaeus in 1753 and now contains roughly 600 species. Its members are most easily recognised by their pinnate or bipinnate, often delicate-looking leaves and by their flowers, which are gathered into dense globular or cylindrical heads that read at a glance as fluffy pom-poms. Individual flowers are small, with a tiny calyx, a five-lobed funnelform corolla and ten or fewer stamens — a count that distinguishes Mimosa from the closely related Acacia and Albizia, both of which have many more stamens per flower.
Most species are sprawling perennial herbs or shrubs, and a large fraction are armed with stout, hooked prickles. Flower colour across the genus is dominated by pinks, roses and lavenders, with the rose-purple heads of the well-known sensitive plant (Mimosa pudica) and its New World relatives being typical. Mimosa is centred in the New World — nearly 600 species occur there — but also extends naturally to eastern Africa (Tanzania, Mozambique, Madagascar), the Indian subcontinent and Indochina. In the Americas its range stretches from North Dakota south to northern Argentina.
The genus is famous for what its leaves do. Mimosa is one of the few plant lineages capable of rapid movement: brush a leaf of Mimosa pudica and the leaflets fold within seconds, driven by changes in turgor pressure at specialised joints called pulvini and propagated by action potentials reminiscent of animal nerve impulses. Many species also raise their leaves during the day and lower them at night, and an early 18th-century experiment by Jean-Jacques d'Ortous de Mairan on one such Mimosa furnished the first evidence that living organisms keep their own internal clocks.
In horticulture and everyday English the name "mimosa" is frequently and confusingly applied to other yellow-flowered legumes — notably Acacia dealbata and Albizia julibrissin — but only species of the true genus Mimosa belong here. The genus includes both prized curiosities, such as the sensitive plant grown worldwide as a children's houseplant, and some of the world's most aggressive invasive weeds, such as the thorny floodplain shrub Mimosa pigra.
Etymology
The genus name Mimosa comes from the Greek mimos, meaning "actor" or "mime", combined with the feminine adjectival suffix -osa, meaning "resembling". The name was meant to evoke the way sensitive leaves of plants such as Mimosa pudica appear to "mimic conscious life" by reacting to touch — folding, drooping and re-opening as if performing.
Distribution
Mimosa is overwhelmingly a New World genus, with nearly 600 species native to the Americas. Its range there extends from North Dakota in the north to northern Argentina in the south, with major centres of diversity in the Neotropics. A smaller but still significant native presence reaches eastern Africa (Tanzania, Mozambique, Madagascar), the Indian subcontinent and Indochina. In the southwestern United States, characteristic native species include Mimosa aculeaticarpa, M. biuncifera, M. borealis and M. berlandieri.
Several species have travelled far beyond these native ranges. Mimosa pudica, originally from the Caribbean and tropical Americas, is now naturalised across tropical Asia, Africa, Australia and the Pacific, and the thorny wetland shrub Mimosa pigra — native from Mexico to northern Argentina — has reached Southeast Asia (Cambodia, Indonesia, Malaysia, Thailand, Vietnam, Sri Lanka) and much of sub-Saharan Africa (Ghana, Kenya, Tanzania, Uganda, South Africa).
Ecology
Mimosa is one of the best-known plant genera for rapid, reversible leaf movement. In Mimosa pudica and several relatives, touching, warming, blowing on or shaking the leaves causes leaflets to fold inward within seconds. The movement is driven by sudden changes in turgor pressure at specialised joints called pulvini, and it propagates along the leaf via action potentials that are functionally similar to nerve impulses in animals. Many Mimosa species also show daily nyctinasty — raising leaves during the day and lowering them at night — and a Mimosa was the subject of Jean-Jacques d'Ortous de Mairan's 1729 experiment that first demonstrated endogenous biological clocks.
The same rapid leaf movement is increasingly studied as an example of plant learning: Mimosa pudica habituates to repeated, harmless stimuli and stops folding, yet remains responsive to novel disturbances.
Several Mimosa species are also notable colonisers. Mimosa pigra produces seeds that float on water, cling to clothing and animal fur, and remain viable for 23 years or more; the species flowers within 4–12 months of germination and can produce mature seed in about five weeks, traits that underpin its capacity to form dense, persistent thickets in wet habitats. Mimosa pudica is similarly fast-cycling and can behave as a weed of hand-cultivated tropical crops, including corn, coconuts, coffee, bananas and sugarcane.
Conservation
The conservation profile of Mimosa is dominated not by threatened species but by the global spread of a handful of aggressive invaders. Mimosa pigra is listed among the world's 100 worst invasive species: in Australia's Northern Territory it is the tenth most problematic declared noxious weed and infests roughly 80,000 hectares of coastal floodplain, forming impenetrable thorny thickets that exclude native vegetation and degrade wetlands. Beyond Australia it is established as a serious weed across much of Southeast Asia (Cambodia, Indonesia, Malaysia, Thailand, Vietnam, Sri Lanka) and sub-Saharan Africa (Ghana, Kenya, Tanzania, Uganda, South Africa), spreading along rivers and in transported sand.
Mimosa pudica, while ornamental and frequently kept as a curiosity, is itself regarded as an invasive weed in Tanzania, parts of Southeast Asia and the Pacific, and is declared problematic in parts of Australia.
Cultural uses
In temperate regions, members of Mimosa are grown chiefly as botanical curiosities — Mimosa pudica is widely sold as a houseplant for its dramatic touch response, and across the tropics it is also grown outdoors. The species is so distinctive that its common names — sensitive plant, touch-me-not, shy plant, shame plant, action plant — all describe the same trait in different idioms.
Several New World Mimosa species carry deeper ethnobotanical and medicinal traditions. Aqueous extracts of Mimosa are widely used in Central and South America for wound healing and burns, and have entered commercial skincare products. Mimosa tenuiflora is particularly important: its root bark contains roughly 1–1.7% dimethyltryptamine (DMT), and it is the central ingredient in the psychoactive decoction known as Jurema (or Yurema) in the Jurema religious tradition of northeastern Brazil. The same species is used in traditional medicine — tea from its leaves and stems for tooth pain, water extracts of its bark for cough and bronchitis.
History
A Mimosa species was at the centre of a landmark experiment in chronobiology. In 1729 the French astronomer Jean-Jacques d'Ortous de Mairan observed that a Mimosa's leaves continued to open and close on a roughly 24-hour rhythm even when the plant was kept in constant darkness — the first experimental evidence that living organisms maintain their own internal biological clocks rather than simply reacting to daylight.
Taxonomy notes
Mimosa was established by Carl Linnaeus in 1753 and is the type genus of the mimosoid clade within Fabaceae, subfamily Caesalpinioideae. GBIF treats Mimosa L. as accepted, with "Sensitive Plant" as a common name. The genus is most reliably distinguished from the closely related Acacia and Albizia by its small flowers having ten or fewer stamens; Acacia and Albizia have many more. Despite this, the common name "mimosa" in horticulture and everyday speech is regularly misapplied to other yellow-flowered legumes — especially Acacia dealbata and Albizia julibrissin — that share Mimosa's feathery bipinnate foliage but do not belong to the genus.
