Mimosas all round? No, definitely not the alcoholic beverage! The mimosa I’m talking about is Mimosa pudica L. [Fabaceae], a creeping herb that’s continuing to fascinate the world – both botanists and non-botanists alike!
Mix one part of thoroughly chilled mimicry with one part of sparkling brilliance, sit back, and watch the leaflets fold…
M. pudica was described by Carl Linnaeus, and is the type species for the genus. The generic name Mimosa is derived from the Greek word mimos (meaning mimic), while the specific epithet comes from the Latin word pudicus (meaning bashful or shrinking to contact). Some may refer to this species as the “sensitive plant” or even as the “shameful plant” because of its very unique and characteristic touch-sensitive leaves. In fact, the species has a huge range of common names in many languages, some of which include:
English: sensitive plant, touch-me-not plant, humble plant, shameful plant
French: amourette herbe, herbe sensible, sensitive épineus, mimosa pudique
Spanish: vergonsoza, dormidera, ten vergüenza
Nahuatl, Mexico: pinahuixtle, quecupatli
Hispaniola: mori vivi
Chinese: hánxiū cǎo (shyness grass)
Sinhala, Sri Lanka: Nidi kumba
M. pudica is widespread in Central America, northern South America and the Caribbean. This taxon has also been introduced to many countries around the world and is not considered to be threatened or in decline. Surprisingly, however, it is included in the Global Invasive Species Database as one of the world’s worst invasive weeds – affecting forestry plantations, croplands, orchards and pastures throughout South Asia, the Pacific Islands and in parts of Africa. The plant is also poisonous to ruminant grazers through the bacterial breakdown of the non-protein amino acid mimosine.
Management of the species is best practised before the plants establish larger populations. Hand-weeding of mature plants is difficult because the plant bears prickles, which can break off and irritate the skin. Herbicides such as foliar sprays of glyphosate are widely used to control the species, but thorough wetting of all leaf surfaces is essential; if plants are disturbed before spraying, the leaves will fold up and the herbicide will be ineffective. Fire can also be used as a management tool, but fire alone may actually increase M. pudica densities by plant regrowth and enhanced seed germination.
Although no work has been undertaken on the biological control of M. pudica, there may be possibilities in view of the successful biological control programmes against the related M. invisa.
M. pudica is an annual or perennial herb, normally growing to 50-70 cm tall, often as a straggling subshrub. Its stems have sparse prickles, around 2-2.5 mm long. The leaves are alternately arranged and bipinnate (twice compound). The pinnae are digitate (shaped like a spread hand), and bear 10-26 pairs of linear-oblong leaflets on each pinna. Stipules are persistent (remaining in place), lanceolate in shape and striately-nerved from the base.
The flowers are lilac or pink (the colour is mainly of the stamen filaments) and are held in ovoid, stalked heads (characteristic of the subfamily Mimosoideae). The calyx is minute, about 0.2 mm long, whereas the corolla is larger (2-2.3 mm long) and containing four stamens. The pods are 1.8 cm × 3-5 mm, densely bristly with brown-tipped tapering hairs along their margins.
Introduction pathways to new locations are via floating seed pods and local dispersal methods on animals (attaching to fur). Once germinated, seedlings grow slowly for two or three months and then accelerate, reaching 0.5-2 m of extension at the end of the first year!
Despite its invasive properties, the sensitive plant is popular in cultivation around the world, and is enjoyed by many as a curiosity due to its highly touch-sensitive leaves. The famous microscopist Robert Hooke was one of the first to investigate the movements of M. pudica, and at that time it had been suggested that plants had nerves and tissues similar to those in animals. But plants don’t have a nervous system or muscles, so how is the rapid movement generated?
Well, it was only later discovered that the leaves fold as a result of the internal movement of water, and the mechanics of the touch-sensitive (thigmonastic) process are now well-documented. These are centred around a motor structure (called the pulvinus), consisting of a rod of sclerenchyma and collenchyma cells (strengthening tissue).
In the extended, unstimulated position, the cells of the entire collenchyma are turgid (distended with water) and the leaflets are held outwards. On receiving the action potential signal through a touch stimulus, the cells in the pulvinus respond by expelling potassium (K+) and chloride ions (Cl–), and taking up calcium ions (Ca2+). The resultant osmotic gradient draws water out of these cells, causing them to shrink temporarily (plasmolysis) and lose structural rigidity; in this contracted position, the pulvinus no longer functions as a support and the petiolules and petioles droop – leaving the plant in its folded state.
There’s no better way to understand this remarkable thigmonastic process than to watch it in action… “Stop, hammer* time!”
* No plants were harmed in the making of this video.
This rapid plant movement is thought to act as a defence against herbivores, which may be deterred by the dramatic response or, if they are small, may be dislodged as the leaves collapse.
M. pudica is not the only member of the Fabaceae family to show thigmonastic movements. Other legumes (e.g. some members of the genera Neptunia, Acacia, Albizia and Samanea) respond to a lesser degree by showing “sleep movements” (nyctinasty). This involves the closing up of the leaves a few hours before dusk, and re-opening a few hours before dawn (controlled by the circadian clock and light signal transduction through phytochromes). It’s thought these nyctinastic movements aid water conservation as well as defence against herbivory.
M. pudica has been used widely in traditional medicine, from treating glandular swellings in India to relieving lower back and kidney pain in the Republic of the Congo. The plant has also been used to treat sleep disorders and, in Senegal, an infusion of the leaves is believed to be calming and sleep-inducing… But that’s not the whole story. While root extracts have shown anti-bacterial properties, too much can be aphrodisiacal (stimulating sexual desire) and, along with the seeds, can function as a strong laxative and induce vomiting!
Before you get too worried (or excited), M. pudica is not yet used in Western medicine as pharmaceutical companies continue to investigate its reported medicinal properties.
M. pudica is similar to M. pigra (giant sensitive plant) and M. diplotricha var. diplotricha (creeping sensitive plant), both of which also bear pink globular inflorescences and prickles. You’ll be relieved to hear that differences in habit, leaf-branching and fruit help distinguish these three species:
- M. pudica is a relatively small plant, with a spreading (prostrate) habit. Leaves are not branched (with one or two pairs of branchlets) and pods are relatively small (1-2.5 cm long), containing only 1-6 one-seeded segments.
- M. pigra is a larger shrub with an upright (erect) growth habit. Leaves are large and much-branched (with 6-16 pairs of branchlets). Its pods are relatively large (3-8 cm long) and contain 14-26 one-seeded segments.
- M. diplotricha is also an upright (erect) shrub, but can also be climbing in its habit. The leaves are branched (with 4-9 pairs of branchlets), but the pods are relatively small (1-3.5 cm long) with only 3-5 one-seeded segments.
Coverage in the wider community…
The fame of M. pudica doesn’t stop there – in fact, non-scientists alike have continued to be intrigued by the plant’s remarkable capabilities. The species even has its own “Vine” (a Twitter-owned mobile app that enables users to create and post short video clips) which has clocked up a remarkable 1.4 million views. The page has also been shared on Facebook over 70,000 times!
One business from the USA has taken the opportunity to exploit the plant’s thigmonastic movements by marketing its seeds as TickleMe Plant®. Complete with a cartoon and colouring pages to entertain the younger audience, the strategy inspires the wider community not to lose “touch” with our living world. There is a real sense of adventure in raising your own TickleMe Plant® and, if no seed grows “to the point where the second set of leaves move when you tickle them”, the company will even replace the seeds. You can’t get a much better deal than that now, can you…?
Alcoholic beverages, problematic weed control, a huge array of medicinal symptoms, social networking prowess… it’s clear this species really does get around! It’s not hard to work out why either. M. pudica has far-reaching interests, not only for botanists but also for the much wider, non-scientific community too. There aren’t many plants out there which have a Facebook page and a Vine dedicated to them! Much of the current research focusses on the species’ many reported medicinal properties with a potential for drug development in the future… So M. pudica could well be coming to a shelf near you! Unbeleafable? You decide!
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. IUCN Species Survival Commission Invasive Species Specialist Group (2010) Mimosa pudica species account. Global Invasive Species Database.
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