Mimosa pudica (also called sensitive plant, sleepy grass, sleepy plant, action plant, humble plant, touch-me-not, touch-and-die, shame plant or shy plant) is a creeping annual or perennial flowering plant of the pea/legume family Fabaceae. It is often grown for its curiosity value: the sensitive compound leaves quickly fold inward and droop when touched or shaken and re-open a few minutes later. For this reason, this species is commonly cited as an example of rapid plant movement. Like a number of other plant species, it undergoes changes in leaf orientation termed "sleep" or nyctinastic movement. The foliage closes during darkness and reopens in light. This was first studied by French scientist Jean-Jacques d'Ortous. In the UK it has gained the Royal Horticultural Society's Award of Garden Merit. The species is native to the Caribbean and South and Central America, but is now a pantropical weed, and can now be found in the Southern United States, South Asia, East Asia, Southeast Asia, Micronesia, Australia, South Africa, and West Africa as well. It is not shade-tolerant and is primarily found on soils with low nutrient concentrations.
Taxonomy Mimosa pudica was first formally described by Carl Linnaeus in Species Plantarum in 1753. The species epithet, pudica, is Latin for 'shame', 'bashful', or 'shrinking', alluding to its shrinking reaction to contact. The species is known by numerous common names including sensitive plant, action plant, humble plant, shameplant, and touch-me-not.
Description
The stem is erect in young plants but becomes creeping or trailing with age. It can hang very low and become floppy. The stem is slender, branching, and sparsely to densely prickly, growing to a length of 1.5 m (5 ft). The erect height of M. pudica usually reaches around ~30 cm (~1 ft). The leaves are bipinnately compound, with one or two pinnae pairs, and 10–26 leaflets per pinna. The petioles are also prickly. Pedunculate (stalked) pale pink or purple flower heads arise from the leaf axils in mid-summer with more and more flowers as the plant gets older. A single flower survives for less than a day, and usually dies completely by the next day. Flowers of M. pudica are very brittle and soft. The globose to ovoid heads are 8–10 mm (0.3–0.4 in) in diameter (excluding the stamens). On close examination, it is seen that the floret petals are red in their upper part and the filaments are pink to lavender. Pollens are circular with approximately 8 microns in diameter.
The fruit consists of clusters of two to eight pods from 1–2 cm (0.4–0.8 in) long each, these being prickly on the margins. The pods break into two to five segments and contain pale brown seeds about 2.5 mm (0.1 in) long. The flowers are insect-pollinated and wind-pollinated. The seeds have hard seed coats which restrict germination and make osmotic pressure and soil acidity less significant hindrances. High temperatures are the main stimuli that cause the seeds to end dormancy. The roots of Mimosa pudica produce carbon disulfide, an antifeedant that prevents certain pathogenic and mycorrhizal fungi from growing within the plant's rhizosphere. This allows the formation of nodules on the roots of the plant that contain endosymbiotic diazotrophs, which fix nitrogen. Mimosa pudica is a tetraploid (2n = 52).
Plant movement
The leaflets close when stimulated in various ways, such as warming, touching, blowing, and shaking, which are all encapsulated within mechanical or electrical stimulation. Movements in response to these stimuli have been termed seismonastic movements. This reflex may have evolved as a defense mechanism to disincentivize predators, or alternatively to shade the plant in order to reduce water loss due to evaporation. The main structure mechanistically responsible for the drooping of the leaves is the pulvinus. The stimulus is transmitted as an action potential from a stimulated leaflet to the leaflet's swollen base (pulvinus), and from there to the pulvini of the other leaflets, which run along the length of the leaf's rachis. The action potential then passes into the petiole, and finally to the large pulvinus at the end of the petiole, where the leaf attaches to the stem. The pulvini cells gain and lose turgor due to water moving in and out of these cells, and multiple ion concentrations play a role in the manipulation of water movement. Similar to the Venus flytrap's trigger hairs, M. pudica's leaves are hypersensitive to touch. In line with the touch-sensing function used for tasks such as defense or nutrient maintenance, these parts have mechanoreceptors linked to mechanosensitive channels that can conduct calcium ions and indirectly relative anions upon touch stimulation, giving rise to depolarization and the initiation of an action potential (AP). They also have voltage-sensitive potassium channels that promote hyperpolarization and turgor formation. Such sensitive plants fire all-or-nothing type APs, similar to those seen in animals. This movement of folding inwards is energetically costly for the plant and also interferes with the process of photosynthesis.
Distribution and habitat
Mimosa pudica is native to the tropical Americas. It can also be found in Asian countries such as Singapore, Bangladesh, Thailand, India, Nepal, Indonesia, Taiwan, Malaysia, the Philippines, Vietnam, Cambodia, Laos, Japan, and Sri Lanka. It has been introduced to many other regions and is regarded as an invasive species in Tanzania, South and Southeast Asia, and many Pacific islands. It is regarded as invasive in parts of Australia and is a declared weed in the Northern Territory, and Western Australia although not naturalized there. Control is recommended in Queensland. It has also been introduced to Uganda, Ghana, Nigeria, Seychelles, Mauritius and East Asia but is not regarded as invasive in those places. In the United States, it grows in Louisiana, Florida, Hawaii, Tennessee, Virginia, Maryland, Puerto Rico, Texas, Alabama, Arkansas, Mississippi, North Carolina, Georgia, the territory of Guam, and the Virgin Islands.
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