Family : Leguminosae
Text © Pietro Puccio
English translation by Mario Beltramini
The precise place of origin is unknown; it is thought to be the tropical America, Brazil in particular, naturalized in many areas with tropical and subtropical climate.
The name of the genus comes from the Greek “mimos” = mime, with reference to the contraction movement of some species, if touched, which seems to imitate afear reaction; the name of the species is the Latin adjective “pudicus, a, um” = shy, bashful, with obvious reference.
Common names: humble plant, sensitive-plant, shameplant, sleeping grass, tickleme plant, tickle me plant, touch-me-not (English), mimosa pudica, mimosa sensitiva (Italian); Marie-la-honte, mimeuse pudique, sensitive (French); arranhadeira, dormideira, dorme-dorme, dorme Maria, malicia-das-mulheres, Maria-fecha-porta (Portuguese); dormidera, dormilona, sensitiva, vergonzosa (Spanish); Gemeine Mimose, Sinnpflanze (German).
The Mimosa pudica L. (1753) is an herbaceous perennial shrub, woody at the base, ramified, with reddish, thorny, stems and with several stiff bristles pointing downwards, about one metre tall.
The leaves are alternate, bipinnate, 3,5-6,5 cm long, with 10 to 20 pairs of oblong leaflets, 1-1,5 cm long and 1-2 mm broad, ciliate, of an intense green colour.
The flower heads (inflorescence formed by a host of flowers without pedicel closely in contact each other), solitary or in pair on a 1-2,5 cm long peduncle covered by a short down, are axillar, globose, of 6-8 mm of diameter; the single tiny flowers have protruding 7 mm long lilac pink stamina, which form the most conspicuous part of the head.
The fruits are flat pods, slightly curved, of brown colour, up to about 1,8 cm long, united in clusters; each contains 3-4 ovoid seeds, of 3,5 mm of diameter, of pale brown colour.
It reproduces in spring by seed, previously kept in water for 24-48 hours, just placed in sandy loam, kept humid at a temperature of 20-24 °C; the seeds, if fresh, germinate in 20-30 days; the growth is fast and the blooming starts three months after the germination.
In the tropical climates, it easily self-disseminates thus becoming a dangerous pest due to its thorns.
It is a widely cultivated species due to the high “sensitivity” of its leaves, which renders it a botanic curiosity, rather than for its flowers.
It is suitable for tropical and subtropical climates as it does not bear temperatures close to 0 °C; its cultivation may be tried in sheltered location in the most humid warm temperate ones. Elsewhere, seen its growth speed, it is utilized as annual, in open ground as well as in pot. It can grow in full sun and also in slight shade and is not particularly demanding for what the soil is concerned, provided the same is well drained; it is therefore advisable to add sand or agri perlite by around the 30% to the mould.
Watering must be regular in summer, but allowing the upper layer of the mould to dry up before giving water again and reduced in winter.
The characteristic which, since ever, has drawn the attention to this leguminous plant is called, in a botanical term, “seismonasty”, which means the movement of a plant as response to an outer stimulus, regardless of the direction from which it comes.
This characteristic, which represents a form of defence against the phytophagous insects, is common to other species, but it is particularly evident in the Mimosa pudica, seen the response speed, as a matter of fact, the leaflets, and possibly the whole leaf depending on the intensity of the contact, close in the time of one second.
In a much simplified way, this movement is due to the presence at the base of each leaflet and of the whole leaf of a thickening, called pulvinus, formed by cells with thin walls whose turgor may quickly vary after a stimulus, especially an impact, on whatever part of the leaf; this originates an electrical impulse which is transmitted to the whole leaf and possibly to the rest of the plant, depending on the intensity of the stimulus.
The impulse causes an alteration, particularly an increase, of the permeability of the cellular membranes, which allows the transfer by osmosis of the water from the lower half to the upper one of the pulvinus. As a consequence, the lower part loses its turgor whilst the upper one’s increases, thus obliging the leaflets and the leaves to bend downwards; the transfer of the water is “guided” by the passage of potassium ions through the cellular membranes. The contrary movement is slower, a few minutes; the water goes back from the upper half to the lower one, thus having the leaf and the leaflets returning in the normal position.
All parts of the plant are toxic: roots and leaves are variously utilized in the traditional medicine.
Synonyms: Mimosa hispidula Kunth (1823).