Mimosa Pudica, the Sensitive Plant

Mimosa Pudica is a perennial herb. It is a creeping plant. It is a native of South America and Central America. It also grows in Asian countries like Thailand, Indonesia, Malaysia, Philippines and Jamaica. It usually grows under shady areas under trees and shrubs. This plant is well known as Touch- me not Plant. It is also referred as sensitive plant, shameful plant, humble plant etc.

Mimosa Pudica is a flowering plant. This plant belongs to pea family. Therefore it is a legume plant. It grow to a height of 50-70 cm. It has a slender stem. Stem is erect in young plants. The stem will start branching and becomes prickly as the plant ages. The leaves are oblong in shape. They are   bipinnate compound leaves. There are 10-26 leaflets in each pinna. The petiole is prickly.

This plant produces pale pink or purple coloured flowers from the leaf axil. They are fluffy to look at. Each flower has 8-10mm diameter. They are spherical in shape. Their petals are red and the filaments are either pink or lavender in colour. The flowers are pollinated either by wind or by insects.

Their fruits come in clusters. They are 1-2 cm long.  They have prickly margin. The fruits contain seeds of length 2.5mm in length. The seeds have hard seed coat.

Mimosa Pudica has tap root. It is a legume plant, therefore it fixes atmospheric nitrogen into the soil with the help of bacteria dwelling in the soil. Their root system contain nodules that contain symbiotic bacteria called Rhizobia. The symbiotic bacteria produces nitrogen compounds that help the plant to grow and compete with other plants. When the plant dies the fixed nitrogen is released making it available for other plants.  

Nitrogen is essential for Photosynthesis and it is an essential component of Chlorophyll. Nitrogen Fixation increases the nitrogen content of the plant and the surrounding soil.  Mimosa Pudica increases the fertility of the soil because it can fix the atmospheric nitrogen into the soil by collaborating with bacteria. It lives in collaboration with the symbiotic bacteria that enriches soil nitrogen.

Most of the plants do not move from place to place. They remain in the same place. They show certain obvious movements like roots move towards water and shoots move towards sunlight. But Mimosa plant is an exception. This plant shows rapid movement of leaves when touched. The foliage of mimosa plant closes during darkness and reopens when sunlight falls on the plant. The leaves also close under various other stimuli, such as touching, warming, blowing, or shaking. These types of movements have been termed “seismonastic movements”. The leaves of the mimosa plant respond to the stimulus of touching.

Mimosa leaves are described as bipinnate. This means that they are compound leaves consisting of four main leaflets called pinnae, which in turn are composed of many even smaller leaflets called pinnules.  (Large leaflets composed of smaller leaflets – bipinnate.)  The tiny leaflets give the plant a distinctly fern-like appearance. The new leaves have a light, yellowish-green colour, while the older ones are a darker green.

Why do the leaves of the Mimosa plant shrink when touched?

If we observe the structure of the compound leaves of Mimosa plant, we will see a white dot like swollen portion at the base of a leaflet. These dots are fluid filled, sac-like structures called pulvini. These are powered by turgor pressure. On the reverse side of the leaf, the leaflet stalks, and main stem, are covered by hairs.  These hairs are prickly, and acts as a defence mechanism against animals that may feed on them. Pulvinus has a group of cells at the base of a leaf or leaflet in Mimosa plant. They lose water molecules when touched and bring about changes in the position of the leaves.

In the sensitive plant Mimosa Pudica, the pulvinus is responsible for the folding of the leaves that occurs when the plant is touched or injured. The cells of the leaves contain water in addition to other cellular contents. They exert a certain amount of force on the cell walls of the plant. This is called Turgor Pressure. This turgor pressure helps the plants to stay upright unless disturbed externally. When we touch or shake the leaves the protein present at the leaf stalk or pulvinus is activated. Simultaneously the plant releases certain chemicals including potassium ions. These chemicals form water. Electrolytes diffuse out of the cells causing a low pressure area in the cells. The cells of the leaves will collapse, hence the leaves close. The “touch stimuli” is transferred to the neighbouring leaves. Similar reaction is transferred to other leaves forcing them to close.

Mimosa Pudica plant respond to touch stimuli in this way. But touching or shaking the leaves of the plant is not good for the plant because the leaves are the centre of Photosynthesis. If the leaves are disturbed by touching or by any other method, the leaves will close and photosynthesis will be prevented and the plant becomes weak and there will be no growth in the plant. So, don’t touch or harm the plant.  This may be the reason why Mimosa Pudica plant is called “Touch- Me-Not” plant.