Mimosa pudica, the "touch-me-not" plant.
Recently, I bought a mimosa plant. You most probably know this plant by one of its many names: sensitive plant, sleepy plant, action plant, touch-me-not, shame-plant, zombie plant, mime (from the greek mímos (μίμος) or shy plant. Fascinated by the well-known response of folding its own leaves when touched, I dug into the underlying mechanism and will share it with you:
The mimosa genus consists of approximately 530 species with Mimosa pudica as its most famous one. The plants are small perennial subshrubs and belong to the fabacea family. A plant family to which also beans and peas belong. The wild plant grows on the American and Asian continent. Sometimes you can find it in southern European countries as an invasive species. Mislead by the examples found in greenhouses, botanical gardens and garden centers, the plant actually grows up to 1 - 1,5 m in height. While it is known for its sensitive leaves, it also produces beautiful pink or purple flowers.
The leaves of mimosa plants show two types of movement: Firstly, the daily closing and opening during the day and the night (controlled by the inner biological clock of the plant) and secondly, the rapid response in terms of mechanical, electrical or thermal stimulation. This fast response can be triggered if the plant is touched, shaken, strong winds blow against it or when it perceives electrical stimulation. It is not yet clear, how and why this reflex evolved. However, many researchers believe it serves as a defense mechanism against animals and insects.
In fact, mimosa has several predators as e.g. the mimosa webworm (Homadaula anisocentra). The mimosa webworm produces webs that it wraps around the leaves of the plant. Consequently, the plant cannot close its leaves anymore. They will start to fossilize and serve as food for freshly hatched larvae of the webworm.
What exactly is causing the collapsing of the leaves?
The general response of plants to touch/vibration is called thigmonasty or seismonasty. Only a few plants are capable of showing a fast response to its environment. This is enabled via specific cells: At the base of each leaf is a thickening similar to a “joint” called pulvinus. This thickening enables growth independent movements. The same thickening and mechanism can be found in the plant prayer family. This family includes plants that e.g. raise its leaves in a prayer-like manner in the evenings. You might think that the very famous Venus Flytrap is also part of this family, but I have to disappoint you: The Venus Flytrap belongs to the Droseraceae, a plant family that includes more than 180 carnivorous plant species. But to come back to the mechanism of the mimosa plants:
The rapid movement is enabled by changes in turgor pressure. Turgor pressure is the pressure inside a cell. The touch of a leaf will be translated to electrical stimulation. Especially potassium ions seem to have a major role in this process. Soon water will be flowing out of the cells, which decreases the turgor pressure and leads to the collapsing of the leaves. If the plant is not stimulate for the next minutes (approximately 20 minutes), the cells will fill again with water and the leaves will open. The picture on the left illustrates the process in the pulvinus. The green cells at the bottom reduce their turgor pressure which leads to the bending of the leaf.
Interestingly, if the plant is intoxicated with anesthetics, no response can be triggered. There is no clear explanation why the response can’t be triggered anymore, but there are several theories on macroscopic, cellular and molecular level.
Mimosa is a very fascinating plant to keep. If you decide to get one yourself, be warned to not touch the leaves too often. It's very tempting, I know. It will stress the plant as the folding of the leaves costs a lot of energy.
Mimosa pudica, Dionaea muscipula and anesthetics, Thiago Paes de Barros de Luccia, 2012
Inhalational and local anesthetics reuce tactile and thermal responses in mimosa pudica – Milne & Beamish 1999
Leaf closing substance of Mimosa pudica; chmical studies on another leaf movement of mimosa II Ueda & Yamamura 1999
Mimosa pudica: Electrical and mechanical stimulation of plant movements. Volkov et al.
MIMOSA WEBWORM - Gibb et al. 2017
Mechanism of the Seismonastic Reaction in Mimosa pudica – Robert D. Allen 1969