Quick answer: When a leaf of the touch-me-not plant (Mimosa pudica) is touched, its leaflets fold up and the leaf droops within seconds. Plants have no nerves or muscles, so the information is passed from cell to cell by electrical-chemical signals. At the base of the leaf, cells in a swelling called the pulvinus quickly lose water, become soft (flaccid) and change shape, so the leaf folds. This movement is not connected with growth, is not directional and is reversible, unlike tropic movements such as a shoot bending towards light.
Two kinds of plant movement
Your NCERT textbook describes two types of movement in plants:
| Type | Linked to growth? | Example |
|---|---|---|
| Movement independent of growth | No | Leaves of the touch-me-not folding on touch |
| Movement due to growth (tropism) | Yes | Shoot bending towards light |
Movements that are not linked to growth and do not depend on the direction of the stimulus are often called nastic movements. The touch response of Mimosa is called thigmonasty (or seismonasty, when the stimulus is a shake or shock).
What happens when you touch a Mimosa leaf
Step 1: The touch is detected by cells of the leaf.
Step 2: An electrical-chemical signal passes from cell to cell. Plants have no specialised nervous tissue, but their cells can still pass such signals along.
Step 3: The signal reaches the pulvinus, a swollen joint at the base of each leaflet and leaf stalk.
Step 4: Cells in one half of the pulvinus (the lower half for the main leaf stalk) lose ions such as potassium (), and water follows them out of the cells by osmosis.
Step 5: These cells lose their turgor (the pressure of water inside them) and become flaccid, while the cells on the other side stay firm. The leaf stalk bends at the pulvinus and droops, and the leaflets fold together.
After several minutes, the ions are moved back in, water re-enters, the cells become turgid again and the leaf slowly reopens.
Key idea from NCERT: Plant cells change shape by changing the amount of water in them, which makes them swell or shrink.
Pulvinus at the base of the leaf stalk
stem ---[ upper cells: stay firm ]--- leaf
[ lower cells: lose water ]
Why the plant may do this
Folding the leaves quickly is thought to help protect the plant, for example by making it look wilted and less attractive to animals that eat leaves. This is an explanation scientists suggest, not something you need to learn for the exam.
Tropic movement vs touch-me-not movement
| Feature | Tropic movement (tropism) | Touch-me-not movement (nastic) |
|---|---|---|
| Linked to growth | Yes: unequal growth of cells | No: change in water content of cells |
| Direction of stimulus | Matters (towards or away) | Does not matter |
| Speed | Slow (hours or days) | Fast (seconds) |
| Reversible | No | Yes, the leaf reopens |
| Controlled by | Plant hormones such as auxin | Movement of ions and water |
| Example | Shoot bending towards light | Mimosa leaves folding on touch |
Summary table
| Idea | Key fact |
|---|---|
| Stimulus | Touch |
| Signal | Electrical-chemical, cell to cell |
| Organ that moves | Pulvinus at the base of the leaf |
| Cause of movement | Cells lose water and become flaccid |
| Recovery | Water re-enters; leaf reopens |
| Growth involved? | No |
Exam tips
- Use the NCERT phrasing: the cells "change shape by changing the amount of water in them".
- When comparing with phototropism, give three differences: growth-linked or not, directional or not, and the cause (auxin vs water movement).
- Mention that plants use electrical-chemical signals but have no nerves or muscles.
Common mistakes
- Saying the plant has nerves or muscles. It has neither.
- Calling the touch-me-not response a tropism. It is not linked to growth and is not directional.
- Confusing it with the coiling of tendrils, which is a growth movement (thigmotropism) towards the support.
- Saying the cells die. They only lose water temporarily and recover.
Frequently Asked Questions
How does the touch-me-not plant move without muscles?
Cells at the base of the leaf lose water, become soft and change shape, so the leaf bends and folds.
How is the touch signal passed in Mimosa?
By electrical-chemical signals that travel from cell to cell. Plants do not have nervous tissue.
What is the pulvinus?
It is a swollen joint at the base of a leaf stalk or leaflet whose cells change their water content to move the leaf.
How is this movement different from phototropism?
It is fast, reversible, not linked to growth and not directional. Phototropism is slow growth towards light controlled by auxin.
Is the folding of Mimosa leaves a tropic movement?
No. It is a nastic movement, because it does not depend on growth or on the direction of the touch.