Quick answer: When a tendril touches a support, the part of the tendril in contact with the support grows more slowly than the part away from it. Because one side grows faster than the other, the tendril bends and coils around the support. This growth movement in response to touch is called thigmotropism. It is different from the quick folding of touch-me-not (Mimosa) leaves, which is caused by changes in water content, not growth.
Climbing plants such as the garden pea, cucumber and grapevine have weak stems and cannot stand upright on their own. They produce thin, thread-like structures called tendrils. When a tendril touches a stick, wire or fence, it slowly coils around it, holding the plant up so that it can climb towards the light.
How does a plant, with no nerves or muscles, respond to touch in this way?
In CBSE Class 10 Science, Chapter 6 (Control and Coordination), how tendrils coil around a support and how this differs from the movement of touch-me-not leaves are frequently asked board exam questions.
What You Will Learn
- What thigmotropism is
- What tendrils are
- How tendrils coil around a support, step by step
- The role of unequal (differential) growth
- How tendril coiling differs from the movement of Mimosa (touch-me-not) leaves
1. What Is Thigmotropism?
Definition: Thigmotropism is the directional growth movement of a plant part in response to touch or contact with a solid object. (Thigma means touch and tropos means turning.)
Tendrils show positive thigmotropism: they grow towards and around the object they touch.
2. How Do Tendrils Coil Around a Support?
Common board question: How do the tendrils of a pea plant coil around a support?
Step 1: Contact. The growing tip of a tendril touches a support, such as a stick or wire.
Step 2: Unequal growth. The part of the tendril in contact with the support does not grow as fast as the part away from the support.
Step 3: Bending. Because the outer side (away from the support) grows faster than the inner side (touching the support), the tendril bends towards the support.
Step 4: Coiling. As this continues, the tendril circles and coils around the support, holding the plant firmly.
Key point: Tendril coiling is caused by differential growth: the side away from the support grows faster than the side touching it.
Role of auxin: Plant hormones, especially auxin, control how fast plant cells grow. Many textbooks explain the faster growth on the outer side of the tendril by a higher concentration of auxin there. For your NCERT answer, the essential point is the unequal growth of the two sides.
3. Tendril Coiling vs Movement of Touch-Me-Not Leaves
| Feature | Tendril coiling (pea) | Folding of touch-me-not (Mimosa) leaves |
|---|---|---|
| Type of movement | Growth movement (directional) | Movement not due to growth |
| Cause | Unequal growth of the two sides of the tendril | Cells quickly lose water and change shape |
| Speed | Slow (hours) | Very fast (seconds) |
| Direction | Depends on where the support is | Does not depend on the direction of touch |
| Reversible? | No; the tendril stays coiled | Yes; the leaves open again after some time |
Remember: In Mimosa, the plant cells change shape by changing the amount of water in them, which makes the leaves fold. There is no growth involved.
4. Summary Table
| Step | What happens |
|---|---|
| 1. Contact | The tendril touches a support |
| 2. Unequal growth | The side touching the support grows more slowly |
| 3. Bending | The faster-growing outer side makes the tendril bend towards the support |
| 4. Coiling | The tendril coils around the support |
5. Exam Tips and Common Mistakes
Exam Tip: In your answer, clearly say that the part of the tendril in contact with the support grows less rapidly than the part away from it.
Exam Tip: Name the movement: thigmotropism, a growth movement in response to touch.
Common Mistake: Saying tendrils coil because of muscles or nerves. Plants have neither; the coiling is caused by unequal growth.
Common Mistake: Giving the same explanation for tendrils and Mimosa. Tendril coiling is a growth movement; Mimosa leaf folding is caused by changes in water content of cells.
Frequently Asked Questions
What is thigmotropism?
Thigmotropism is the directional growth movement of a plant part in response to touch, such as a tendril coiling around a support.
How do tendrils coil around a support?
The part of the tendril touching the support grows more slowly than the part away from it, so the tendril bends and coils around the support.
What is the difference between tendril coiling and the movement of touch-me-not leaves?
Tendril coiling is a slow growth movement caused by unequal growth. Touch-me-not leaves fold quickly because their cells lose water and change shape, with no growth involved.
Which plants have tendrils?
Climbing plants such as the garden pea, cucumber and grapevine have tendrils.