Tropism
- CBSE · NCERT Class 10, Ch 6 Control and Coordination
- ICSE Class 10, Unit 2 Plant Physiology
A tropism is a slow movement of a plant part by growth, towards or away from a stimulus such as light, gravity, water, touch or a chemical.
The name of each tropism tells you what the plant responds to:
- Phototropism: light. Shoots bend towards light, and roots bend away from it.
- Geotropism: gravity. Roots grow down, and shoots usually grow up.
- Hydrotropism: water. Roots grow towards moist soil.
- Thigmotropism: touch. Tendrils coil round a support.
- Chemotropism: chemicals. A pollen tube grows towards the ovules.
The bending towards light comes from a plant hormone called auxin, made at the shoot tip. It gathers on the shady side, and the cells there grow longer.
A small example
Keep bean seedlings on a wire mesh over a flask of water, in a box open on one side to a window. After 2 or 3 days, the shoots bend towards the light and the roots away from it.
Where marks go
In CBSE, Question 11 of the 2026-27 Science sample paper, option B, asks how the sensitive plant responds to touch. NCERT says no growth is involved there, so it is not a tropism. The board's Science marking scheme for 2026-27 wants two ideas. The touch passes from cell to cell by electrical-chemical means. Then cells change shape by gaining or losing water.
In ICSE, the 2024 Biology paper showed a pea tendril on a support. The answer was thigmotropism. The marking scheme defined it as a "growth movement of plant parts toward touch stimulus". CISCE's report on the 2024 ICSE Biology paper says answers missed those keywords. It also names "chemical tropism" as an incorrect answer, so use the exact term.
Sources
Facts last checked against these sources on 30 September 2026.
- Science, Class X (2026-27), Chapter 6: Control and Coordination · NCERT
- Science (086) Marking Scheme, Class X 2026-27 - Question 11 · CBSE Academic Unit
- Science (086) Sample Question Paper, Class X 2026-27 - Question 11 · CBSE Academic Unit
- ICSE Biology syllabus, Examination Year 2028 edition (January 2026), Class X unit 2 · CISCE
- Analysis of Pupil Performance, ICSE Biology 2024 (November 2024) - Questions 2(i) and 3(v) · CISCE
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