Coordination in Plants: Phytohormones
NCERT Class 10 Science – Chapter 7 (As per NCERT 2025–26)
Table of Contents
1. Introduction
Unlike animals, plants do not have a nervous system or muscles. They respond to stimuli using Chemical Coordination (Phytohormones) and specific movements.
2. Types of Movements in Plants
Plant movements are broadly classified into two types:
A. Nastic Movements (Growth Independent)
These movements are immediate and non-directional.
Example: The sensitive plant (Mimosa pudica). When touched, the leaves fold up due to a change in turgor pressure in the cells.
B. Tropic Movements (Growth Dependent)
These are directional movements where the direction of growth depends on the direction of the stimulus.
| Stimulus | Type of Tropism | Response |
|---|---|---|
| Light | Phototropism | Shoot bends towards light (+ve). |
| Gravity | Geotropism | Roots grow downwards (+ve). |
| Chemicals | Chemotropism | Pollen tube grows towards ovule. |
| Water | Hydrotropism | Roots grow towards water. |
3. Plant Hormones (Phytohormones)
Phytohormones are chemical substances that coordinate growth, development, and responses to the environment.
⚡ Exam Cheat Sheet: Functions
- Auxin: Synthesized at shoot tip, helps in cell elongation and Phototropism.
- Gibberellin: Helps in the growth of the stem.
- Cytokinin: Promotes rapid cell division (present in fruits and seeds).
- Abscisic Acid (ABA): Inhibits growth; causes wilting of leaves and induces dormancy (Stress hormone).
Fig 1: Phototropism: Shoot bending towards light due to Auxin diffusion.
4. Chapter Assessment
A. Very Short Answer Type Questions (1 Mark)
- Q1. Which plant hormone promotes cell division?
Ans: Cytokinin. - Q2. Which hormone is responsible for the wilting of leaves?
Ans: Abscisic Acid (ABA). - Q3. Give one example of a chemotropic movement.
Ans: Growth of pollen tube towards ovule. - Q4. Where is Auxin synthesized?
Ans: At the shoot tip. - Q5. Which hormone helps in the growth of the stem?
Ans: Gibberellin.
B. Short Answer Type Questions
Q6. How does phototropism occur in plants?
Auxin is synthesized at the shoot tip. When light comes from one side, auxin diffuses towards the shady side of the shoot. This concentration of auxin stimulates the cells on the shady side to grow longer than the cells on the light side, causing the plant to bend towards the light source.
Q7. Differentiate between Tropic and Nastic movements.
| Tropic Movement | Nastic Movement |
|---|---|
| Directional (depends on stimulus). | Non-directional (independent of stimulus). |
| Growth dependent (slow). | Growth independent (immediate). |
| Ex: Phototropism. | Ex: Thigmonasty (Touch-me-not). |
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