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Tropic Movements in Plants
Unlike animals, plants cannot walk or run from one place to another when their environment changes. Instead, they respond to environmental changes by growing or moving specific parts of their body—such as their roots, stems, or leaves—toward or away from a stimulus. These directional growth behaviors are called Tropic Movements.
In Class 10 Biology (Chapter: Control and Coordination), understanding how plants navigate their surroundings using growth movements is a core syllabus requirement.
1.0What are Tropic Movements?
Tropism are directional growth movements shown by plants toward or away from a stimulus.
In simple words: “Tropic movements are growth movements of plants caused by external stimuli such as light, water, gravity, or touch.” These movements are usually slow because they involve plant growth.
2.0Characteristics of Tropic Movements
- Directional in nature
- Growth-based movements
- Controlled by plant hormones
- Occur in response to stimuli
- Usually slow movements
Tropic movements help plants adjust to their environment.
3.0Types of Tropic Movements in Plants
Plants show different types of tropic movements depending on the stimulus involved.
- Phototropism
- Geotropism
- Hydrotropism
- Thigmotropism
- Chemotropism
A. Phototropism (Response to Light)
Phototropism is the growth movement of a plant part in response to light.
- Positive Phototropism: Shoots/stems bend and grow toward light to maximize photosynthesis.
- Negative Phototropism: Roots grow away from light, burrowing deep into the soil.
- The Mechanism (Role of Auxin): When a plant shoot receives light from one side only, the auxin hormone synthesizes at the tip and diffuses down to the shady side of the stem. Auxin stimulates rapid cell elongation. Because the cells on the shady side grow longer than the cells on the sunny side, the stem naturally bends toward the light source.
B. Geotropism / Gravitropism (Response to Gravity)
Geotropism is the growth movement of a plant part in response to the pull of gravity.
- Positive Geotropism: Roots grow downward, deep into the earth, ensuring the plant anchors securely and accesses deep water table networks.
- Negative Geotropism: Shoots grow upward against gravity, ascending toward air and sunlight.
C. Hydrotropism (Response to Water)
Hydrotropism is the growth movement of plant roots toward moisture or water.
- Observation: If a plant is grown in dry soil with a water source placed nearby, the roots will curve and grow horizontally toward the water source, even if it means defying the downward pull of gravity. This shows that hydrotropism can be stronger than geotropism.
D. Chemotropism (Response to Chemicals)
Chemotropism is the directional growth of a plant part in response to specific chemical substances.
- Classic Board Exam Example: The growth of a pollen tube down through the stigma and style toward the ovule during plant fertilization. The ovule secretes specific sugars and chemicals that guide the pollen tube directly to it.
E. Thigmotropism (Response to Touch)
Thigmotropism is the directional growth of a plant part in response to physical contact or touch with a solid object.
- Observation: Climbing plants like peas, gourds, and grapevines have thread-like structures called tendrils. When a tendril comes into contact with a support structure (like a stick or a fence), the side of the tendril touching the object grows slower than the outer side. This unequal growth causes the tendril to coil tightly around the support, allowing the plant to climb upward.