CBSE · Class 10 · Science · Control and CoordinationWhat is plant tropism? Explain any four major types of tropic movements in plants with appropriate examples, highlighting the role of plant hormones (phytohormones) in directional growth.

Introduction to Plant Tropism\nPlants do not have a nervous system or muscles, yet they respond to environmental stimuli such as light, gravity, water, and chemical substances. These directional growth movements in plants in response to external stimuli are known as tropic movements (or tropism). Tropic movements can be either positive (towards the stimulus) or negative (away from the stimulus).

Four Major Types of Tropic Movements

1. Phototropism (Response to Light):

   • Description: The directional growth movement of plant parts in response to light.
   • Example: Plant shoots grow towards the source of light (positive phototropism), while roots generally grow away from light (negative phototropism).

2. Geotropism or Gravitropism (Response to Gravity):

   • Description: The movement of plant organs in response to the force of gravity.
   • Example: Roots grow downwards into the soil, following gravity (positive geotropism), whereas main stems grow upwards against gravity (negative geotropism).

3. Hydrotropism (Response to Water):

   • Description: The directional growth of plant roots towards a source of moisture or water.
   • Example: Plant roots bend and grow towards areas containing damp soil or water channels (positive hydrotropism).

4. Thigmotropism (Response to Touch):

   • Description: The directional growth response of plants in response to contact with a solid object.
   • Example: Tendrils of climbing plants (like peas or grapevine) coil around a support structure when they touch it, helping the plant climb.

Role of Phytohormones (Auxin)\nDirectional movements in plants are regulated by plant hormones called phytohormones. Auxin is the primary hormone responsible for phototropism and geotropism. When sunlight falls on one side of a plant stem, auxin diffuses towards the shaded side. Higher concentration of auxin on the shaded side stimulates cells to grow longer than those on the illuminated side, causing the stem to bend towards the light source.

Conclusion\nTropic movements enable plants to secure optimal sunlight, water, and structural support, thereby ensuring their survival and growth in a changing environment.