Plant Tropisms

• A tropism is the growth response of a plant to a stimulus.
• A positive tropism occurs when a plant grows towards the stimulus.
• A negative tropism occurs when a plant grows away from the stimulus.

These are the tropisms we will study:

1. Phototropism
2. Geotropism
3. Thigmotropism
4. Hydrotropism
5. Chemotropism

1. Phototropism

• Phototropism is the growth response of a plant to light.
• Stems are positively phototropic.
  • They growing towards light to ensure that leaves receive the maximum amount of light for photosynthesis.
• Roots are negatively phototropic.
  • They grow away from light.

2. Geotropism

• Geotropism is the growth response of a plant to gravity.
  • Roots are positively geotropic, growing towards gravity.
  • Stems are negatively geotropic, growing away from gravity.

3. Thigmotropism

• Thigmotropism is the growth response of a plant to touch.
  • Climbing plants like ivy, vines, and tomatoes produce specialised parts called tendrils that wrap around supporting structures.
  • Tendrils are positively thigmotropic .
  • They grow around any object they touch for support.

4. Hydrotropism

• Hydrotropism is a plant's growth response to water.
  • Roots and pollen tubes grow towards water.

5. Chemotropism

• Chemotropism is a plant's growth response to chemicals.
  • Roots grow towards minerals.
  • Pollen tubes grow towards chemicals released by the ovule (positive chemotropism).
  • Roots are negatively chemotropic to harmful substances like acids or heavy metals (e.g., lead, zinc).