Lecture on Plant Tropisms

Introduction to Tropisms

• Tropisms: Growth responses of plants to environmental stimuli.
• Two primary tropisms in plants:
  • Phototropism: Growth response to light.
  • Gravitropism: Growth response to gravity.

Understanding Stimuli

• Stimulus: Any environmental factor that induces a response.
• Examples of stimuli humans respond to: cold (put on a sweater), hunger (eat food).
• Importance: Helps organisms survive by responding to environmental changes.

Phototropism

• Positive Phototropism: Growth of a plant towards light.
  • Plants maximize photosynthesis by growing towards light.
  • Controlled by a chemical called auxin.
  • Auxin moves to the side of the plant furthest from light, causing cells to elongate and plant to bend towards light.

Key Experiments in Phototropism

1. Charles Darwin and Francis Darwin Experiment:

   • Used oat seedlings to determine the location of the phototropic response.
   • Manipulated variables: cutting the tip, opaque cover, transparent cap, and base cover.
   • Conclusion: Phototropic response originates at the tip of the plant.

2. Peter Boysen-Jensen Experiment:

   • Investigated the movement of auxin using permeable (gelatin) and non-permeable (mica) barriers.
   • Conclusion: Auxin must be able to move within the plant for phototropism to occur.

3. Frits Went Experiment:

   • Used agar to demonstrate auxin's role in directing growth.
   • Conclusion: Auxin distribution determines the direction of plant growth towards or away from light.

Gravitropism

• Gravitropism: Growth response of a plant to gravity.
  • Positive Gravitropism: Growth towards gravity (e.g., roots).
  • Negative Gravitropism: Growth away from gravity (e.g., stems, leaves).
  • Even when trees are tilted, they eventually grow upward due to negative gravitropism.

Mechanism of Gravitropism

• Auxin also plays a role in gravitropism.
• Auxin distribution within a plant was influenced by gravity, affecting growth direction.

Important Terms

• Auxin: Chemical controlling plant growth responses.
• Positive Tropism: Growth towards the stimulus.
• Negative Tropism: Growth away from the stimulus.

Key Takeaways

• Understanding of plant responses to light and gravity.
• Role of auxin in directing plant growth.
• Contributions of key researchers: Charles and Francis Darwin, Peter Boysen-Jensen, Frits Went.

Conclusion

• Tropisms are crucial for plant survival and adaptation.
• Further studies in plant biology continue to explore the complexities of these growth mechanisms.

The lecture concluded with an invitation for questions and further discussion on the topic.