auxin darwin experiment

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 Here’s a step-by-step explanation of Darwin’s classic experiment on auxin (phototropism) 👇

🌱 Title: Darwin’s Experiment on Phototropism (Demonstrating the Role of the Tip in Auxin Action)

🧠 Objective:

To study how the tip of the coleoptile (shoot tip) perceives light and controls bending (phototropic response).

🧪 Materials Used:

  • Oat (Avena) or canary grass seedlings

  • Light source

  • Knife or blade

  • Opaque caps (to cover tips)

  • Transparent caps

  • Microscopic slides or glass plates

🔬 Step-by-Step Procedure:

Step 1: Control Experiment

  • Darwin took normal oat seedlings and allowed them to grow vertically in uniform light.

  • Then, he exposed them to unilateral (one-sided) light.

  • Observation: The seedlings bent towards the light source (showing positive phototropism).

Step 2: Tip Removal

  • He cut off the coleoptile tips of some seedlings.

  • The decapitated seedlings were then exposed to one-sided light.

  • Observation: No bending occurred.

  • Inference: The tip is necessary for perceiving light.

Step 3: Covering the Tip with Opaque Cap

  • The tip of some seedlings was covered with a black opaque cap (to block light).

  • Exposed to unilateral light.

  • Observation: No bending towards light occurred.

  • Inference: The tip perceives light, and when it cannot, the plant doesn’t respond.

Step 4: Covering the Base with Opaque Cap

  • The base of some seedlings was covered, leaving the tip exposed.

  • Exposed to one-sided light.

  • Observation: Normal bending occurred.

  • Inference: The tip, not the base, is responsible for light perception.

Step 5: Transparent Cap on the Tip

  • The tip of some seedlings was covered with a transparent cap that allowed light to pass through.

  • Exposed to one-sided light.

  • Observation: Normal bending occurred.

  • Inference: It is light perception (not covering itself) that matters — the tip detects light stimulus.

🧩 Conclusion:

  • The coleoptile tip perceives light and produces a chemical substance (later identified as auxin).

  • This substance moves from the tip to the shaded side, promoting cell elongation there, causing the bending towards light.

🧠 Significance:

This experiment by Charles Darwin and Francis Darwin (1880) laid the foundation for discovering auxin, the first plant hormone, and for understanding phototropism.

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