Darwing discovered that canary grass coleoptile bends towards light only when .

Options

A. It is 5 cm long
B. The seedling is growing in blue light
C. Coeloptile tip is intact and exposed to light
D. Cooler nights

Answer

To answer the question "Darwin discovered that canary grass coleoptile bends towards light only when...", we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Coleoptile**: The coleoptile is a protective sheath covering the emerging shoot of monocotyledonous plants, such as canary grass. It plays a crucial role in guiding the growth of the plant towards light.

2. **Identifying the Experiment**: Charles Darwin conducted experiments on canary grass coleoptiles to understand how they respond to light. He observed that the coleoptiles bend towards light, a phenomenon known as phototropism.

3. **Key Discovery**: Darwin discovered that the bending towards light occurs only when the tip of the coleoptile is intact and exposed to light. This indicates that the tip is responsible for detecting light and sending signals for growth direction.

4. **Analyzing the Options**:
- **Option A**: "It is 5 centimeters long." - This does not relate to the light detection mechanism.
- **Option B**: "The seedling is growing in blue light." - While blue light is effective for phototropism, the key factor is the condition of the coleoptile tip.
- **Option C**: "Coleoptile tip is intact and exposed to light." - This is the correct answer, as it directly relates to Darwin's findings.
- **Option D**: "Cooler nights." - This does not influence the bending towards light.

5. **Conclusion**: Based on the observations and experiments conducted by Darwin, the correct answer is **Option C: the coleoptile tip is intact and exposed to light**.

### Final Answer:
Darwin discovered that canary grass coleoptile bends towards light only when the coleoptile tip is intact and exposed to light (Option C).

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