Some early experiments on phototropic curvature in grasses led to discovery of

Options

A. Auxin
B. Gibberellins
C. Cytokinins
D. None of the above.

Answer

To solve the question about early experiments on phototropic curvature in grasses leading to the discovery of a specific substance, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Phototropism**:
- Phototropism is the growth of a plant in response to light direction. Plants tend to bend towards the light source.

**Hint**: Remember that "photo" refers to light and "tropism" refers to growth direction.

2. **Historical Context**:
- The early experiments on phototropism were conducted by Charles Darwin and his son Francis Darwin. They observed that the tip of the plant stem is responsible for sensing light.

**Hint**: Think about who contributed significantly to the understanding of plant movements and growth.

3. **Role of the Stem Tip**:
- In the experiments, it was found that when the stem tip is exposed to light, it bends towards the light source. This bending is due to differential growth on the shaded side of the plant.

**Hint**: Consider what happens to the plant when one side grows faster than the other.

4. **Discovery of Auxin**:
- The bending of the plant was attributed to a hormone that was produced in the tip of the stem. This hormone was later identified as auxin, which promotes cell elongation on the side of the plant that is away from the light.

**Hint**: Recall that hormones are chemical messengers that regulate growth and development in plants.

5. **Evaluating the Options**:
- The options provided in the question are auxin, gibberellin, cytokinin, and none of the above. Based on the experiments, auxin was indeed discovered in relation to phototropic curvature.

**Hint**: Think about which of the options is directly related to the bending response of plants to light.

6. **Conclusion**:
- Therefore, the correct answer to the question is auxin, as it was discovered through early experiments on phototropic curvature in grasses.

**Hint**: Summarize your findings to confirm the answer.

### Final Answer:
The early experiments on phototropic curvature in grasses led to the discovery of **auxin**.