The photoperiod in plants is perceived at

Explanation

**Step-by-Step Solution:**

1. **Understanding Photoperiodism**: Photoperiodism refers to the response of plants to the length of day or night. It is crucial for processes such as flowering and growth.

2. **Identifying the Perception Site**: The question asks where the photoperiod is perceived in plants. This is typically at the leaves.

3. **Role of Fully Developed Leaves**: Fully developed leaves are the primary sites for perceiving light stimuli. They have the necessary structures to detect changes in light duration.

4. **Young Leaves and Sensitivity**: While young leaves are sensitive to light, they do not play a significant role in perceiving photoperiod. Instead, they are more responsive to other light-related stimuli.

5. **Other Plant Parts**: Other parts of the plant, such as the meristem and floral buds, do not perceive the photoperiod. They rely on signals from the leaves.

6. **Conclusion**: Therefore, the photoperiod in plants is perceived at the fully developed leaves.

**Final Answer**: The photoperiod in plants is perceived at the fully developed leaves.

---