In photoperiodism/vernalization the stimulus is perceived by green leaves only.

Explanation

### Step-by-Step Solution:

1. **Understanding Photoperiodism**:
- Photoperiodism is the response of plants to the length of day and night. It is a physiological reaction to the duration of light and darkness that affects flowering and other developmental processes in plants.

2. **Role of Green Leaves**:
- In photoperiodism, the stimulus (light) is perceived by the green leaves of the plant. The chlorophyll in the leaves captures light, which is essential for the plant to determine the length of the day.

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