In short day plants, what is the critical period, the length of the day or night?

Explanation

**Step-by-Step Solution:**

1. **Understanding Photoperiodism**: Photoperiodism is the response of organisms, including plants, to the length of day and night. It is crucial for processes like flowering and growth.

2. **Identifying Short Day Plants (SDP)**: Short day plants are those that flower when the length of daylight is shorter than a certain critical period. They require longer nights to trigger the flowering process.

3. **Determining the Critical Period**: For short day plants, the critical period refers to the length of the night. They need a specific duration of darkness to initiate flowering.

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