Photoperiodism is effect of relative day length on flowering.

Explanation

### Step-by-Step Solution:

1. **Understanding Photoperiodism**:
- Photoperiodism is the phenomenon where the flowering of plants is influenced by the relative lengths of day and night. It was discovered by W.W. Garner and H.A. Allard.

2. **Key Components**:
- The two main components of photoperiodism are the light period (day length) and the dark period (night length). Plants respond to the duration of these periods.

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