Give one significant contribution of each of the following scientists :
W. W. Garner and H. A. Allard

Answer

### Step-by-Step Solution:

1. **Identify the Scientists**: The question asks for the significant contributions of W. W. Garner and H. A. Allard.

2. **Understand Their Contribution**: Both scientists made a notable contribution to the field of botany, specifically in understanding how light affects plant growth and flowering.

3. **Photoperiodism**: In 1920, Garner and Allard described the concept of photoperiodism, which is the response of plants to the relative lengths of light and dark periods in a day.

4. **Critical Factor for Flowering**: They concluded that the length of day (photoperiod) is a critical factor that determines whether a plant will flower or not.

5. **Research on Tobacco Plants**: Their research primarily focused on tobacco plants, where they observed the effects of different light durations on flowering.

6. **Classification of Plants**: Based on their findings, they classified plants into two groups:
- **Short Day Plants**: These plants require a shorter duration of light to flower. Examples include rice, onion, and soybeans.
- **Long Day Plants**: These plants require a longer duration of light to flower. Examples include radish and potato.

7. **Summarize the Contribution**: The significant contribution of W. W. Garner and H. A. Allard is the establishment of photoperiodism in plants, where they identified the importance of day length in flowering and classified plants into short day and long day categories.