Hormonal substance migrates from leaves to shoot apices for inducing flowering only when the plants are exposed to the necessary____

Answer

To solve the question regarding the hormonal substance that migrates from leaves to shoot apices for inducing flowering, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about a necessary condition for hormonal migration that induces flowering in plants. This indicates that we need to identify an external factor that influences this process.

2. **Identifying Internal and External Factors**: Recognize that flowering in plants is influenced by both internal factors (like hormones) and external factors (like light). The question specifically asks for an external factor.

3. **Exploring the Role of Light**: The key external factor that affects flowering is light. Plants require specific light conditions to trigger the hormonal changes necessary for flowering.

4. **Defining Photoperiod**: The term "photoperiod" refers to the duration of light exposure that plants receive. It is crucial in determining the flowering response in different plant species.

5. **Categorizing Plants**: Based on their light requirements, plants can be categorized into two groups:
- **Long Day Plants**: These plants require longer periods of light exposure than a critical threshold to induce flowering (e.g., spinach, certain wheat varieties).
- **Short Day Plants**: These plants require shorter periods of light exposure than a critical threshold to induce flowering (e.g., rice, cotton).

6. **Conclusion**: The necessary condition for the hormonal substance to migrate from leaves to shoot apices, thus inducing flowering, is the exposure to the necessary "inductive photoperiod."

### Final Answer:
The necessary condition is the "inductive photoperiod."

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