Assertion: The external factors that affected photosynthesis are number, size age and orientation of leaves, mesophyll cells and chloroplasts and the amount of chlorophyll.
Reason: The internal factors that affect photosynthesis are availability of sunlight, temperature `CO_(2)` concentration and water.

Explanation

To analyze the assertion and reason provided in the question, we need to break down the concepts of external and internal factors affecting photosynthesis.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that external factors affecting photosynthesis include the number, size, age, and orientation of leaves, mesophyll cells, chloroplasts, and the amount of chlorophyll.
- **Analysis**: These factors are actually internal characteristics of the plant. External factors typically refer to environmental conditions that influence photosynthesis, such as light intensity, temperature, carbon dioxide concentration, and water availability.

2. **Understanding the Reason**:
- The reason states that internal factors affecting photosynthesis include the availability of sunlight, temperature, carbon dioxide concentration, and water.
- **Analysis**: This is incorrect because sunlight, temperature, carbon dioxide concentration, and water are external factors. Internal factors would include the physiological and structural characteristics of the plant itself, such as the number of chloroplasts, leaf structure, and genetic factors.

3. **Conclusion**:
- Both the assertion and the reason are incorrect. The assertion incorrectly categorizes internal factors as external, and the reason incorrectly categorizes external factors as internal.

### Final Answer:
Both the assertion and the reason are false statements.

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