In Lathyrus aphaca, the entire leaf is modified into tendrils.

Answer

### Step-by-Step Solution:

1. **Identify the Plant**: The question refers to the plant Lathyrus aphaca, commonly known as yellow wedge.

2. **Understand Leaf Modification**: In Lathyrus aphaca, the entire leaf structure is modified into tendrils. This adaptation helps the plant to climb and support itself.

3. **Role of Stipules**: While the leaves are modified into tendrils, the stipules (small leaf-like structures at the base of the leaf stalk) become enlarged. These enlarged stipules take over the function of photosynthesis, which is crucial for the plant's energy production.

4. **Function of Tendrils**: The tendrils help the plant to cling onto nearby structures, allowing it to grow upwards and access more sunlight, which is essential for its survival.

5. **Conclusion**: In summary, in Lathyrus aphaca, the entire leaf is transformed into tendrils for climbing, while the stipules are enlarged to perform photosynthesis.