Which one of the following parts of the pitcher plant becomes modified into a pitcher?

1. Pitcher plants are carnivorous plants that have modified structures to trap prey. 2. The characteristic pitcher structure, which holds digestive fluids, is a modification of the Leaf. 3. Different parts of the leaf may be involved depending on the species (e.g., the lamina, petiole, or tendril), but fundamentally, it's the leaf that forms the pitcher. 4. The stem, stipule, or petiole (alone, without the rest of the leaf blade being involved in pitcher formation) are not the primary parts modified into the pitcher.