The correct answer to why leaves of many grasses are capable of folding and unfolding is option 4, they have specialized bulli-form cells. Bulli-form cells are a unique cell type found in certain grasses that allow the leaves to fold and unfold. These cells have a high water content and are located at the base of the leaf. They expand and contract based on the water pressure inside them, causing the leaf to curl or straighten. This mechanism helps the grasses to conserve water during dry periods by reducing the surface area exposed to the air and therefore reducing water loss through transpiration. When there is sufficient water available, the bulli-form cells fill with water and the leaf unfolds to capture more sunlight for photosynthesis.

Options 1, 2, and 3 are not the correct answers because:

Option 1: The differentiation of mesophyll into palisade and spongy parenchyma is unrelated to the folding and unfolding capability of the grass leaves.

Option 2: Having stomata on both sides of the leaf is a common feature in many plant leaves and does not specifically contribute to folding and unfolding ability.

Option 3: High levels of silica, although present in some grasses, is not the primary reason for their ability