Answer:
Due to presence of waxy layer.
Explanation:
A leaf cannot be tested for starch by adding iodine solution directly because waxy cuticle covering the leaves that prevents the penetration of iodine in the leave cells. First, the waxy layer should be removed by boiling the leaves that causes disruption in the cell membranes that make easier for the iodine to penetrate in the leave cells so that's why iodine can not be tested for starch directly.
