This experiment includes several important steps. The first step, involving boiling the leaf in water, kills the cells of the leaf. Next, the now dead leaf is boiled in ethanol. This step allows for chlorophyll to be extracted (this chlorophyll can be used in other experiments). Both boiling steps help to soften the waxy cuticle covering the leaf, disrupt the cell membranes, and make it easier for the iodine to penetrate the cells.
Chlorophyll in plants allows plants to trap sunlight for a process known as photosynthesis. Photosynthesis uses water, carbon dioxide, and light energy to produce a sugar known as glucose. Much of the glucose made (that which is not used immediately to supply energy to the plant) must be stored. In humans, we store the glucose in our liver and muscle cells as a polymer known as glycogen, or as fat. In plants the glucose is stored as a similar polymer known as starch.
When iodine comes into contact with starch, the color changes from a yellow-brown to a very dark blue. Spreading the leaf out on a white tile and then adding the iodine in drops until the leaf is covered allows the observer to see exactly were the starch is stored within the leaf.