place the leaves in boiling water for six minutes to soften the cells, and then in ethanol (alcohol). when the leaves are drained of colour, spread them out on a flat surface. soak the leaves in iodine and the green parts will turn blue/black and the non green parts will stay the yellowish brown colour of iodine liquid. the green parts contain starch, which makes iodine turn blue/black. the blue/black stain is what shows starch is present. you could take "before &amp; after" photos to prove the black areas were green at the start.