the iodine test

The iodine solution turns blue-black all over the leaf.
This result indicates that starch is present in the whole leaf.
(邊個有blue-black就代表果度有starch)
..............turns brown ...............
..............indicates that there is no or a few starch in the leaf.
(若為brown色,代表葉子內的那部份沒有或只有少量starch存在)

first of all, iodine will react with starch and the colour of solution will change from brown to dark blue.
when the leaves become brown, this indicate there is no starch on the leaves. the brown colour is the original colour of iodine.
when the leaves become dark blue, this indicate there is starch presence on the leaves. when the leaves carry out photosynthesis, glucose is produced and changed into starch and then sucrose for transporting and storing. so when we apply iodine test, the starch made react with iodine to produce a dark blue colour.