Question

why does a leaf covered with black paper not show positive test with iodine? (in short) ​

Answer 1

If the leaf is covered by black piece of paper on both top and bottom. It should turn yellow after a week, reason behind this is that chlorophyll will no longer have access to sunlight.

Answer 2

A leaf covered with a black paper not show positive test with Iodine because covered part was not able to to prepare their food in absence of Sunlight water it was unable to do photosynthesis.