One leaf covered

Take away the light from one part of one leaf, leave everything else exactly as it was, and see what the iodine says.

You need

A potted plant that has been kept in the dark for 48 hours, the materials for the starch test, and strips of black and of colourless polythene.

Method

From the potted plant that has been kept in the dark for 48 hours, choose two leaves of equal size — call them leaf A and leaf B. Cover part of leaf A with black polythene, and part of leaf B with colourless polythene.
Then leave this arrangement for 3 to 5 hours in a place where sunlight falls on it.
Carry out the starch test on leaves A and B as described in activity 2.2.

Figure to be added

Figure 2.5 — a plant in a clay pot. One leaf, labelled A, has a black polythene square clipped over part of it; another leaf, labelled B, has a colourless polythene square over part of it in the same way.

Why leaf B is covered too

It would have been simpler to cover one leaf and leave the other bare. It would also have been a worse experiment. Covering both means the two leaves differ in exactly one thing — whether light reaches them — and are alike in everything else, including being wrapped in plastic. Leaf B is the control.

Without the control, an objection is available

Suppose only leaf A were covered, and it produced no starch. Someone could reasonably say the polythene had stopped the air getting to it, or trapped heat, or damaged it. Leaf B answers all of those at once: it has polythene on it too, and it behaved differently. The only thing left that could explain the difference is the light.