What the two leaves showed

The iodine draws the shape of the polythene onto the leaf. That is the whole result, and it is hard to argue with.

Figure to be added

Figure 2.6 — the two leaves after the starch test. Leaf A is dark blue over most of its surface, with a pale rectangle exactly where the black polythene had been, labelled as showing no colour change. Leaf B is dark purple all over, with no pale patch, labelled dark purple or blue.

The observation

When iodine solution was added to leaf A, no colour change was seen in the part that had been covered with black polythene.

When iodine solution was added to leaf B, the part that had been covered with colourless polythene turned dark purple or blue, just like the rest of the leaf.

Reading the result

Because it had been covered with black polythene, that part of leaf A did not receive sunlight. Photosynthesis therefore did not take place in that part, so no starch was produced there, and no colour change occurred with the iodine solution.

The leaf covered with colourless polythene received sunlight, so photosynthesis took place and starch was produced. The polythene was there; the light got through it; the starch appeared.

The conclusion

According to this, it can be concluded that sunlight is necessary for photosynthesis.

No light, no starch

A destarched leaf covered with black polythene shows no colour change with iodine; one covered with colourless polythene does. Sunlight is necessary for photosynthesis.