Sunlight and the green patches

Bhaskar knows plants need water and sunlight to grow, but he wonders: is sunlight needed to make starch itself?

Activity 10.3 answers it neatly. He takes two similar potted plants whose leaves have both green and non-green (pale) patches. One plant stays in sunlight; the other is kept in the dark for 36 hours. From each he takes a leaf, traces its patches on tracing paper (so he can compare later), and does the iodine test.

On the leaf from the sunlit plant, the green patches turned blue-black: starch was there. The non-green patches did not turn blue-black.

The leaf from the plant kept in the dark showed no blue-black colour anywhere, not even on its green patches. No starch had been made.

Put the clues together. Starch appears only where there is green colour (chlorophyll) and where the plant has had sunlight. So chlorophyll prepares starch in the presence of sunlight, and both are essential.

Do the pale patches have no chlorophyll at all? The book is careful: they may just not have enough chlorophyll to make starch that the iodine test can detect.

What about leaves that look red, violet or brown? They contain more of those coloured pigments than chlorophyll, which hides the green. Some of those pigments help in photosynthesis too, and the iodine test can still show starch in such leaves.