After staining a leaf with Lug’s solution, how would the leaf appear?

• A. Uniformly black
• B. White
• C. Mostly black except for a white area corresponding to the presence of the disk
• D. Mostly white except for a black area corresponding to the presence of the disk

Answer: (C)

Lugol’s iodine stains starch blue-black. In a leaf that has been photosynthesizing, starch forms where light is available. A disk placed on the leaf blocks light in that patch, so little or no starch is produced there. When you apply Lugol’s solution, starch-containing areas turn blue-black, while the shaded patch remains pale. So the leaf would be mostly dark blue-black with a white area under the disk where starch wasn’t formed.