National League for Nursing (NLN PAX) Practice Exam

When a small foil disk is attached to a leaf and the plant is placed in a well-lit area, the light facilitates photosynthesis in the leaf. During this process, chlorophyll in the leaf captures light energy, leading to the production of glucose and oxygen. Staining the leaf with Lugol's solution, which is an iodine solution, is a common method to detect the presence of starch, a product of photosynthesis.

In the area where the foil disk is located, photosynthesis cannot occur because the disk blocks light. Consequently, that specific area will not produce starch and therefore will not change color when treated with Lugol's solution. The regions of the leaf that were exposed to light and were able to photosynthesize would contain starch, resulting in a black coloration when stained with iodine. Thus, when the leaf is stained, it would show a mostly black appearance due to the presence of starch in the areas that underwent photosynthesis, except for the area corresponding to the disk, which will remain a lighter color, indicating the absence of starch. This creates the contrast described, where the leaf is mostly black with a white or lighter area corresponding to the disk's location.