Answer:
Because they are yet to perform photosynthesis, which requires sunlight
Explanation:
The leaves of plants are the site of a unique process called PHOTOSYNTHESIS. Photosynthesis is the process by which plants synthesize their food in form of glucose in the presence of sunlight. The glucose they synthesize is however, stored as STARCH (a carbohydrate polymer) in the leaves. This is why the leaves of a plant will naturally test positive for starch at certain times.
However, the leaves of plants will not test positive for starch early in the morning but do so at the end of the day because the leaves of that particular plant is YET to undergo photosynthesis due to lack of light from sun. However, at the end of the day, it would have performed photosynthesis and synthesize sugar and hence, test positive.
