Starch is primarily stored in the chloroplasts of a leaf. Chloroplasts are the organelles responsible for photosynthesis, and they contain the green pigment chlorophyll. During the process of photosynthesis, plants convert sunlight into chemical energy, producing glucose as a byproduct. Some of this glucose is then converted into starch for storage.
In leaves, starch is often stored in the form of small granules within the chloroplasts. When the plant needs energy and cannot photosynthesize (for example, during the night or in non-ideal weather conditions), it can break down the stored starch into glucose and utilize it for respiration and growth.