Leaf is the main lateral organ of the upper part of a plant whose main function is photosynthesis. A leaf is a part of a plant that has a constant volume.
Leaf blades are generally flattened and thin to allow sunlight to pass through the cells and reach all the cells in the leaf that contain chloroplasts.
Apart from this, physiological functions such as respiration, evapotranspiration etc. are done in the leaves. Leaves can also serve as food and water reservoirs. In some plants, the shape of the leaves may change due to special needs.