The leaves of a tree are held by the branches. Leaves are usually held at the ends of the branches. The, although some have leaves along the branches. The main functions of leaves are photosynthesis and gas exchange. A leaf is often flat, so it absorbs the most light, and thin, so that the sunlight can get to the green parts in the cells, which convert sunlight, carbon dioxide from the atmosphere, and water from the roots, into glucose and oxygen. Most of a tree's biomass comes from this process. 