There are many function of a leaf in plants such as Making food for plant by the process of Photosynthesis, Gaseous exchange and Transpiration.
Making food: The main function of a leaf is to make food. The food is made by a leaf in the presence of sunlight, Carbon dioxide, water and Chlorophyll. The process by which a leaf make food is called Photosynthesis.
Gaseous exchange: Stomata are tiny pores present in the lower surface of any leaves. Through these Stomata plants take in Carbon dioxide and release Oxygen during day time. At night plants take in Oxygen and release Carbon dioxide.
Transpiration: The excess water taken by plants is release into air in the form of water vapour through stomata present in the leaves. This process is called Transpiration. It makes trees cool.