Anything exposed to the sunlight for some time becomes very hot especially during the summer months. A scrap of paper or a piece of metal kept in the sunlight may even become too hot to touch after a while. A plant leaf is made up of several layers of cells. The upper epidermis covers the top surface of the leaf and the lower epidermis cover the underside. The lower epidermis has many openings called stomata which act as valves. They regulate the exchange of gases between the leaf and air.
When they are open they allow carbon dioxide to go into the leaf. They also release oxygen and water vapour. When the stomata are closed inhaling or exhaling cannot take place. Each stomata is surrounded by two sausage-shaped guard cells which control the size of openings. The stomata are usually open during the day and closed at night. The water vapour that is lost by the leaf through the stomata is replaced by water from the roots. This process is called transpiration. So when water evaporates it cools the leaf. Hence this enables the plant to keep cool in the sunlight.
