Carbon dioxide enters the leaf through tiny openings called the stomata. These specialized apertures open during the day to allow for the exchange of carbon dioxide and water in a process known as transpiration.
The leaf of a plant is protected by a waxy substance called the cuticle. This prevents the air, containing carbon dioxide, from entering the leaf. Since carbon dioxide is one of the primary components that drive photosynthesis, plant leaves adapted and evolved these highly specialized structures to allow the passage of carbon dioxide. In some unicellular and marine autotrophic plants, carbon dioxide freely enters without passing through any specialized part.