All gas exchange in a plant takes place in it's stomata. Stomata are microscopic openings, kind of like little mouths, that open and close and take in CO2 and transpire O2 at the discretion of...
Through pores in the green parts of plants (usually under leaves) called 'stomata'.
Through the open stomata on the underside of the leaves
stomata
carbon dioxide enter a plant for the leafs