In plants, oxygen and carbon dioxide diffuse through the stomata and the intercellular spaces of the leaves, and the lenticels of the bark.Lenticels are small openings in the pits of the bark. The exchange of gases takes place through the lenticels also, apart from the exchange through the openings in the leaves.The direction of exchange of gases between a plant and its surroundings depends upon the time of the day and the usage of the gases by the plant. Plants respire throughout the day, while photosynthesis takes place only in the presence of sunlight. In daytime, carbon dioxide produced in respiration by the plants is used by them in photosynthesis. So, carbon dioxide is not released into the environment.In fact, plants take in additional carbon dioxide from the air for photosynthesis. Out of the oxygen produced in photosynthesis, some amount is consumed by the plants in respiration.The rest is released into the air through the stomata. At night, when there is no photosynthesis, no oxygen is released. Also, the carbon dioxide produced in respiration is not used by the plants. So, it is released in the air.