Plants do not have any special respiratory organs. Roots take up oxygen from air trapped in the soil by means of root hairs. Root hairs are embedded in the soil. Oxygen in the air surrounding them diffuses into the root hair and from there into the roots.
Carbon dioxide given out, similarly, diffuses out through roots. Stomata in leaves opens to let in oxygen and release carbon dioxide. In the older parts of roots or bark of woody plants, tiny openings called lenticels are present. It is through these lenticels that oxygen reaches the inner living tissues and carbon dioxide moves out.
Guard cells help in the opening and closing of stomata. When guard cells get filled up with water, they swell and become turgid. The two guard cells curve away from each other opening the stomata. When guard cells become flaccid, stoma closes. Minerals also play a role in making guard cells turgid or flaccid.