Plants - breathe oxygen and carbon dioxide. In daylight they produce their own food at the same time by a process known as photosynthesis. During this process the plants release more oxygen into the atmosphere than they breathe in. The effect of the plants feeding and breathing system is that they exhale oxygen during the day and carbon dioxide during the night. This is the reason for the old custom in hospitals of removing plants and flowers from the wards at night and bringing them back in the morning. Without enough plants, the world's atmosphere would not be replenished with sufficient oxygen for the human race to survive. Generally plants breathe more slowly the colder it becomes. Some plants, including many trees, go into a form of hibernation during the winter, with their breathing reduced to a minimum.
